Physical World/Astronomy
Planets
The International Astronomical Union (IAU) defines a planet, for the purposes of our solar system only, as a celestial body that:
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit
Inferior planet – used in reference to Mercury and Venus, which are closer to the Sun than the Earth is
Superior planet – used in reference to Mars, Jupiter, Saturn, Uranus, Neptune, and all the known minor and dwarf planets, including Ceres and Pluto, which are farther from the Sun than the Earth is
Inner planets – those planets which lie inside the asteroid belt, i.e. Mercury, Venus, Earth and Mars
Outer planets – those that lie outside the asteroid belt, i.e. the four gas giants
Following the discovery of Neptune, there was considerable speculation that another planet might exist beyond its orbit. The search culminated at the start of the 20th century with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the gas giants
Rogue planet – also known as orphan planet, is a planetary-mass object which has either been ejected from its system or was never gravitationally bound to any star and that therefore orbits the galaxy directly
Circumbinary planet – a planet that orbits two stars instead of one
Vulcan was a small planet proposed to exist in an orbit between Mercury and the Sun. Attempting to explain peculiarities of Mercury's orbit, the 19th-century French mathematician Urbain Le Verrier hypothesized that they were the result of another planet, which he named ‘Vulcan’
Kepler's laws of planetary motion are three scientific laws describing the motion of planets around the Sun:
First law – The orbit of every planet is an ellipse with the Sun at a focus
Second law – A line joining a planet and the Sun sweeps out equal areas during equal intervals of time
Third law – The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit (mean distance from the Sun)
Titius-Bode law (sometimes termed just Bode's law) is a hypothesis that the semi-major axes of planets in the solar system follow a simple rule. It was discredited with the discovery of Neptune
Equatorial bulge – a difference between the equatorial and polar diameters of a planet, due to the centrifugal force of its rotation. A rotating body tends to form an oblate spheroid rather than a sphere. The Earth has an equatorial bulge of 42.7 km. The Sun does not have an equatorial bulge
Mercury
| Name | Mercury |
| Diameter | 4879 km |
| Mass | 0.33 x 1024 kg |
| Density | 5,427 kg/m3 |
| Average distance from Sun | 57.9 million km (0.39 AU) |
| Orbital period (year) | 88 days |
| Sidereal rotation period | 58.6 days |
| Average temperature | 167oC |
| Atmosphere | Very thin and blown away by solar winds |
| Number of moons | None |
| Largest moons |
Exosphere of Mercury contains oxygen
Second densest planet, after Earth
Mercury is the second hottest planet, after Venus, as Mercury has no atmosphere to help regulate the temperature
Mercury has just 38% the gravity of Earth
Mercury has a large iron core
Ganymede and Titan are larger than Mercury
Transit of Mercury occurs every 3 to 13 years and occur in May or November
Mercury has the highest orbital eccentricity of all the planets. Mercury’s orbit is inclined by 7 degrees to the plane of Earth’s orbit. Caused by curvature of space-time
Mercury moves at an average speed of 48 km/s, making it the fastest planet
Names for features on Mercury come from a variety of sources. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field
Caloris Basin, also called Caloris Planitia, is a large crater on Mercury about 1,550 km in diameter, one of the largest impact basins in the Solar System
Venus
| Name | Venus |
| Diameter | 12,104 km |
| Mass | 4.87 x 1024 kg |
| Density | 5,243 kg/m3 |
| Average distance from Sun | 108.2 million km (0.73 AU) |
| Orbital period (year) | 225 days |
| Sidereal rotation period | 243 days |
| Average temperature | 462oC |
| Atmosphere | Carbon dioxide 96.5% |
| Number of moons | None |
| Largest moons |
Venus is the closest planet to the earth. The minimum distance between Earth and Venus is 25.5 million miles
Venus is sometimes called Hesperus (evening star) or Phosphorus or Lucifer (morning star)
Venus has retrograde rotation
A day on Venus lasts longer than a year
Venus has acid rain (sulphuric acid)
Venus is the hottest planet
Venus has 50,000 dormant volcanoes
After the Moon, Venus is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6
Venus has lowest orbital eccentricity (closest planet to a circular orbit)
Transit of Venus first observed by Jeremiah Horrocks in 1639
Mikhail Lomonosov was the first person to hypothesize the existence of an atmosphere on Venus based on his observation of the transit of Venus of 1761
Transits of Venus are among the rarest of predictable astronomical phenomena and currently occur in a pattern that repeats every 243 years, with pairs of transits eight years apart. Last in 2004 and 2012, next in 2117 and 2125
For the 1769 transit, observations were made from Tahiti at a location still known as Point Venus. This occurred on the first voyage of James Cook, who was one of the observers to use parallax to work out the distance from the Earth to the Sun
During a transit there are four ‘contacts’, when the circumference of the small circle (small body disk) touches the circumference of the large circle (large body disk) at a single point
Black drop effect is an optical phenomenon visible during a transit of Venus. Just after second contact, and again just before third contact during the transit, a small black ‘teardrop’ appears to connect Venus' disk to the limb of the Sun, making it impossible to accurately time the exact moment of second or third contact
Maxwell Montes is a mountain massif on Venus, part of which contains the highest point on the planet's surface. Located on Ishtar Terra, it is 11,000 m high. It is named for James Clerk Maxwell, and is, along with Alpha Regio and Beta Regio, one of three exceptions of the rule that Venusian surface features are named for women or goddesses
Maat mons is the highest volcano on Venus
Venus is associated with copper
Earth
| Name | Earth |
| Diameter | 12,756 km |
| Mass | 5.97 x 1024 kg |
| Density | 5,514 kg/m3 |
| Average distance from Sun | 149.6 million km (1.0 AU) |
| Orbital period (year) | 365.2564 days |
| Sidereal rotation period | 23 hours 56 minutes 4 seconds |
| Average temperature | 15oC |
| Atmosphere | Nitrogen 78.1% Oxygen 20.9% Argon 0.93% Carbon dioxide 0.04% |
| Number of moons | 1 |
| Largest moons | Moon |
Earth is the only planet not named after a god
The orbital speed of Earth averages about 29.78 km/s (107,200 km/h; 66,600 mph)
When Earth was formed, day length was six hours, as rotation speed was faster
The Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane
Earth comes closest to the Sun every year around January 3. It is farthest from the Sun every year around July 4
Earth is the densest planet in the Solar System
The age of the Earth is around 4.54 billion years
Temperature at Earth’s core is 4000o C
Temperature in space near Earth is 2.7 Kelvin
Earth's crust occupies less than 1% of Earth's volume
Earth’s crust (by weight) – oxygen 46%, silicon 27%, aluminium 8%
Van Allen radiation belt is a torus of energetic charged particles (plasma) around Earth, which is held in place by Earth's magnetic field. It is split into two distinct belts, with energetic electrons forming the outer belt and a combination of protons and electrons creating the inner belt
South Atlantic Anomaly – refers to the area where the Earth's inner Van Allen radiation belt comes closest to the Earth's surface. This leads to an increased flux of energetic particles in this region and exposes orbiting satellites to higher than usual levels of radiation
Moon
Moon is 240,000 miles from Earth and has a diameter of 2160 miles
14th largest body in Solar System (between Io and Europa)
Moon is 1.3 light seconds from earth
Moon daytime temperature 107oC, nighttime temperature -153oC
Moon orbital period is 27.32 days
The South Pole-Aitken basin is an impact crater on the Moon. Roughly 2500 kilometers in diameter and 13 kilometers deep, it is the largest known impact crater not only on the Moon, but also in the entire solar system
Terminator – the edge of the shadow between night and day on the moon
The dark areas visible on the moon, the lunar maria, are plains of flood basaltic lava flows. They were dubbed maria, Latin for ‘seas’, by early astronomers who mistook them for actual seas
Nearly the entire lunar surface is covered with regolith (a layer of loose, heterogeneous material covering solid rock)
Oceanus Procellarum, Latin for ‘Ocean of Storms’, is a vast lunar mare on the western edge of the near side of the Moon
Moon’s surface is deformed by 20 m by the gravitational effect of the Earth
Features on the moon – mare (seas), lacus (lakes), palus (marshes), sinus (bays)
Tidal forces between Earth and the Moon have slowed the moon's rotation so that the same side is always facing the Earth. The other face, which is never visible from the Earth in its entirety, is therefore called the ‘far side of the Moon’. The far side should not be confused with the ‘dark side’ (the hemisphere that is not illuminated by the Sun at a given point in time), as the two are the same only during a full moon
The Moon's rotation and orbital periods are both just under four weeks, so no matter when the Moon is observed from the Earth the same hemisphere of the Moon is always seen. Due to tidal locking
40% oxygen, 19% silicon, 14% iron in moon rocks
Mendeleev Crater – on dark side of the moon
Mare Nubium – ‘sea of clouds’ on the Moon
The giant impact hypothesis is the now-dominant scientific theory for the formation of the Moon, which is thought to have formed as a result of a collision between the young Earth and a Mars-sized body over 4.5 billion years ago that is sometimes called Theia, derived from Greek mythology, as Theia was the Titan who gave birth to the Moon goddess Selene. The hypothesis was first published in Icarus in 1975 by Hartmann and Davis
Presence of the moon prevents the earth from wobbling
At a major lunar standstill, which takes place every 18.6 years, the range of the declination of the Moon reaches a maximum. As a result, at high latitudes, the Moon's greatest altitude changes in just two weeks from high in the sky to low over the horizon
The Moon is thought to have formed nearly 4.5 billion years ago, not long after the Earth
Phases of the moon – new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent
Waxing – amount of lit area of the moon increases over time
Waning – amount of lit area of the moon decreases over time
Crescent moon – part way between a half moon and a new moon, or between a new moon and a half moon
Gibbous moon – between a full moon and a half moon, or between a half moon and a full moon
Blue moon – when two full moons occur in a single month, the second full moon is called a blue Moon
Harvest moon – the full moon nearest the autumnal equinox (in October)
Hunter’s moon – follows the harvest moon, in November
Selenography – study of the moon's surface
Mars
| Name | Mars |
| Diameter | 6,792 km |
| Mass | 6.4 x 1023 kg |
| Density | 3,933 kg/m3 |
| Average distance from Sun | 227.9 million km (1.38 AU) |
| Orbital period (year) | 687 days |
| Sidereal rotation period | 24.6 hours |
| Average temperature | -63oC |
| Atmosphere | Carbon dioxide 95.3% |
| Number of moons | 2 |
| Largest moons | Phobos, Deimos |
Mars has just 37% the gravity of Earth
At the closest points of their orbits, Earth and Mars are 34.8 million miles apart
Mars lost its magnetosphere four billion years ago, so the solar wind interacts directly with the Martian ionosphere, keeping the atmosphere thinner than it would otherwise be by stripping away atoms from the outer layer. The atmosphere on Mars consists of 95% carbon dioxide, 3% nitrogen, 1.6% argon and contains traces of oxygen and water
Olympus Mons is the tallest known volcano and mountain in the Solar System
For a time in the late 19th and early 20th centuries, it was erroneously believed that there were canals on Mars. These were a network of long straight lines that appeared in drawings of the planet Mars, first observed by the Italian astronomer Giovanni Schiaparelli in 1877
Hellas Planitia is a huge, roughly circular impact basin located in the southern hemisphere of Mars. It is the largest visible impact crater known in the Solar System
Utopia Planitia is the largest recognized impact basin on Mars and in the Solar System
Valles Marineris is a system that runs along the Martian surface east of the Tharsis region. At more than 4000 km long, 200 km wide and up to 7 km deep, the Valles Marineris rift system is larger than any of Earth's largest canyons, and is the largest known canyon in the solar system
Mars has red skies during the day and blue skies at night
Elysium Planitia is the second largest volcanic region on Mars, after Tharsis
Moons
Phobos (larger, with a diameter of 22 km) and Deimos. Named after sons of Ares and Aphrodite. Phobos is the god of fear and panic. Deimos is the god of dread and terror. Discovered by Asaph Hall in 1877
Jupiter
| Name | Jupiter |
| Diameter | 142,984 km |
| Mass | 1.9 x 1027 kg |
| Density | 1,326 kg/m3 |
| Average distance from Sun | 778.3 million km (5.20 AU) |
| Orbital period (year) | 4,333 days (11.9 years) |
| Sidereal rotation period | 9.9 hours |
| Average temperature | -108oC |
| Atmosphere | Hydrogen 89.6% Helium 10.1% |
| Number of moons | 80 |
| Largest moons | Ganymede, Callisto, Io, Europa |
Jupiter has the shortest day of any planet
Jupiter’s atmosphere was first observed by Voyager in 1979
Jupiter and the other gas giants have small amounts of methane and other trace gases in the atmosphere
Flashes of lightning have been detected in the atmosphere of Jupiter
Jupiter has the highest gravity of the planets, 2.5 times greater than gravity on Earth
Jupiter’s gravitational field pushes asteroids towards earth
The visible surface of Jupiter is divided in a number of bands parallel to the equator. There are two types of bands: lightly coloured ‘zones’ and relatively dark ‘belts’
Great Red Spot is a huge storm on Jupiter. The first observation of the Great Red Spot was in the seventeenth century. The red spot spins anticlockwise and takes six (Earth) days to rotate completely
Jupiter has a faint planetary ring system
Moons
Moons of Jupiter are named after figures who figured in the life of Zeus
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Moons of Jupiter are named after figures who figured in the life of Zeus
Galilean moons are named after Zeus’s lovers. Discovered in 1610 by Galileo
Ganymede is the largest moon in the Solar System (5262 km diameter), Titan is the second largest (5151 km diameter), Callisto is the third largest and is thought to have the oldest surface in the solar system
Ganymede has a magnetic field and auroras
Callisto is the third-largest moon in the Solar System
Valhalla is the largest multi-ring impact crater on Callisto and in the Solar System
Io is named after a fire goddess. Io's surface is dotted with volcanic depressions known as paterae. Io is the most active volcanic moon in the solar system. May be melted by Jupiter’s gravitational pull. Innermost of the Galilean moons
Ole Romer was a Danish astronomer who in 1676 made the first quantitative measurements of the speed of light by studying eclipses of Io
Europa has the smoothest surface of any known solid object in the Solar System. It may have water below the frozen surface
Amalthea is the largest of the inner satellites of Jupiter. Irregularly shaped and reddish in colour, it is thought to consist of porous water ice. Last satellite to be discovered by direct visual observation
Saturn
| Name | Saturn |
| Diameter | 120,536 km |
| Mass | 5.7 x 1026 kg |
| Density | 687 kg/m3 |
| Average distance from Sun | 1,426.7 million km (9.58 AU) |
| Orbital period (year) | 10,756 days (29.5 years) |
| Sidereal rotation period | 10.7 hours |
| Average temperature | -139oC |
| Atmosphere | Hydrogen 96.3% Helium 3.2% |
| Number of moons | 83 |
| Largest moons | Titan, Rhea, Iapetus, Dione, Tethys |
Saturn has the lowest density of the planets
Saturn has the largest equatorial bulge
Giant White Spot appears every c. 30 years
Saturn is the most distant planet that can be seen with the naked eye
Saturn’s upper atmosphere is divided into bands of clouds. The top layers are mostly ammonia ice
Galileo was the first to observe the rings of Saturn in 1610 using his telescope, but was unable to identify them as such
Christian Huygens was the first to suggest that Saturn was surrounded by a ring detached from the planet
Cassini Division is a 2980 mile wide region between the A Ring and B Ring. It was discovered in 1675 by Giovanni Cassini. The inner edge of the Cassini Division is governed by a strong orbital resonance with the moon Mimas
Huygens Gap is located at the inner edge of the Cassini Division
The densest parts of the Saturnian ring system are the A and B Rings. Along with the C Ring, which was discovered in 1850 and is similar in character to the Cassini Division, these regions comprise the main rings
The Encke Gap is within the A Ring. It is caused by the presence of the small moon Pan, which orbits within it
The Keeler Gap is within the A Ring. The small moon Daphnis orbits within it
The separation between the A Ring and the F Ring has been named the Roche Division in honor of the French physicist Edouard Roche
The F Ring is the outermost discrete ring of Saturn and perhaps the most active ring in the Solar system. It is located 3000 km beyond the outer edge of the A Ring. It was discovered in 1979 by Pioneer 11. It is just a few hundred kilometres wide, and is held together by two shepherd moons, Prometheus and Pandora, which orbit inside and outside it
Jet stream of Saturn is a persisting hexagonal wave pattern around the north polar vortex
Moons
Moons of Saturn are named after mythological Greek titans, and giants from Inuit, Norse and Gallic mythology
Titan is the second largest moon in the Solar System
Atmosphere of Titan is 98% nitrogen
Titan has lakes of liquid methane
Titan is the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found
Iapetus is the ‘yin and yang’ moon of Saturn – half clean ice, half black deposits
Hyperion is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance. It was the first non-round moon to be discovered, in 1848
Enceladus is most reflective object, in Saturn’s E ring. Has ice fountains (geysers). It was discovered in 1789 by William Herschel and is named after a child of Gaia
Tiger stripes of Enceladus consist of four sub-parallel, linear depressions in the south polar region
Calypso is moon of Saturn named after a naiad (nymph) in Greek mythology
Mimas is a moon of Saturn that was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I. Mimas closely resembles the Death Star in the Star Wars films
Phoebe is a moon of Saturn which revolves in opposite direction to other moons. The first moon to be discovered photographically, by William Pickering in 1899
Saturn has the only known trojan satellites: Saturn's moon Tethys has two trojan moons (Telesto and Calypso), and Dione also has two trojan moons (Helene and Polydeuces)
Uranus
| Name | Uranus |
| Diameter | 51,118 km |
| Mass | 8.7 x 1025 kg |
| Density | 1,271 kg/m3 |
| Average distance from Sun | 2,870.7 million km (19.22 AU) |
| Orbital period (year) | 30,687 days (84 years) |
| Sidereal rotation period | 17.2 hours |
| Average temperature | -197oC |
| Atmosphere | Hydrogen 82.5% Helium 15.2% |
| Number of moons | 27 |
| Largest moons | Titania, Oberon, Umbriel, Ariel, Miranda |
Uranus was originally known as Georgium Sidus (the Georgian Planet), in honour of King George III. Only planet whose name is derived from Greek mythology
William Herschel observed Uranus in 1781
Uranus is blue-green due to the absorption of red light by methane in the upper atmosphere
Uranus has retrograde rotation
Uranus's axis of rotation lies on its side with respect to the plane of the Solar System, with an axial tilt of 97.8 degrees. This gives it seasonal changes completely unlike those of the other major planets
Uranus and Neptune are sometimes referred to as “ice giants”
Uranus has 13 narrow rings, the brightest being the epsilon ring
Ring system confirmed around Uranus in 1977 using the Kuiper Airborne Observatory
Moons
Moons of Uranus are all named after characters from the works of William Shakespeare and Alexander Pope
Titania is the eighth largest moon in the Solar System. Discovered by William Herschel in 1787
Three of the moons of Uranus are named after characters from The Rape of the Lock: Belinda, Umbriel, and Ariel, the latter name shared with Shakespeare's The Tempest
Miranda has one of the most extreme and varied topographies of any object in the Solar System, including Verona Rupes, a 20 km high scarp that is the highest cliff in the Solar System
Neptune
| Name | Neptune |
| Diameter | 49,528 km |
| Mass | 1.02 x 1026 kg |
| Density | 1,638 kg/m3 |
| Average distance from Sun | 4,498.4 million km (30.1 AU) |
| Orbital period (year) | 60,190 days (164.8 years) |
| Sidereal rotation period | 16.1 hours |
| Average temperature | -201oC |
| Atmosphere | Hydrogen 79.5% Helium 18.5% |
| Number of moons | 14 |
| Largest moons | Triton, Proteus, Nereid |
Urbain Le Verrier predicted the existence of the then unknown planet Neptune, using only mathematics and astronomical observations of the known planet Uranus. At the same time, but unknown to Le Verrier, similar calculations were made by John Couch Adams in Cambridge. Le Verrier transmitted his own prediction in a letter to Johann Galle of the Berlin Observatory, and the planet was found with the Berlin Fraunhofer refractor that same evening, 23 September 1846, by Galle and Heinrich d'Arrest
Methane in the atmosphere absorbs red light, which makes the planet appear blue
Neptune has the strongest sustained winds of any planet in the Solar System
Great Dark Spot is a dark spot on Neptune similar in appearance to Jupiter's Great Red Spot. It was detected in 1989 by NASA’s Voyager 2 probe. When the spot was to be photographed again in 1994 by the Hubble Space Telescope, the spot had disappeared completely. However, an almost identical spot emerged in Neptune's northern hemisphere. This new spot is called the Northern Great Dark Spot
Neptune has a planetary ring system, though one much less substantial than that of Saturn. The rings of Neptune consist primarily of five principal rings and were first discovered in 1984. Neptune's rings are named after astronomers who contributed important work on the planet: Galle, Le Verrier, Lassell, Arago, and Adams
Moons
Moons of Neptune are named after water nymphs
Triton was discovered by British brewer William Lassell
Triton has an atmosphere containing nitrogen
Triton is unique among all large moons in the solar system for its retrograde orbit around the planet (i.e., it orbits in a direction opposite to the planet's rotation). Most of the outer moons of Jupiter and Saturn also have retrograde orbits, as do some of Uranus' outer moons. But all of those other retrograde moons are quite small; the largest of them (Phoebe, a moon of Saturn) has only 8% of the diameter (and 0.03% of the mass) of Triton
Cryovolcano (ice volcano) is a volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. Collectively referred to as cryomagma or ice-volcanic melt, these substances are usually liquids and form plumes, but can also be in vapour form. First observed on Triton
Moons
A natural satellite, or moon, is a celestial body that orbits another body (a planet, dwarf planet, or small Solar System body), which is called its primary, and that is not artificial. There are 207 known natural satellites orbiting planets in the Solar System
Shepherd moons – small moon that orbits near the outer edges of planetary rings or within gaps in the rings. The gravity of shepherd moons serves to maintain a sharply defined edge to the ring
Tidal acceleration – an effect of the tidal forces between an orbiting natural satellite (i.e. a moon), and the planet (called the primary) that it orbits. The ‘acceleration’ is usually negative, as it causes a gradual slowing and recession of a satellite, and a corresponding slowdown of the primary's rotation. The process eventually leads to tidal locking of first the smaller, and later the larger body. The Earth-Moon system is the best studied case
Tidal locking – (or captured rotation) occurs when the gravitational gradient makes one side of an astronomical body always face another, an effect known as synchronous rotation. For example, the same side of the Moon always faces the Earth
Trojan – a minor planet or natural satellite (moon) that shares an orbit with a larger planet or moon, but does not collide with it. The term originally referred to the Trojan asteroids orbiting around Jupiter's Lagrangian points. Both Tethys and Dione have two trojan moons
Late Heavy Bombardment – period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well
A ray system comprises radial streaks of fine ejecta thrown out during the formation of an impact crater, looking like many thin spokes coming from the hub of a wheel
Exoplanets
An exoplanet or extrasolar planet is a planet that does not orbit the Sun and instead orbits a different star, stellar remnant, or brown dwarf. More than 1800 exoplanets have been discovered (December 2014). There are also rogue planets, which do not orbit any star and which tend to be considered separately
The Kepler space telescope has also detected a few thousand candidate planets
In 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi)
Hot Jupiters are a class of extrasolar planets whose characteristics are similar to Jupiter, but which have high surface temperatures because they orbit very close to their parent stars
Super-Earth – an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's smaller gas giants Uranus and Neptune
SuperWASP (Wide Angle Search for Planets) is an international academic organization performing an ultra-wide angle search for transiting extrasolar planets. WASP-1b was found in 2006
A habitable zone (HZ) is a region of space where conditions are favorable for life as it can be found on Earth. There are two regions that must be favorable, one within a solar system and the other within the galaxy. Planets and moons in these regions are the likeliest candidates to be habitable and thus capable of bearing extraterrestrial life. Life is most likely to form within the circumstellar habitable zone (CHZ) within a solar system, and the galactic habitable zone (GHZ) of the larger galaxy.The HZ may also be referred to as the ‘life zone’, ‘Green Belt’ or the ‘Goldilocks Zone’
Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets from radial velocity measurements via observation of Doppler shifts in the spectrum of the star around which the planet orbits. Otto Struve proposed in 1952 the use of powerful spectrographs to detect distant planets. He described how a very large planet would cause its parent star to wobble slightly as the two objects orbit around their centre of mass
Dwarf planets
A dwarf planet is defined, for the purposes of our solar system only, as a celestial body that:
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite
The IAU currently recognizes five dwarf planets—Ceres, Pluto, Haumea, Makemake, and Eris. However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition
Ceres is a dwarf planet in the asteroid belt. Its name is derived from the Roman goddess Ceres, the goddess of growing plants and of motherly love. It was the first asteroid to be discovered, in 1801 by Piazzi in Palermo
Eris orbits the Sun in the scattered disk accompanied by a moon, Dysnomia. Discovered by Mike Brown at the Mount Palomar observatory. Formerly known as 2003UB313, Xena and Lila. Named after Eris, the Greek mythological goddess personifying discord. Her Latin name is Discordia. Largest dwarf planet
Haumea is a dwarf planet in the Kuiper belt. Its mass is one-third the mass of Pluto. It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory. In 2008, it was accepted as a dwarf planet and named after Haumea, the Hawaiian goddess of childbirth
Haumea has two moons – Hi’iaka and Namaka
Makemake was discovered in 2005 by Mike Brown. Its diameter is roughly three-quarters that of Pluto. Makemake is named after the creator of humanity and god of fertility in the mythos of the Rapanui, the native people of Easter Island
Pluto was discovered by Clyde Tombaugh in 1930. The name Pluto was proposed by Venetia Burney, an 11-year-old schoolgirl from Oxford
Pluto was classified as a planet until 2006
Pluto and Charon are now considered to be a double dwarf planet
Charon was discovered by James Christie in 1978
Pluto has four smaller moons – Nix and Hydra, both discovered in 2005, and Kerberos and Styx, both named in 2013
Small Solar System Bodies
Small Solar System Body (SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neither planets nor dwarf planets:
This encompasses:
- all minor planets apart from the dwarf planets, i.e.
- the classical asteroids, (except for 1 Ceres, the largest);
- the Centaurs and Neptune Trojans; the trans-Neptunian Objects (except for dwarf planets such as Pluto, Makemake and Eris);
- all comets
Minor planet – an astronomical object in direct orbit around the Sun that is neither a planet nor originally classified as a comet. Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, and other trans-Neptunian objects. The orbits of 620,000 minor planets were archived at the Minor Planet Center by 2013
Trans-Neptunian objects
A trans-Neptunian object (TNO) is any object in the solar system that orbits the Sun at a greater distance on average than Neptune. The first trans-Neptunian object to be discovered was Pluto. The largest known trans-Neptunian objects are Eris and Pluto, followed by Makemake and Haumea. The Kuiper belt, Scattered disc, and Oort cloud are names for three divisions of this volume of space
Sedna is a trans-Neptunian object which may qualify as a dwarf planet. Sedna is named after the Inuit goddess who created the sea creatures of the Arctic. Passes through the Kuiper asteroid belt
Orcus is a TNO in the Kuiper belt with a large moon, Vanth. It is a large plutino (an object in 2:3 orbital resonance with Neptune) and is likely a dwarf planet
1992 QB1, was the first trans-Neptunian object to be discovered after Pluto and Charon. It was discovered in 1992 at the Mauna Kea Observatory, Hawaii
Asteroids
Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids
In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was developed by Chapman, Morrison, and Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. Originally, they classified only three types of asteroids:
- C-type asteroids – carbonaceous, 75% of known asteroids
- S-type asteroids – silicaceous, 17% of known asteroids
- M-type asteroids – metallic, 8% of known asteroids
Kirkwood gap – a gap in the distribution of the orbital periods of the orbits of main-belt asteroids. They correspond to the locations of orbital resonances with Jupiter
Ceres was the first asteroid to be discovered, in 1801
Pallas is the second asteroid to have been discovered, and it is one of the largest asteroids in the Solar System
Vesta is the second-most-massive asteroid after the dwarf planet Ceres and it comprises an estimated 9% of the mass of the asteroid belt. The less-massive Pallas is slightly larger, making Vesta third in volume. Vesta is the only asteroid that can be seen by the naked eye
433 Eros was the first near-Earth asteroid discovered
1036 Ganymed is the largest near-Earth asteroid
1566 Icarus is an Apollo asteroid that at perihelion comes closer to the Sun than Mercury
Asteroid belt – the region of the Solar System located roughly between the orbits of Mars and Jupiter
Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, the first asteroid of this group which was discovered by Karl Wilhelm Reinmuth. They are Earth-crosser asteroids
3753 Cruithne is an asteroid in orbit around the Sun in 1:1 orbital resonance with that of the Earth. It has been incorrectly called ‘Earth's second moon’. Named after a people of early medieval Ireland
Centaurs
Centaurs are Small Solar System Bodies with a semi-major axis between those of the outer planets. They have unstable orbits that cross or have crossed the orbits of one or more of the giant planets
The first centaur to be discovered was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest confirmed centaur is 10199 Chariklo
Three centaurs (including Chiron) have been found to display cometary comas and are therefore classified as both asteroids and comets
Neptune trojans
Neptune trojans are bodies in orbit around the Sun that orbit near one of the stable Lagrangian points of Neptune. They therefore have approximately the same orbital period as Neptune and follow roughly the same orbital path. Nine Neptune trojans are currently known
Comets
Comet is an icy Small Solar System Body that, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei are themselves loose collections of ice, dust, and small rocky particles, ranging from a few hundred metres to tens of kilometers across
Short-period comets are generally defined as having orbital periods of less than 200 years
Long-period comets have highly eccentric (elongated) orbits and periods ranging from 200 years to thousands or even millions of years
Comet tail always points directly away from the sun, pushed by the solar wind
Sungrazer – a comet that passes extremely close to the Sun at perihelion
Halley’s comet returns c. every 76 years, last seen in 1986, will next appear in 2061
Hale-Bopp was most widely observed comet of the 20th century, and was dubbed the Great Comet of 1997
Great Comet of 1860, also known as Newton's Comet, has the distinction of being the first comet discovered by telescope
Encke's Comet is a periodic comet that completes an orbit of the Sun once every three years – the shortest period of any known comet
Shoemaker-Levy 9 comet was torn into 21 fragments known as ‘string of pearls’, which crashed into Jupiter in 1994
Comets could have provided up to half the water in the earth’s oceans
Main-belt comets are bodies orbiting within the main asteroid belt which have shown cometary activity during part of their orbit
Comet Elst–Pizarro is a body that displays characteristics of both asteroids and comets, and is the prototype of main-belt comets. Its orbit keeps it within the asteroid belt, yet it displayed a dust tail like a comet while near perihelion
Maria Mitchell was an American astronomer, who in 1847, by using a telescope, discovered a comet which as a result became known as the ‘Miss Mitchell's Comet’
Outer Solar System
Kuiper belt – a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice. It extends between 30 and 50 AU from the Sun. Though it is estimated to contain anything from dozens to thousands of dwarf planets, it is composed mainly of small Solar System bodies
Scattered disc – a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered disc overlaps the Kuiper belt but extends much further outwards, and is thought to be the source of short-period comets
Oort cloud – a hypothetical spherical cloud of icy objects that is believed to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU. It is believed to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets
Heliopause – the theoretical boundary where the Sun's solar wind is stopped by the interstellar medium; where the solar wind's strength is no longer great enough to push back the stellar winds of the surrounding stars
Meteors and meteor showers
2008 TC3 was a meteoroid 2 to 5 m in diameter that entered Earth's atmosphere in 2008. The meteoroid was notable as the first such body to be observed and tracked prior to reaching Earth. It exploded an estimated 37 km above the Nubian Desert in Sudan
Murchison meteorite is named after Murchison, Victoria, in Australia. It is one of the most studied meteorites due to its large mass (>100 kg), the fact that it was an observed fall, and it belongs to a group of meteorites rich in organic compounds. Murchison contains common amino acids such as glycine, alanine and glutamic acid
Chelyabinsk meteor entered Earth's atmosphere over Russia in 2013. The object exploded in an air burst, producing many surviving small fragmentary meteorites, as well as a large shock wave. The largest found fragments having a total mass of 654 kg were raised from the bottom of the Chebarkul lake
Neumann lines or Neumann bands are fine patterns of parallel lines seen in cross-sections of many iron meteorites
Most meteorites are stony meteorites, classed as chondrites and achondrites. Only about 6% of meteorites are iron meteorites or a blend of rock and metal, the stony-iron meteorites
Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. Most meteorites that are recovered on Earth are chondrites
Hexahedrites are a type of iron meteorite
Radiant or apparent radiant of a meteor shower is the point in the sky, from which (to a planetary observer) meteors appear to originate
Zenithal Hourly Rate (ZHR) of a meteor shower is the number of meteors a single observer would see in one hour under a clear, dark sky
Leonids are a prolific meteor shower associated with the comet Tempel-Tuttle. The Leonids get their name from the location of their radiant in the constellation Leo: the meteors appear to radiate from that point in the sky. Visible in November
Comet Swift-Tuttle is the parent body of the Perseid meteor shower. The shower is visible from mid-July each year, with the peak in activity being between August 9 and 14
Quadrantids are an easily visible January meteor shower
Geminids – a meteor shower caused by the object 3200 Phaethon, which is thought to be a Palladian asteroid. This would make the Geminids, together with the Quadrantids, the only major meteor showers not originating from a comet. The meteors from this shower can be seen in December
Orionid meteor shower, usually shortened to the Orionids, is the most prolific meteor shower associated with Halley's Comet. Orionids are an annual meteor shower which last approximately one week in late-October
Giovanni Schiaparelli demonstrated that the Perseids and Leonids meteor showers were associated with comets
Auroras
Auroras result from emissions of photons in the Earth's upper atmosphere, from ionized nitrogen molecules regaining an electron, and oxygen atoms and nitrogen molecules returning from an excited state to ground state. They are ionized or excited by the collision of solar wind and magnetospheric particles being funneled down and accelerated along the Earth's magnetic field lines; excitation energy is lost by the emission of a photon, or by collision with another atom or molecule. Oxygen emissions are green, nitrogen emissions are blue or red
In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Galileo. Its southern counterpart, the aurora australis (or the southern lights)
Kristian Birkeland first elucidated the nature of the Aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland-Eyde process of fixing nitrogen from the air
Sun
Sun takes 220 million years to travel round the galaxy
Diameter of Sun – 1.4 million km
Sun is 93 million miles from Earth (1 AU). Light takes 8 minutes to reach Earth
Core of the Sun has a temperature of close to 13.6 million kelvin (K). By contrast, the Sun's surface temperature is approximately 5800 K
Sun is 71% hydrogen, 27% helium
The Sun formed about 4.6 billion years ago from the gravitational collapse of a region within a large molecular cloud
Photosphere of an astronomical object is the region from which externally received light originates. The visible surface of the Sun
Chromosphere – the lower level of the solar atmosphere between the photosphere and the corona
Solar transition region is a region of the Sun's atmosphere, between the chromosphere and corona. It is visible from space using telescopes that can sense ultraviolet
Corona – the set of coloured rings around the Sun (or moon) created when it shines through a thin cloud. Corona is hotter than the surface of the Sun
Coronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. Coronal loops are often found with sunspots at their footpoints
Footpoint – the intersection of tubes of magnetic field lines with the surface of the photosphere
Prominence – a large bright feature extending outward from the Sun's surface, often in a loop configuration. Prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the Sun's corona. Large eruptions of luminous hydrogen gas that rise thousands of kilometers above the chromosphere
Sunspot – a cooler darker spot appearing periodically on the Sun's photosphere; associated with a strong magnetic field
Solar flare – a sudden eruption of hydrogen gas on the surface of the Sun, usually associated with sunspots
Intense solar flares are known as ‘X-class’ flares
Spicule – a dynamic jet of about 500 km diameter on the Sun. It moves upwards at about 20 km/s from the photosphere
Solar maximum is a normal period of greatest solar activity in the 11 year solar cycle of the Sun. During solar maximum, large numbers of sunspots appear and the sun's irradiance output grows by about 0.1%
Sunspots were first observed telescopically in late 1610 by the English astronomer Thomas Harriot and Frisian astronomers Johannes and David Fabricius
Galileo discovered that the Sun rotated by observing sunspots
Wolf number – a quantity that measures the number of sunspots and groups of sunspots present on the surface of the Sun
Grand solar minima occur when several solar cycles exhibit lesser than average activity for decades or centuries. Solar cycles still occur during these grand solar minimum periods but the intensity of those cycles are less intense
Maunder Minimum – the period roughly spanning 1645 to 1715 when sunspots became exceedingly rare. The Maunder Minimum coincided with a period of lower-than-average global temperatures
Dalton Minimum – the period roughly spanning 1790 to 1830 when sunspots became exceedingly rare
Solar wind – a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons. These particles can escape the Sun's gravity because of their high kinetic energy and the high temperature of the corona. The solar wind creates the heliosphere, a vast bubble in the interstellar medium that surrounds the solar system. Other phenomena include geomagnetic storms that can knock out power grids on Earth, the aurorae (northern and southern lights), and the plasma tails of comets that always point away from the Sun
The solar wind flows outward from the Sun until encountering the termination shock, where motion slows abruptly. The heliosheath is the region of the heliosphere beyond the termination shock. The outermost edge of the heliosphere is called the heliopause
A coronal mass ejection (CME) is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space
1859 Solar Superstorm, or the Carrington Event, was a powerful geomagnetic solar storm in. A solar flare and/or coronal mass ejection hit Earth's magnetosphere and induced the largest known solar storm
Solar mass, 1.989 x 1030 kg, is a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies. It is equal to the mass of the Sun
Gravity at the surface of the Sun is 28 times higher than the gravity on earth
Sun’s radiation – ultraviolet A, ultraviolet B, ultraviolet C
Helioseismology – the study of the propagation of wave oscillations, particularly acoustic pressure waves, in the Sun
Galileo discovered that Sun rotated by tracking movement of sunspots
Fred Hoyle realised that the Sun would become a red giant, then a white dwarf
Yarkovsky effect – photons from the Sun affect path of asteroids
Helioscope – an instrument used in observing the Sun. The helioscope was first used by Benedetto Castelli and refined by Galileo. The method involves projecting an image of the Sun onto a white sheet of paper suspended in a darkened room with the use of a telescope
George Hale is is known for inventing the spectroheliograph, with which he made his discoveries of the solar vortices and magnetic fields of sunspots
Stars
A star is a massive, luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun
Stars contain 11% of atoms in the universe
Most elements up to atomic number 26 (iron) were made inside stars
Magnitude – a measure of a star’s brightness; the higher the number, the fainter the star
Apparent magnitude – a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere
Absolute magnitude – the apparent magnitude an object would have if it were at a standard luminosity distance (10 parsecs, or 1 AU, depending on object type) away from the observer
Dwarf – a small, low-luminosity star
White dwarfs are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth
Giant – a large and luminous star with a low density
A white dwarf is produced when a low or medium mass star dies. These stars are not heavy enough to generate the core temperatures required to fuse carbon in nucleosynthesis reactions
Chandrasekhar limit – the theoretical upper limit to the mass of a stable white dwarf star, 2.8 x 1030 kg, i.e. 1.4 times the mass of the Sun. Above the Chandrasekhar limit, white dwarfs collapse into neutron stars. Only stars more than three times the Sun’s mass can shrink further to produce black holes
A red giant is a star of 0.4–10 times the mass of the Sun which has exhausted the supply of hydrogen in the core and switched to fusing hydrogen in a shell outside the core. They have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is lower in temperature, giving them an orange hue
Red giants produce carbon and oxygen. Become white dwarves
Sun will become a red giant in c. five billion years
Red dwarf stars are small, cool, faint stars on the main sequence that constitute the vast majority of stars in the Milky Way and have a diameter and mass of less than one-third that of the Sun. Proxima Centauri and Barnard’s Star are both red dwarf stars
Proxima Centauri is rich in magnesium. Will become a white dwarf
Neutron star – the collapsed core of a massive star remaining after a supernova explosion. Such stars are composed almost entirely of neutrons
Magnetar – type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma rays
Supergiant – a star of enormous size, low density and great luminosity
Black dwarf is a hypothetical stellar remnant, created when a white dwarf becomes sufficiently cool to no longer emit significant heat or light
Brown dwarfs are sub-stellar objects which are too low in mass to sustain stable hydrogen fusion
The Sun is a G2V star – a main-sequence dwarf star of moderate temperature (yellow dwarf star of spectral type G and luminosity class V)
Most stars are currently classified using the letters O, B, A, F, G, K, and M, where O stars are the hottest (blue) and the letter sequence indicates successively cooler stars up to the coolest M class (red)
Double star – a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope
Cepheid – pulsating variable star, used to measure distances to other galaxies. Henrietta Leavitt's discovery of the period-luminosity relation of Cepheid variables radically changed the theory of modern astronomy, an accomplishment for which she received almost no recognition during her lifetime. Her work was the basis for the work of astronomer Edwin Hubble
Compact star – used to refer collectively to white dwarfs, neutron stars, other exotic dense stars, and black holes. Most compact stars are at the endpoint of stellar evolution and are thus called stellar remnants. A compact star which is not a black hole, may be called a degenerate star
Nova – a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner
Supernova – a stellar explosion that is more energetic than a nova. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy. It may undergo sudden gravitational collapse into a neutron star or black hole
SN 1572 (Tycho's Supernova) was a supernova of Type Ia in the constellation Cassiopeia, one of about eight supernovae visible to the naked eye in historical records. It burst forth in 1572
Supernovae can be triggered in one of two ways: by the sudden reignition of nuclear fusion in a degenerate star; or by the collapse of the core of a massive star
In 1934 Fritz Zwicky and Walter Baade coined the term ‘supernova’ and hypothesized that they were the transition of normal stars into neutron stars, as well as the origin of cosmic rays
If a supernova's spectrum contains lines of hydrogen (known as the Balmer series in the visual portion of the spectrum) it is classified Type II; otherwise it is Type I
Type Ia supernovas all explode with the same brightness, so they are used to calculate distances from Earth
Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosity versus their spectral types or classifications and effective temperatures. Stars on this band are known as main-sequence stars or ‘dwarf’ stars. Stars tend to fall only into certain regions of the diagram. The most prominent is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence. In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants, giants and supergiants
Pulsars are highly magnetized, rotating neutron stars that emit a beam of electromagnetic radiation in the form of radio waves. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name
The first pulsar was discovered in 1967, by Jocelyn Bell Burnell and Antony Hewish of the University of Cambridge. While using a radio array to study the scintillation of quasars, they found a very regular signal, consisting of pulses of radiation at a rate of one in every few seconds. Initially baffled as to the seemingly unnatural regularity of its emissions, the pair dubbed their discovery LGM-1, for ‘little green men’
Crab Pulsar is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova which was observed on Earth in the year 1054. Discovered in 1968, the pulsar was the first to be connected with a supernova remnant
Quasar (contraction of QUASi-stellAR radio source) is an astronomical source of electromagnetic energy, including radio waves and visible light. A compact region in the centre of a massive galaxy that surrounds its central supermassive black hole
The first quasars were discovered with radio telescopes in the late 1950s
The term quasar was coined by Chinese-born U.S. astrophysicist Hong-Yee Chiu in 1964
Donald Lynden-Bell is an English astrophysicist, best known for his theories that galaxies contain massive black holes at their centre, and that such black holes are the principal source of energy in quasars
Wolf-Rayet (WR) stars are evolved, massive stars (over 20 solar masses initially), which are losing mass rapidly by means of a very strong stellar wind. Wolf–Rayet stars are extremely hot
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity
Population III, or metal-free stars, are a hypothetical extinct population of extremely massive and hot stars with virtually no surface metals
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen
One of James Jeans' major discoveries, named Jeans length, is a critical radius of an interstellar cloud in space
Sirius is the brightest star in the night sky, almost twice as bright as Canopus, the next brightest star. It is known colloquially as the ‘Dog Star’, reflecting its prominence in its constellation, Canis Major
Sirius is a binary star system consisting of two stars (Sirius A and Sirius B) orbiting each other with a separation of about 20 astronomical units
Sirius B is a white dwarf. This star is primarily composed of a carbon–oxygen mixture that was generated by helium fusion
Apparent magnitudes – Sun -26.8, Sirius A -1.46, Canopus -0.72, Arcturus -0.06
Absolute magnitudes – Sun 4.83, Canopus – 5.53
Alpha Centauri is the closest star system to our own solar system at 4.37 light-years distant. Proxima Centauri, usually regarded as part of the system, is the closest star (apart from the Sun) at 4.22 light-years distant
Proxima Centauri – apparent magnitude 11
Alpha Centauri – apparent magnitude 0
Regulus is the brightest star in Leo
Barnard’s Star – six light-years away from Earth in the constellation of Ophiuchus
Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus
Altair is the largest star in the constellation Aquila
Vega will become a Pole Star
Summer Triangle of stars – Vega, Altair and Deneb
Deneb is the brightest star in the constellation Cygnus. A blue-white supergiant, Deneb is also one of the most luminous nearby stars
Winter Hexagon has vertices at Rigel, Aldebaran, Capella, Pollux, Procyon, and Sirius. Smaller and more regularly shaped is the Winter Triangle, an approximately equilateral triangle that shares two vertices (Sirius and Procyon) with the larger asterism. The third vertex is Betelgeuse
Lucy is a diamond star in constellation Centaurus
Antares is a red supergiant star in Scorpius constellation
Rigel is the brightest star in the constellation Orion
Betelgeuse is in the constellation Orion
Betelgeuse is a red supergiant. It is likely that the star will continue to burn and fuse elements until its core is iron, at which point Betelgeuse will explode as a type II supernova. During this event the core will collapse, leaving behind a neutron star
Arcturus is the brightest star in the constellation Bootes. Red giant
Pleiades, or Seven Sisters (Messier object 45 or M45), is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus
Hyades is a star cluster in Taurus
Aldebaran is an orange giant star in Taurus. Known as the ‘eye’ of Taurus
Almach is a double star in Andromeda
Plough or the Big Dipper is an asterism of seven stars that has been recognized as a distinct grouping in many cultures from time immemorial. The comprising stars are the seven brightest of the formal constellation Ursa Major
Thuban – Pole Star in Egyptian times
Gamma Cephei will succeed Polaris as the Earth's northern pole star, c. 5200, due to the precession of the equinoxes
Nebulae
A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases
Orion nebula – the closest region of massive star formation to Earth
Nebulae are often star-forming regions. In these regions the formations of gas, dust, and other materials clump together to form larger masses, which attract further matter, and eventually will become massive enough to form stars
Pillars of Creation is a photograph taken by the Hubble Telescope of elephant trunks (formations of interstellar matter found in space) of interstellar gas and dust in the Eagle Nebula, some 7000 light years from Earth
Coalsack Dark Nebula is the most prominent dark nebula in the skies, easily visible to the naked eye as a dark patch silhouetted against the southern Milky Way. It was observed by Vincente Yanez Pinzon in 1499. The Coalsack is located at a distance of approximately 600 light years away from Earth, in the Crux constellation
Crab nebula – a supernova remnant and pulsar in the constellation of Taurus. The nebula was first observed in 1731 by John Bevis
Rosette nebula – in Monoceros constellation
The Horsehead Nebula is a dark nebula in the Orion constellation. The nebula is located just below Alnitak, the easternmost star of Orion's Belt, and is part of the much larger Trapezium, or Orion Trapezium Cluster is a group of stars in the heart of the Orion Nebula, in the constellation of Orion. It was discovered by Galileo
Tarantula Nebula is in the Large Magellanic Cloud. Tarantula Nebula is the most active starburst region known in the Local Group of galaxies
Nebular hypothesis – the most widely accepted model explaining the formation and evolution of the Solar System. According to the nebular hypothesis, stars form in massive and dense clouds of molecular hydrogen – giant molecular clouds (GMC). They are gravitationally unstable, and matter coalesces to smaller denser clumps within, which then proceed to collapse and form stars
Dark nebula – a type of interstellar cloud that is so dense that it obscures the light from the background emission or reflection nebula (e.g. the Horsehead Nebula) or that it blocks out background stars (e.g., the Coalsack Nebula). The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of larger molecular clouds
Frost line, also known as the snow line or ice line, refers to a particular distance in the solar nebula from the central sun where it is cold enough for hydrogen compounds such as water, ammona, and methane to condense into solid ice grains. The frost line of the Solar System is around 5 AU
Constellations
A constellation is a group of stars into which the sky is divided for the purposes of identifying and naming celestial objects. There are 88 constellations. The largest constellation is Hydra (the water snake), the smallest constellation is Crux (the southern cross)
Apus – Bird of Paradise
Auriga – Charioteer
Bootes – herdsman
Camelopardalis – Giraffe
Canes Venatici – Hunting Dogs
Cetus – Whale
Circinus – Compasses
Columba – Dove
Cygnus – Swan
Delphinus – Dolphin
Dorado – Swordfish
Draco – Dragon
Grus – Crane
Lacerta – Lizard
Lepus – Hare
Monoceros – Unicorn
Musca – Fly
Ophiuchus – Serpent-bearer
Pavo – Peacock
Pyxix – Mariner’s Compass
Reticulum – Net
Scutum – Shield
Vela – Sails
Volans – Flying Fish
Vulpecala – Fox
Cancer the crab – faintest constellation
Alpha – brightest star in a constellation
Cassiopeia – W shaped constellation, formed by five bright stars
Zodiac – the ring of constellations that lines the ecliptic, which is the apparent path of the Sun across the sky over the course of the year. The Moon and planets also lie within the ecliptic, and so are also within the constellations of the zodiac
Ursa Minor – constellation that is the location of the north celestial pole, that currently has nearly the same coordinates as the bright star Polaris
Polaris (commonly North(ern) Star or Pole Star) is the brightest star in the constellation Ursa Minor
The constellation Octans is the location of the south celestial pole
Virgo is the second largest constellation in the sky. It can be easily found through its brightest star, Spica
Orion Molecular Cloud Complex. It is approximately 1500 light years from Earth
Hunting dogs of Orion – Canis Major and Canis Minor
Procyon is the brightest star in the constellation Canis Minor
Stars in Orion’s belt – Alnitak (zeta), Alnilam (epsilon), and Mintaka (delta)
Hubble Ultra Deep Field is an image of a small region of space in the constellation Fornax. It is the deepest image of the universe ever taken, looking back approximately 13 billion years
Constellation of Ophiuchus occupies most of the sign of Sagittarius
Galaxies
Galaxy – large aggregation of stars, gas, and dust, typically containing billions of stars
Starburst galaxy – a galaxy undergoing an exceptionally high rate of star formation
Active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. A galaxy hosting an AGN is called an active galaxy
Local Group – the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies (including dwarf galaxies), with its gravitational centre located somewhere between the Milky Way and the Andromeda Galaxy
Andromeda is the largest galaxy of the Local Group
Milky Way has 200 billion stars
Andromeda – nearest galaxy to the Milky Way
Andromeda is 2.5 million light years away
Milky Way and Andromeda are locked by gravity
Hubble discovered that Andromeda was a separate galaxy
Pinwheel Galaxy is in the constellation Ursa Major
Antennae Galaxies are a pair of interacting galaxies in the constellation Corvus. They are currently going through a phase of starburst. They were discovered by William Herschel in 1785
Seyfert galaxies are a class of galaxies with nuclei that produce spectral line emission from highly ionized gas. Named after Carl Seyfert
Large Magellanic Cloud (LMC) is a nearby irregular galaxy, and is a satellite of the Milky Way. It is the third closest galaxy to the Milky Way
Small Magellanic Cloud (SMC) is a dwarf galaxy. It contains several hundred million stars
Radio galaxies are types of active galaxy that are very luminous at radio wavelengths. The radio emission is due to the synchrotron process. Cygnus A is one of the most famous radio galaxies
The Hubble sequence is a classification of galaxy types developed by Edwin Hubble in 1925. It is also called the tuning-fork diagram due to the shape of its graphical representation. Types of galaxy – elliptical (most galaxies), spiral (e.g. the Milky Way, which is a barred spiral) and irregular
Dwarf spheroidal galaxies are low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy
Dwarf elliptical galaxies elliptical galaxies that are much smaller than other galzxies
Radio galaxy – intense source of cosmic radio waves
Whirlpool Galaxy is a spiral galaxy that is estimated to be 23 million light-years from the Milky Way
Sombrero Galaxy is an unbarred spiral galaxy in the constellation Virgo
Jacobus Kapteyn was a Dutch astronomer, best known for his extensive studies of the Milky Way and as the first discoverer of evidence for galactic rotation in 1904
Eclipses
The Sun's distance from the Earth is about 390 times the Moon's distance, and the Sun's diameter is about 400 times the Moon's diameter. Because these ratios are approximately the same, the Sun and the Moon as seen from Earth appear to be approximately the same size in a solar eclipse
A total eclipse occurs when the Sun is completely obscured by the Moon. The intensely bright disk of the Sun is replaced by the dark outline of the Moon, and the much fainter corona is visible. During any one eclipse, totality is visible only from at most a narrow track on the surface of the Earth
Next UK total eclipse will be in 2090
An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a very bright ring, or annulus, surrounding the outline of the Moon
A hybrid eclipse shifts between a total and annular eclipse
Solar eclipse can only occur during a new moon
Total lunar eclipse occurs when Moon passes into Earth's shadow
Danjon Scale of lunar eclipse brightness is a five-point scale useful for measuring the appearance and luminosity of the Moon during a lunar eclipse. An eclipse's rating on the Danjon Scale is denoted by the letter L (0 to 4)
Saros – a period of 223 synodic months that can be used to predict eclipses of the Sun and Moon
The umbra (Latin for ‘shadow’) is the darkest part of the shadow, where the light source is completely blocked by the occluding body. An observer in the umbra experiences a total eclipse
The penumbra (from the Latin paene ‘almost, nearly’ and umbra ‘shadow’) is the region in which only a portion of the light source is obscured by the occluding body. An observer in the penumbra experiences a partial eclipse
A penumbral eclipse occurs when the Moon passes through the Earth’s penumbra. The penumbra causes a subtle darkening of the Moon's surface
The antumbra is the region from which the occluding body appears entirely contained within the disc of the light source. If an observer in the antumbra moves closer to the light source, the apparent size of the occluding body increases until it causes a full umbra. An observer in this region experiences an annular eclipse
A selenelion or selenehelion occurs when both the Sun and the eclipsed Moon can be observed at the same time
Black holes
John Wheeler is widely credited with coining the term black hole in 1967. Originally a black hole was known as a frozen star. John Wheeler also worked with Niels Bohr in explaining the basic principles behind nuclear fission
Event horizon – boundary of a black hole
Singularity – the zero-volume, infinitely dense region at the centre of a black hole, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and spacetime has infinite curvature
Schwarzschild radius – the radius of a sphere in space, that if containing a correspondingly sufficient amount of mass (and therefore, reaches a certain density), the force of gravity from the contained mass would be so great that no known force could stop the mass from continuing to collapse in volume into a point of infinite density: a gravitational singularity (a black hole). R=2GM/c squared, where G is the gravitational constant, M is the object mass, and c is the speed of light
Rotating black holes have an ergosphere, a region bounded by: on the outside, an oblate spheroid which coincides with the event horizon at the poles and is noticeably wider around the ‘equator’. This boundary is sometimes called the ‘ergosurface’, but it is just a boundary and has no more solidity than the event horizon. At points exactly on the ergosurface, space-time is dragged around at the speed of light
Rotating black holes were first described by Roy Kerr
There is a supermassive black hole at the centre of every galaxy
Sagittarius A* is a bright and very compact astronomical radio source at the centre of the Milky Way Galaxy, near the border of the constellations Sagittarius and Scorpius. It is believed to be the location of a supermassive black hole
Stellar mass black hole – a black hole formed by the gravitational collapse of a massive star (more massive than about 20 solar masses) at the end of its lifetime. The process is observed as a supernova explosion or as a gamma ray burst
Hawking radiation – thermal radiation thought to be emitted by black holes due to quantum effects. It is named after Stephen Hawking who worked out the theoretical argument for its existence in 1974, becoming the first insight into quantum gravity
S = A/4 – Hawking entropy equation. The black hole entropy (S) is proportional to the area of its event horizon (A)
Spaghettification – a body falling into a black hole would be stretched into long thin shapes like spaghetti
Cygnus X-1 was the first X-ray source widely accepted to be a black hole candidate. Discovered by Paul Murdin
Accretion disc – a structure formed by diffuse material in orbital motion around a central body. The central body is typically a young star, a white dwarf, a neutron star, or a black hole. Gravity causes material in the disc to spiral inward towards the central body
Cosmology
Current interpretations of astronomical observations indicate that the age of the Universe is 13.8 billion years
Big Bang Theory – universe came out of a singularity. Quarks and leptons, together with their antiparticles, dominated the universe less than a second after its beginning. A slight imbalance between quarks and antiquarks then resulted in the annihilation of most antiparticles, creating an immense amount of electromagnetic radiation. After a microsecond the universe cooled sufficiently for the quarks to combine to form protons and neutrons
Big Bang nucleosynthesis – the production of nuclei other than those of the lightest isotope of hydrogen during the early phases of the universe. Primordial nucleosynthesis is believed to have taken place between approximately 10 seconds until 20 minutes after the Big Bang, and is calculated to be responsible for the formation of most of the universe's helium as isotope He-4, along with small amounts of deuterium (H-2 or D), the helium isotope He-3, and a very small amount of the lithium isotope Li-7
Cosmic inflation – the theorized exponential expansion of the universe at the end of the grand unification epoch, 10−36 seconds after the Big Bang. The inflationary hypothesis was originally proposed in 1980 by American physicist Alan Guth
Inflation replaced the hot big bang model
300,000 years after the universe was formed, at the end of the photon epoch, the ambient temperature had fallen below 10,000 degrees. Electrons and positively-charged atomic nuclei combined to form neutral atoms. Electromagnetic radiation was set free and light could roam across the universe
Hydrogen and helium atoms began to form as the density of the universe falls. This is thought to have occurred about 377,000 years after the Big Bang
Baryon asymmetry problem – the apparent fact that there is an imbalance in baryonic matter and antibaryonic matter in the universe. Neither the standard model of particle physics, nor the theory of general relativity provide an obvious explanation for why this should be so; and it is a natural assumption that the universe be neutral with all conserved charges. The Big Bang should have produced equal amounts of matter and antimatter, as such, there should have been total cancellation of both. In other words, protons should have cancelled with antiprotons, electrons with antielectrons (positrons), neutrons with antineutrons, and so on for all elementary particles. This would have resulted in a sea of photons in the universe with no matter. Since this is evidently not the case, after the Big Bang, some physical laws must have acted differently for matter and antimatter
Baryogenesis – generic term for hypothetical physical processes that produced an asymmetry between baryons and antibaryons in the very early universe, resulting in the substantial amounts of residual matter that make up the universe today
Steady State theory (also known as the Infinite Universe theory or continuous creation) is a model, now discredited, developed in 1948 by Fred Hoyle, Thomas Gold, Hermann Bondi and others as an alternative to the Big Bang theory
Georges Lemaitre was the first person to propose the theory of the expansion of the Universe, widely misattributed to Edwin Hubble. Lemaitre also proposed what became known as the Big Bang theory of the origin of the Universe, which he called his 'hypothesis of the primeval atom'
In physical cosmology, the Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately ending as a black hole singularity
Big Freeze – Observations suggest that the expansion of the universe will continue forever. If so, the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario is popularly called the Big Freeze. Also known as Heat death of the universe, its final thermodynamic state in which it has diminished to a state of no thermodynamic free energy to sustain motion or life. It has reached maximum entropy
Big Rip – the matter of the universe, from stars and galaxies to atoms and subatomic particles, is progressively torn apart by the expansion of the universe at a certain time in the future
Big Bounce – Big Crunch followed by a Big Bang
Dark energy – a hypothetical form of energy which permeates all of space and has strong negative pressure. According to the theory of relativity, the effect of such a negative pressure is qualitatively similar to a force acting in opposition to gravity at large scales. Invoking such an effect is currently the most popular method for explaining recent observations that the universe appears to be expanding at an accelerating rate, as well as accounting for a significant portion of the missing mass in the universe. Term was coined by Michael Turner. In the standard model of cosmology, dark energy currently accounts for 74% of the total mass-energy of the universe
Quintessence – a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. It has been proposed by some physicists to be a fifth fundamental force
Dark matter – a conjectured form of matter that is undetectable by its emitted electromagnetic radiation, but whose presence can be inferred from gravitational effects on visible matter and background radiation
Dark matter is estimated to constitute 84% of the matter in the universe and 23% of the mass-energy
Dark matter was postulated by Fritz Zwicky in 1934 to account for evidence of ‘missing mass’ in the orbital velocities of galaxies in clusters
Vera Rubin uncovering the discrepancy between the predicted angular motion of galaxies and the observed motion. This phenomenon became known as the galaxy rotation problem. This discrepancy can be accounted for by a large amount of dark matter that permeates the galaxy
Dark flow – a net motion of galaxy clusters with respect to the cosmic microwave background radiation which was found in a 2008 study. May indicate a multiverse
When a galaxy or a planetary system forms, its material takes the shape of a disk. Most of the material orbits and rotates in one direction. This uniformity of motion is due to the collapse of a gas cloud. The nature of the collapse is explained by the principle called conservation of angular momentum
Cosmic microwave background radiation (most often abbreviated CMB, also referred to as relic radiation) is a form of electromagnetic radiation discovered in 1965 that fills the entire universe. The high degree of uniformity throughout the observable universe and its faint but measured anisotropy lend strong support for the Big Bang model in general
COsmic Background Explorer (COBE), also referred to as Explorer 66, was a satellite dedicated to cosmology. Its goals were to investigate the cosmic microwave background radiation of the universe. This work provided evidence that supported the Big Bang theory of the universe: that the CMB was a near-perfect black-body spectrum and that it had very faint anisotropies
Wilkinson Microwave Anisotropy Probe (WMAP), also referred to as Explorer 80, is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat, the Cosmic Microwave Background Radiation , across the full sky
Planck is a space observatory launched in 2009 by the European Space Agency (ESA) and designed to observe the anisotropies of the cosmic microwave background (CMB) over the entire sky, at a high sensitivity and angular resolution. Planck is the successor to WMAP
BICEP (Background Imaging of Cosmic Extragalactic Polarization) and the Keck Array are a series of CMB experiments. They aim to measure the polarization of the CMB; in particular, measuring the B-mode of the CMB. In 2014, the collaboration announced that BICEP2 had made the first detection of so-called B-mode polarization, a possible signature of inflation (gravitational waves) in the very early Universe. Data from the Planck spacecraft showed that 75% of the signal was due to galactic dust
Temperature of Cosmic Microwave Background Radiation is 3oK
The multiverse is the hypothetical set of multiple possible universes that together comprise everything that exists and can exist: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them. The term was coined in 1895 by the American philosopher and psychologist William James. The various universes within the multiverse are sometimes called parallel universes
Hydrogen line, 21 centimetre line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. The line is of great interest in big bang cosmology because it is the only known way to probe the ’dark ages’ from recombination to reionization
Hubble's law is the statement in physical cosmology that the redshift in light coming from distant galaxies is proportional to their distance. Galaxies recede from us with speeds that increase with their distance. The law is often expressed by the equation v = H0D, with H0 the constant of proportionality (the Hubble constant) between the distance D to a galaxy and its velocity v
Hubble constant – 70 (km/s)/Mpc (Megaparsec)
Hubble's law can be easily depicted in a Hubble Diagram in which the velocity (assumed approximately proportional to the redshift) of an object is plotted with respect to its distance from the observer
The cosmological constant (usually denoted by the Greek capital letter lambda: Λ) is the value of the energy density of the vacuum of space. It was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe. Einstein abandoned the concept after the observation of the Hubble redshift indicated that the universe might not be stationary, as he had based his theory on the idea that the universe is unchanging
ΛCDM (Lambda Cold Dark Matter) or Lambda-CDM model is a parametrization of the Big Bang cosmological model in which the universe contains a cosmological constant, denoted by Lambda, associated with dark energy, and cold dark matter (abbreviated CDM). It is frequently referred to as the standard model of Big Bang cosmology
WIMPs (Weakly Interacting Massive Particles) are hypothetical particles serving as one possible solution to the dark matter problem. These particles interact through the weak nuclear force and gravity, and possibly through other interactions no stronger than the weak force. Because they do not interact with electromagnetism they cannot be seen directly, and because they do not interact with the strong nuclear force they do not react strongly with atomic nuclei
MACHO (Massive Astrophysical Compact Halo Object) is the general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. The term was chosen whimsically by contrast with WIMP
DarkSide collaboration is an international affiliation of universities and labs seeking to directly detect dark matter in the form of WIMPs
Gamma-ray bursts – (GRB) the most luminous events known in the universe since the Big Bang. They are flashes of gamma rays coming from seemingly random places in deep space at random times
Roger Penrose won the 1988 Wolf Prize for physics, which he shared with Stephen Hawking for their contribution to our understanding of the universe
Most abundant elements in the universe – hydrogen, helium, oxygen
Tables and catalogues
Alfonsine Tables provided data for computing the position of the Sun, Moon and planets relative to the fixed stars. They are named for Alfonso X of Castile, upon whose order they were prepared in Toledo, Spain around 1252 to 1270
The Rudolphine Tables consist of a star catalog and planetary tables published by Johannes Kepler in 1627. Named after Emperor Rudolf II, they contain positions for the 1006 stars measured by Tycho Brahe, and 400 and more stars from Ptolemy and Johann Bayer, with directions and tables for locating the planets of the solar system
Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1564 stars. Most of the brighter stars were assigned their first systematic names by the German astronomer Johann Bayer in 1603, in his star atlas Uranometria
Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in his Catalogue of Nebulae and Star Clusters published in 1771
There are 110 Messier objects, including M1 (Crab nebula), M31 (Andromeda galaxy), M42 (Orion nebula) and M45 (Pleiades)
New General Catalogue of Nebulae and Clusters of Stars (abbreviated as NGC) is a catalogue of 7840 deep-sky objects in astronomy compiled by John Louis Emil Dreyer in 1888, as a new version of John Herschel's Catalogue of Nebulae and Clusters of Stars. NGC 1 is a spiral galaxy located 190 million light-years away in the constellation Pegasus
The Gliese catalogue attempts to list all stars within 20 parsecs of Earth ordered by right ascension
Catalogue of Galaxies and of Clusters of Galaxies was compiled by Fritz Zwicky in 1961 – 1968
Carte du Ciel (literally, 'Map of the Sky') and the Astrographic Catalogue were two distinct but connected components of a massive international astronomical project, initiated in the late 19th century, to catalogue and map the positions of millions of stars
Henry Draper Catalogue is a star catalogue published between 1918 and 1924, giving spectroscopic classifications for 225,300 stars
Father Angelo Secchi was a pioneer in astronomical spectroscopy, and was one of the first scientists to state authoritatively that the Sun is a star. Published La Stelle (‘The Stars’) in 1877
Observation
Charles II founded the Royal Greenwich Observatory in 1675 and appointed the first Astronomer Royal John Flamsteed. Astronomer Royal was originally the title of the director of the Royal Greenwich Observatory, but since 1972 it has simply been an honorary title. Edmond Halley was the second Astronomer Royal. Martin Rees is the current Astronomer Royal
Edmond Halley edited Newton’s Principia Mathematica (1687)
John Flamsteed was refusing to publish work that had been commissioned by the king, and in 1712 Newton and Edmond Halley published a preliminary version of Flamsteed's Historia Coelestis Britannica without crediting the author. Some years later, Flamsteed managed to buy many copies of the book, and publicly burnt them in front of the Royal Observatory
Nevil Maskelyne was the fifth English Astronomer Royal, from 1765 to 1811. Helped to measure longitude, latitude, and calculated the density of the earth
George Airy was Astronomer Royal from 1835 to 1881. He established Greenwich as the location of the prime meridian
Uraniborg was an astronomical observatory operated by Tycho Brahe; built c. 1580 on Hven, an island in the Oresund between Zealand and Scania, at that time belonging to Denmark
During the course of his career, Herschel constructed more than four hundred telescopes. The largest and most famous of these was a reflecting telescope with a 49+ inches diameter primary mirror and a 40 feet focal length
Leviathan of Parsonstown is the unofficial name of the Rosse six foot telescope. This is a historic reflecting telescope of 72 in (1.8 m) aperture, which was the largest telescope in the world from 1845 until the construction of the 100 in (2.5 m) Hooker Telescope in 1917. The Rosse six foot telescope was built by William Parsons, 3rd Earl of Rosse, on his estate, Birr Castle, at Parsonstown (now Birr in County Offaly, Ireland)
Hooker telescope is at Mount Wilson Observatory, California. Hubble determined that some nebulae were actually galaxies outside our own Milky Way, and discovered the presence of the redshift that indicated the universe is expanding
Hale telescope (60 in) at Mount Wilson Observatory built to test Hubble’s theory that the universe was expanding. Mirror manufactured in New York and transported to California by train
Yerkes Observatory is operated by the University of Chicago in Williams Bay, Wisconsin. The Observatory has the world's largest refracting telescope
Bolshoi telescope is in the Caucasus. Largest telescope in the world when it opened in 1975
Large Binocular Telescope (LBT) is at Mount Graham, Arizona. Largest telescope. 2 x 8.4m mirrors
James Webb Space Telescope – replacement for Hubble Space Telescope. Due to be launched in 2018. It will have a large and very cold 6.5 m diameter mirror covered in gold, an observing position far from Earth, orbiting the Earth–Sun L2 point
Stratospheric Observatory for Infrared Astronomy (SOFIA) is a joint project of NASA and the German Aerospace Center (DLR) to construct and maintain an airborne observatory. SOFIA is based on a Boeing 747 wide-body aircraft that has been modified to include a large door in the aft fuselage that can be opened in flight to allow a reflecting telescope access to the sky
ANTARES is the name of a neutrino detector residing 2.5 km under the Mediterranean Sea off the coast of Toulon. It is designed to be used as a directional Neutrino Telescope to locate and observe neutrino flux from cosmic origins in the direction of the Southern Hemisphere of the Earth, a complement to the northern hemisphere neutrino detector IceCube
IceCube is a neutrino telescope constructed at the Amundsen-Scott South Pole Station in Antarctica. Similar to its predecessor, the Antarctic Muon And Neutrino Detector Array (AMANDA)
Lovell Telescope is a radio telescope at Jodrell Bank Observatory, near Goostrey, Cheshire. When it was constructed in the mid 1950s, the telescope was the largest steerable dish radio telescope in the world. Bernard Lovell was the first Director of Jodrell Bank
Multi-Element Radio Linked Interferometer Network (MERLIN) is an interferometer array of radio telescopes spread across England. The array is run from Jodrell Bank Observatory
Lowell Observatory Near-Earth Object Search (LONEOS) was a project designed to discover asteroids and comets that orbit near the Earth that ran until 2008
The Very Large Array (VLA) is a radio astronomy observatory located on the Plains of San Augustin, 50 miles west of Socorro, New Mexico
The Very Large Telescope Project (VLT) is a system of four separate optical telescopes organized in an array formation. The project is organized by the European Southern Observatory. VLT is located at the Paranal Observatory on Cerro Paranal, a mountain in the Atacama Desert in northern Chile
Palomar Observatory is located in San Diego County, California, 90 miles southeast of Mount Wilson Observatory, on Palomar Mountain. It is owned and operated by the California Institute of Technology (Caltech). The observatory currently consists of four main instruments, including the 200 inch Hale Telescope
The Arecibo Observatory in Puerto Rico is operated by Cornell University under cooperative agreement with the National Science Foundation. The observatory's 305m radio telescope is the largest single-aperture telescope ever constructed. It was featured in the film GoldenEye
The Atacama Large Millimetre Array (ALMA) is an international astronomy project that consists of an astronomical interferometer formed from an array of radio telescopes, located in the Atacama Desert in northern Chile
The Galaxy Evolution Explorer (GALEX) is an orbiting ultraviolet space telescope that was launched in 2003. The first observation (called ‘First Light’) was dedicated to the crew of the Space Shuttle Columbia
The Lick Observatory is an astronomical observatory, owned and operated by the University of California. It is situated on the summit of Mount Hamilton
Along with the Faulkes Telescope North and the Faulkes Telescope South, the Liverpool Telescope is available for use by school children around the world over the internet
The Observatories at Mauna Kea, (MKO), are an independent collection of astronomical research facilities located on the summit of Mauna Kea on the Big Island of Hawaii. The facilities are located in a 500-acre special land use zone known as the ‘Astronomy Precinct’
James Clerk Maxwell Telescope (JCMT) is an infrared telescope with a primary mirror diameter of 15 metres. It is located at Mauna Kea Observatory in Hawaii. It is the largest astronomical telescope in the world designed specifically to operate in the submillimetre regime (between the far-infrared and the microwave regions of the electromagnetic spectrum)
UKIRT, the United Kingdom Infra-Red Telescope, is a 3.8 metre infrared reflecting telescope, the largest dedicated infrared telescope in the world. It is located on Mauna Kea, Hawaii as part of Mauna Kea Observatory
Southern African Large Telescope (SALT) is a 66m2 area optical telescope. It is located close to the town of Sutherland in the semi-desert region of the Karoo
Square Kilometre Array (SKA) is a radio telescope project to be built in Australia and South Africa which would have a total collecting area of approximately one square kilometre. It will operate over a wide range of frequencies and its size will make it 50 times more sensitive than any other radio instrument. Construction of the SKA is scheduled to begin in 2018
Herschel Space Observatory is a European Space Agency space observatory sensitive to the far infrared and submillimetre wavebands. It is the largest space telescope ever launched, carrying a single mirror of 3.5 metres in diameter. The observatory was carried into orbit in May 2009, reaching the second Lagrangian point (L2) of the Earth-Sun system, 1,500,000 kilometres (930,000 mi) from the Earth, two months later
LOFAR is the LOw Frequency ARray for radio astronomy. LOFAR will be the largest radio telescope ever built, using a new concept based on a vast array of omni-directional antennas. The project is based on an interferometric array of radio telescopes using 15,000 small antennas and 77 larger stations, mostly in the Netherlands
Solar and Heliospheric Observatory (SOHO) is a spacecraft built Astrium that was launched in 1995 to study the Sun, and has discovered over 2700 comets
Solar Dynamics Observatory (SDO) is a NASA mission which will observe the Sun for over five years. Launched in 2010, the observatory is part of the Living With a Star (LWS) program
Space Weather Prediction Center (SWPC), formerly the Space Environment Center (SEC), is a laboratory and service center of the National Oceanic and Atmospheric Administration (NOAA) located in Boulder, Colorado. SWPC continually monitors and forecasts Earth's space environment, providing solar-terrestrial information
The Corona program was a series of American strategic reconnaissance satellites. The Corona satellites were used for photographic surveillance of the Soviet Union, the People's Republic of China, and other areas beginning in 1959 and ending in 1972
MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov Telescopes) is a system of two telescopes situated at the Roque de los Muchachos Observatory on La Palma, one of the Canary Islands. MAGIC detects particle showers released by gamma rays, using the Cherenkov radiation, i.e., faint light radiated by the charged particles in the showers
Kepler space observatory was launched in 2009. Kepler mission is ‘specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets’
European Extremely Large Telescope (E-ELT) is currently being built by the European Southern Observatory (ESO) on top of Cerro Armazones in the Atacama Desert of northern Chile
Giant Magellan Telescope (GMT) is a ground-based extremely large telescope in Chile planned for completion in 2020
CryoSat is an ESA programme which will monitor variations in the extent and thickness of polar ice through use of a satellite in low Earth orbit
Minor Planet Center operates at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. It is the official organization in charge of collecting observational data for minor planets and comets
Sentinel is a space observatory currently being developed by Ball Aerospace for the private philanthropic B612 Foundation. The foundation is dedicated to the protection of Earth from asteroid strikes
National Solar Observatory operates facilities at two locations – one at Sacramento Peak near Sunspot in New Mexico, and the other at Kitt Peak in Arizona
Liverpool Telescope – fully robotic telescope on La Palma
Isaac Newton Group of Telescopes operates the William Herschel Telescope, Isaac Newton Telescope and Jacobus Kapteyn Telescope optical telescopes on La Palma
Most large telescopes have spectrometers, which are used either to measure the chemical composition and physical properties of astronomical objects or to measure their velocities from the Doppler shift of their spectral lines
Balloon satellite (also referred to as a satelloon) is a satellite that is inflated with gas after it has been put into orbit
Sloan Digital Sky Survey is a redshift survey which aims to map 25% of the sky and obtain observations on around 100 million objects and spectra for 1 million objects
Gaia is a space observatory of the European Space Agency. The mission aims to construct a 3D space catalog of approximately 1 billion astronomical objects. It has a billion pixel camera
The Wow! signal was a strong narrowband radio signal detected by Jerry Ehman in 1977, while working on a SETI project at The Big Ear radio telescope of The Ohio State University. The signal bore expected hallmarks of potential non-terrestrial and non-solar system origin. It lasted for the full 72 second duration that Big Ear observed it, but has not been detected again
Glossary
Aberration of light – (also referred to as astronomical aberration or stellar aberration) is an astronomical phenomenon which produces an apparent motion of celestial objects about their locations dependent on the velocity of the observer
Absorption spectroscopy – spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength. Shows what elements exist in a star or the atmosphere of an exoplanet
Albedo – the ratio of the outgoing solar radiation reflected by an object to the incoming solar radiation incident upon it
Aphelion – point where a planet is furthest from the Sun
Apogee – point in the orbit of the moon or of an artificial satellite most distant from the centre of the earth
Apparition – period of time a planet is visible
Asterism – a pattern of stars seen in Earth's sky which is not an official constellation
Astronomical unit – (AU) standard unit of measure to represent the distances of objects within our solar system from the Sun. One AU is 149,597,870 kilometers, c. 93 million miles, (the mean distance between the Earth and the Sun over one Earth orbit)
Barycentre – the point between two celestial objects where they balance each other
Bolide – an exploding meteor, or fireball
Celestial equator – a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. It is a projection of the terrestrial equator out into space. As a result of the Earth's axial tilt, the celestial equator is inclined by 23.4° with respect to the ecliptic plane
Celestial mechanics – the branch of astronomy that deals with the motions of celestial objects
Celestial sphere – an imaginary rotating sphere of ‘gigantic radius’, concentric and coaxial with the Earth. All objects in the sky can be thought of as lying upon the sphere
Conjunction – as seen from some place (usually the Earth), two celestial bodies appear near one another in the sky
Cosmic dawn – the time, a few hundred million years after the Big Bang, when the first stars and galaxies burst into existence
Cosmic rays – energetic particles originating from outer space that impinge on Earth's atmosphere. About 89% of all the incoming cosmic ray particles are simple protons, with nearly 10% being helium nuclei (alpha particles), and slightly under 1% are heavier elements; electrons (beta particles) constitute about 1% of galactic cosmic rays. The term ‘ray’ is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles
Declination – the celestial sphere equivalent of terrestrial latitude
Earthshine – reflected earthlight visible on the Moon's night side. It is also known as the Moon's ashen glow or as ‘the old Moon in the new Moon's arms’
Ecliptic – the apparent path the Sun traces out along the sky, independent of Earth's rotation in the course of the year
Elongation – the angular distance between the Sun and any other object in the sky
Equinox – from the Latin for ‘equal’ and ‘night’. Vernal equinox in March, autumn equinox in September, in the northern hemisphere
First light – when a telescope (or, in general, a new instrument) is first used to take an astronomical image after it has been constructed
Geostationary orbit – a geosynchronous orbit directly above the Earth's equator (0° latitude), with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. A geostationary satellite has an altitude of 22,200 miles above the Earth’s surface
Geosynshronous orbit – an orbit around a planet or moon with an orbital period that matches the planet or moon's sidereal rotation period
Heliometer – an instrument originally designed for measuring the variation of the Sun's diameter at different seasons of the year
Inclination – of the orbit of a planet is defined as the angle between the plane of the orbit of the planet and the ecliptic, which is the plane containing Earth's orbital path
Interstellar cloud – the generic name given to an accumulation of gas, plasma and dust in our and other galaxies
Lagrangian points – the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects (such as a satellite with respect to the Earth and Moon). The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centripetal force required to orbit with them
Lamb shift, named after Willis Lamb, is a small difference in energy between two energy levels of the hydrogen atom. Used by astronomers to detect minor wobbles in a stars’ position
Libration – an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth
Light year – the distance travelled by light in space during a year. 5.88 x 1012 miles
Luminosity – measures the total amount of energy emitted by a star or other astronomical object per unit time. The Sun has 1.00 solar luminosity
Lunation – the mean time for one lunar phase cycle (i.e., the synodic period of the Moon)
Magnetosphere – the area of space near an astronomical object in which charged particles are controlled by that object's magnetic field. Near the surface of the object, the magnetic field lines resemble those of a magnetic dipole
Meteor – a bright trail or streak that appears in the sky when a meteoroid is heated to incandescence by friction with the earth's atmosphere. Also known as a shooting star
Meteor stream – a transient shower of meteors when a meteor swarm enters the earth's atmosphere
Meteor swarm – a very powerful and spectacular spell that is similar to fireball
Meteorite – stony or metallic object that is the remains of a meteoroid that has reached the earth's surface
Meteoroid – any of the small solid extraterrestrial bodies that hits the earth's atmosphere
Metonic cycle – moon returns to the same position amongst the stars every 19 years
Nadir – a point on the celestial sphere directly below the observer
Observable universe – the galaxies and other matter that we can in principle observe from Earth in the present day, because light (or other signals) from those objects has had time to reach us since the beginning of the cosmological expansion
Occultation – an event that occurs when one celestial object is hidden by another celestial object that passes between it and the observer
Olbers' paradox – the argument that the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe. It is one of the pieces of evidence for a non-static universe such as the current Big Bang model. Thomas Digges was the first to expound the Copernican system in English in 1576 but discarded the notion of a fixed shell of immoveable stars to postulate infinitely many stars at varying distances; he was also first to postulate the ‘dark night sky paradox’ (now known as Olbers’ paradox)
Opposition – celestial bodies are said to be in opposition when they are on opposite sides of the sky, viewed from a given place
Orbital eccentricity – a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptical orbit, 1 is a parabolic escape orbit, and > 1 is a hyperbolic escape orbit
Orbital resonance – occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other
Orbital velocity – the speed of a satellite in orbit around a planet
Parallax – an apparent displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. Friedrich Bessel is credited with being the first to use parallax in calculating the distance to a star
Parsec – the distance from the Sun which would result in a parallax of 1 second of arc as seen from Earth. From parallax and second, 3.25 light years
Perigee – point in the orbit of the moon or of an artificial satellite closest to the centre of the earth
Perihelion – point where a planet is nearest to the Sun
Planisphere – a star chart analog computing instrument in the form of two adjustable disks that rotate on a common pivot. It can be adjusted to display the visible stars for any time and date
Precession of the equinoxes – westward motion of the equinoxes along the ecliptic, discovered by Hipparchus. Variation in the direction of the Earth’s axis of rotation. Also known as axial precession
Prograde – all the planets and almost all of the other objects that revolve in orbit around the Sun, with the exception of many comets, do so in the ‘prograde’ direction, i.e the same sense as the rotation of the Sun. Most planets also rotate in the same prograde direction, the exceptions being Venus and Uranus, which have retrograde rotations
Quadrature – that aspect of a heavenly body in which it makes a right angle with the direction of the Sun
Redshift – change in the wavelength of light, first observed by Hubble
Sidereal period – the time that it takes the object to make one full orbit around the Sun, relative to the stars. This is considered to be an object's true orbital period
Right ascension – the celestial sphere equivalent of terrestrial longitude
Roche limit – the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction. Typically, the Roche limit applies to a satellite disintegrating due to tidal forces induced by its primary, the body about which it orbits
Scintillation or twinkling – generic terms for rapid variations in apparent brightness or colour of a distant luminous object viewed through a medium, most commonly the atmosphere
Solstice – either of the two times of the year when the Sun is at its greatest distance from the celestial equator
Sun dog or parhelion – an atmospheric phenomenon that creates bright spots of light in the sky, often on a luminous ring or halo on either side of the sun
Synodic – conjunction of two bodies, as seen from the Sun
Synodic period – the time that it takes for the object to reappear at the same point in the sky, relative to the Sun, as observed from Earth; i.e. returns to the same elongation
Syzygy – the alignment of three celestial bodies in the same gravitational system along a straight line
Vacuum energy – an underlying background energy that exists in space throughout the entire universe
White hole – a hypothetical region of spacetime which cannot be entered from the outside; but matter and light can escape from it. The reverse of a black hole
Zenith – a point on the celestial sphere directly above the observer
Zodiacal light – a faint, roughly triangular, whitish glow seen in the night sky which appears to extend up from the vicinity of the Sun along the ecliptic or zodiac
