- Accretion disc (or accretion disk): It is a structure formed by diffuse material in orbital motion around a central body. The central body is typically either a young star, a protostar, a white dwarf, a neutron star, or a black hole. Instabilities within the disc redistribute angular momentum, causing material in the disc to spiral inward towards the central body. Gravitational energy released in that process is transformed into heat and emitted at the disk surface in the form of electromagnetic radiation. Accretion discs of young stars and protostars radiate in the infrared, those around neutron stars and black holes in the X-ray part of the spectrum.
- Asterism: It is a pattern of stars seen in Earth's sky which is not an official constellation. Like constellations, they are composed of stars which, while they are in the same general direction, are not physically related, often being at significantly different distances from Earth. An asterism may be composed of stars from one or more constellations. They are mostly simple shapes and few stars make these patterns easy to identify.
- Asteroids also called minor planets or planetoids, are a class of astronomical objects. The term asteroid is generally used to indicate a diverse group of small celestial bodies in the solar system that orbit around the Sun. Asteroids, or minor planets, are small celestial bodies composed of rock, ice, and some metal. Ceres, Juno, Pallas, Vesta are among the better known. As of 2008 there are 181,699 numbered minor planets -or asteroids-, and many more not yet numbered. But most asteroids are ordinary and not particularly noteworthy -less than 15,000 of them have been named
- Asteroid belt: This is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter where 98.5% of the known minor planets' orbits can be found. This region is termed the main belt when contrasted with other concentrations of minor planets, since these may also be termed asteroid belts.
- Black hole: It is a region of space in which the gravitational field is so powerful that nothing can escape after having fallen past the event horizon. Even electromagnetic radiation (e.g. light) is unable to escape, rendering the interior invisible. However, black holes can be detected if they interact with matter outside the event horizon. Black holes were first described by Einstein's theory of general relativity that predicts that when a large enough amount of mass is present within a sufficiently small region of space, all paths through space are warped inwards towards the centre of the volume, forcing all matter and radiation to fall inward. General relativity describes a black hole as a region of empty space with a pointlike singularity at the centre and an event horizon at the outer edge; the description changes when the effects of quantum mechanics are taken into account. Accordingly rather than holding captured matter forever, black holes may slowly leak a form of thermal energy called Hawking radiation.
- Celestial sphere: This is 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.
- Coma: In astronomy, a coma (from the Latin word for "hair") is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the sun; as the comet warms, parts of it sublimate. The coma is generally made of ice and dust. Larger dust particles are left along the comet's orbital path while smaller particles are pushed away from the Sun into the comet's tail by light pressure. This gives a comet a "fuzzy" appearance when viewed in telescopes and distinguishes it from stars.
- Comet dust: refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets' origin.
- Cosmic dust: It is a type of dust composed of particles in space which are a few molecules to 0.1 mm in size. Cosmic dust is found in various locations: intergalactic dust, interstellar dust, circumplanetary dust, dust clouds around other stars, etc.
- Crab Nebula: It is a supernova remnant and pulsar wind nebula in the constellation of Taurus. Located at a distance of about 6,500 light-years (2 kpc) from Earth, the nebula has a diameter of 11 ly (3.4 pc) and is expanding at a rate of about 1,500 kilometres per second. At the centre of the nebula lies the Crab Pulsar, a rotating neutron star, which emits pulses of radiation from gamma rays to radio waves with a spin rate of 30.2 times per second. The nebula was the first astronomical object identified with a historical supernova explosion.
- Dark nebula: It is 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. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Isolated small dark nebulae are called Bok globules. The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way. In the inner regions of dark nebulae important events take place, such as the formation of stars and masers.
- Dark matter: It is matter that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter. Dark matter accounts for the vast majority of mass in the observable universe. The composition of dark matter is unknown, but may include ordinary and heavy neutrinos, recently postulated elementary particles such as WIMPs and axions, astronomical bodies such as dwarf stars and planets (collectively called MACHOs), and clouds of nonluminous gas.
- Emission nebula: It is a cloud of ionized gas (a plasma) emitting light of various colours. The most common source for ionization are high-energy photons emitted from a nearby hot star. Among the several different types of emission nebula are H II regions, in which star formation is taking place and young, massive stars are the source of the ionising photons; and planetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.
- Gamma-ray bursts (GRBs): They are the most luminous electromagnetic events occurring in the universe since the Big Bang. They are flashes of gamma rays emanating from seemingly random places in deep space at random times. The duration of a gamma-ray burst is typically a few seconds, but can range from a few milliseconds to several minutes, and the initial burst is usually followed by a longer-lived "afterglow" emitting at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio). Gamma-ray bursts are detected by orbiting satellites about two to three times per week.
- Gas giant (or Jovian planet) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System; Jupiter, Saturn, Uranus, and Neptune. Uranus and Neptune are 'ice giants', as they are mostly composed of ice, rock, as well as gases of water, ammonia and methane. Gas giants may have a rocky or metallic core but the majority of its mass is in the form of the gaseous hydrogen and helium, with traces of water, methane, ammonia, and other hydrogen compounds. (compressed into liquids or solids deep in a gas giant's atmosphere.)
- Globular clusters: They are tight groups of hundreds of thousands of very old stars.
- Intergalactic space: This is the physical space between galaxies. Generally free of dust and debris, intergalactic space is very close to a total vacuum. The average density of the Universe is less than one atom per cubic meter. Average temperature is only 3 Kelvin. The density of the Universe is clearly not uniform; it ranges from relatively high density in galaxies (including very high density in planets, stars, and black holes) to conditions in vast voids that have much lower density than the Universe's average.
- Intermediate-mass black hole (IMBH): It is a black hole whose mass is significantly more than stellar black holes (a few tens of the mass of the Sun) yet far less than supermassive black holes (a few millions of the mass of the Sun). There is less evidence for their existence than for the other two types. Evidence for the existence of IMBHs can be obtained from observation of gravitational radiation, emitted by the compact remnant that orbits the IMBH. However it is not clear how such a black hole would form. They are too massive to be formed by the collapse of a single star, which is how the stellar black holes are thought to form but their environments lack the extreme conditions -i.e., high density and velocities observed at the centres of galaxies- which seemingly lead to the formation of supermassive black holes. There are two scenarios for the formation of IMBHs. The first, is the merging of stellar mass black holes and other compact objects by means of gravitational radiation. The second one is the runaway collision of massive stars in dense stellar clusters and the collapse of the collision product into an IMBH.
- Interstellar medium (or ISM - gas or dust): It is the name given to the gas and dust that pervade interstellar space. The interstellar medium consists of an extremely dilute mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and magnetic fields. The matter consists of about 99% gas and 1% dust by mass. The ISM's densities range from a few thousand to a few hundred million particles per cubic meter, and an average value in the Milky Way Galaxy of a million particles per cubic meter. The gas is roughly 90% hydrogen and 10% helium by number of nuclei, with additional heavier elements ("metals") present in trace amounts.
- Kuiper belt, sometimes called the Edgeworth-Kuiper belt: It is a region of the Solar System beyond the planets. It is similar to the asteroid belt, although it is far larger. It consists mainly of small bodies (remnants from the Solar System's formation) and at least one dwarf planet -Pluto. The Kuiper belt objects are composed largely of frozen volatiles (dubbed "ices"), such as methane, ammonia and water.
- Macho or Massive astrophysical compact halo object: It is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a small chunk of normal baryonic matter, which emits little or no radiation and drifts through interstellar space unassociated with any solar system. Since MACHOs would not emit any light of their own, they would be very hard to detect. MACHOs may sometimes be black holes or neutron stars as well as brown dwarfs or unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs.
- Meteor: The visible path of a meteoroid that enters Earth's (or another body's) atmosphere is a meteor, commonly called a "shooting star" or "falling star".
- Meteoroid: It is small sand to boulder-sized particle of debris in the Solar system. The visible path of a meteoroid that enters Earth's (or another body's) atmosphere is a meteor, commonly called a "shooting star" or "falling star".
- Meteor shower: A meteor shower, also known as a "meteor storm" or "meteor outburst", occurs when a group of meteors are observed to radiate from one point in the sky. These meteors are small fragments of cosmic debris entering Earth's atmosphere at extremely high speed. They vaporize due to friction with the air, leaving a streak of light that very quickly disappears. For bodies with a size larger than the atmospheric mean free path (10 cm to several metres), this visible light is due to the heat produced by the ram pressure (not friction, as is commonly assumed) of atmospheric entry. Most of the small fragments of cosmic debris are smaller than a grain of sand, so almost all fragments disintegrate and never hit the earth's surface. Fragments which do contact Earth's surface are called meteorites.
- Micro black hole or quantum mechanical black hole or mini black hole: It is simply a tiny black hole for which quantum mechanical effects play an important role. In theory, a black hole can have any size [mass]. The smaller the size of the micro black hole, the faster the evaporation rate, resulting in a sudden burst of particles as the micro black hole suddenly explodes.
- Nebula: It is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star's cycle. Originally nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way. Nebulae often form star-forming regions, such as in the Eagle Nebula. 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 big enough to form stars. The remaining materials are then believed to form planets, and other solar system objects.
- Open clusters: They generally contain less than a few hundred stars, which are often very young.
- Planetary nebula: It is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years. About 1,500 are known to exist in the Milky Way Galaxy. Planetary nebulae are important objects in astronomy because they play a crucial role in the chemical evolution of the galaxy, returning material to the interstellar medium which has been enriched in heavy elements and other products of nucleosynthesis (such as carbon, nitrogen, oxygen and calcium). In other galaxies, planetary nebulae may be the only objects observable enough to yield useful information about chemical abundances.
- Planetary System: A planetary system consists of the various non-stellar objects orbiting a star such as planets, moons, asteroids, meteoroids, comets, and dust. The Sun together with its planetary system, which includes Earth, is known as the Solar System.
- Primordial black hole: It is a hypothetical type of black hole that is formed not by the gravitational collapse of a star but by the extreme density of matter present during the universe's early expansion. According to the Big Bang Model (also called the Standard Model), during the first few moments after the Big Bang, pressure and temperature were extremely great. Under these conditions, simple fluctuations in the density of matter may have resulted in local regions dense enough to create black holes. Although most regions of high density would be quickly dispersed by the expansion of the universe, a primordial black hole would be stable, persisting to the present. Stephen Hawking theorized that due to quantum effects, primordial black holes could "evaporate" by a theoretical process now referred to as Hawking Radiation in which particles of matter would be emitted.
- Radio sources: They are objects in outer space that emit strong radio waves. Radio emission comes from hot gas, electrons spiralling in magnetic fields and specific wavelengths emitted by atoms and molecules in space. There are many sources of radio waves beyond our galaxy, most of which are found in other galaxies. A few examples include neutral hydrogen and carbon monoxide, mostly found in spiral galaxies, and quasars, which are point-like sources of radio waves and other kinds of electromagnetic radiation that are thought to be powered by supermassive black holes in the centres of elliptical galaxies. Scientists have also discovered weak background radiation left over from the Big Bang.
- Rotating black hole: It is a black hole that possesses angular momentum.
- Seyfert galaxies: They are a class of galaxies with nuclei that produce spectral line emission from highly ionized gas. They are a subclass of active galactic nuclei (AGN), and are thought to contain supermassive black holes.
- Solar system: The Solar System consists of the Sun and the other celestial objects gravitationally bound to it: the eight planets, their 166 known moons, three dwarf planets (Ceres, Pluto, and Eris and their four known moons), and billions of small bodies: asteroids, Kuiper belt objects, comets, meteoroids, and interplanetary dust. In broad terms, the charted regions of the Solar System consist of the Sun, four terrestrial inner planets, an asteroid belt composed of small rocky bodies, four gas giant outer planets, and a second belt, called the Kuiper belt, composed of icy objects. Beyond the Kuiper belt lies the scattered disc, the heliopause, and ultimately the hypothetical Oort cloud.
- Solar wind: The solar wind is a stream of charged particles (i.e., a plasma) which are ejected from the upper atmosphere of the sun. It consists mostly of high-energy electrons and protons (about 1 keV) that are able to escape the sun's gravity. Many phenomena are directly related to the solar wind, including geomagnetic storms that can knock out power grids on Earth, aurorae (e.g., Northern Lights) and the plasma tail of a comet always pointing away from the sun.
- Stellar association, or moving group: This is a very loose star cluster, looser than both open clusters and globular clusters. Stellar associations will normally contain from 10 to 100 or more stars. The stars share a common origin, but have become gravitationally unbound and are still moving together through space.
- Stellar black hole: It is a black hole formed by the gravitational collapse of a massive star (20 or more solar masses) at the end of its lifetime. The process is observed as a supernova explosion or as a gamma ray burst. The collapse of a star is a natural process to produce a black hole. It is inevitable at the end of the life of a star, when all stellar energy sources are exhausted. If the mass of the collapsing part of the star is below a certain critical value, the end product is a compact star, either a white dwarf or a neutron star. Both these stars have a maximum mass. So if the collapsing star has a mass exceeding this limit, the collapse will continue forever (catastrophic gravitational collapse) and form a black hole.
- Stellar wind: It is a stream of charged particles (a plasma) which are ejected from the upper atmosphere of a star. When originating from the Earth's Sun, the phenomenon is called the solar wind. It consists mostly of high-energy electrons and protons that are able to escape the star's gravity in part because of the high temperature of the corona and their high kinetic energy.
- Supermassive black hole: It is a black hole with a mass of an order of magnitude between 105 and 1010 of solar masses. It is currently thought that most, if not all galaxies, including the Milky Way, contain supermassive black holes at their galactic centres. There is also evidence that two supermassive black holes can co-exist in the same galaxy for a certain amount of time.
- Universe: It is defined as the summation of all particles and energy that exist and the space-time in which all events occur. Based on observations of the portion of the Universe that is observable, physicists attempt to describe the whole of space-time, including all matter and energy and events which occur, as a single system.
- White hole: It is the time reversal of a black hole. While a black hole
acts as an absorber for any matter that crosses the event horizon, a white
hole acts as a source that ejects matter from its event horizon. The sign
of the acceleration is invariant under time reversal, so both black and
white holes attract matter. The only potential difference between them is
in the behaviour at the horizon. Stephen Hawking has argued that white holes
are the same as black holes, once quantum mechanics is taken into account.