Astronomy Glossary

A rotating disk of gas, dust, and other material that forms around a massive object such as a black hole or protostar as matter spirals inward due to gravity and angular momentum conservation.

A small rocky body orbiting the Sun, mostly found in the asteroid belt between Mars and Jupiter. Asteroids are remnants from the early solar system that never coalesced into a planet.

A system consisting of two stars orbiting their common center of mass. More than half of all star systems in the Milky Way are thought to be binary or multiple-star systems.

A small icy body that, when passing close to the Sun, heats up and releases gases in a visible atmosphere (coma) and sometimes a tail. Comets originate from the Kuiper Belt or the Oort Cloud.

A recognizable pattern of stars on the celestial sphere, traditionally named after mythological figures, animals, or objects. The International Astronomical Union recognizes 88 official constellations covering the entire sky.

The thermal radiation left over from the epoch of recombination in Big Bang cosmology, approximately 380,000 years after the Big Bang. It is the oldest electromagnetic radiation in the universe, observed at a nearly uniform temperature of 2.725 K.

A hypothetical form of energy that permeates all of space and drives the observed accelerating expansion of the universe. It accounts for approximately 68% of the total energy density of the universe.

Matter that does not interact with electromagnetic radiation and is therefore invisible, but whose existence is inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe.

A celestial body orbiting the Sun that is massive enough for gravity to give it a roughly spherical shape but has not cleared its orbital neighborhood of other debris. Pluto, Eris, and Ceres are examples.

The apparent path of the Sun across the celestial sphere over the course of a year, which corresponds to the plane of Earth's orbit around the Sun. The planets of the solar system orbit roughly along the ecliptic plane.

The boundary surrounding a black hole beyond which no light or matter can escape. It represents the point of no return; any object crossing the event horizon is irrevocably captured by the black hole's gravity.

A gravitationally bound system consisting of stars, stellar remnants, interstellar gas and dust, and dark matter. Galaxies vary enormously in size and shape, from small irregular galaxies to giant ellipticals containing trillions of stars.

The bending of light from distant objects by the gravitational field of a massive intervening object, as predicted by general relativity. It can magnify, distort, or create multiple images of background sources.

A region of the solar system beyond Neptune's orbit, extending from about 30 to 55 AU from the Sun, populated by small icy bodies and dwarf planets including Pluto. It is a source of short-period comets.

A type of neutron star with an extraordinarily powerful magnetic field, typically a thousand times stronger than ordinary neutron stars. Magnetars can produce intense bursts of X-rays and gamma rays.

A large cloud of gas and dust in interstellar space. Nebulae can be regions of active star formation (emission nebulae), the remnants of dead stars (planetary nebulae and supernova remnants), or clouds that reflect nearby starlight (reflection nebulae).

The extremely dense remnant of a massive star that has undergone a supernova explosion, composed almost entirely of neutrons. Typical neutron stars have masses of 1.4 to 2 solar masses compressed into a sphere about 20 km in diameter.

A unit of distance equal to approximately 3.26 light-years, defined as the distance at which one astronomical unit subtends an angle of one arcsecond. It is the standard unit for expressing distances to stars and galaxies in professional astronomy.

An early stage in star formation in which a dense region within a molecular cloud collapses under gravity and heats up but has not yet begun hydrogen fusion. Protostars are embedded in surrounding gas and dust and are primarily observable in infrared.

An extremely luminous active galactic nucleus powered by a supermassive black hole accreting matter. Quasars are among the brightest and most distant objects in the observable universe, some emitting more energy than entire galaxies.

A luminous giant star in a late phase of stellar evolution that has exhausted hydrogen in its core and expanded to many times its original size. The Sun will become a red giant in about 5 billion years.

The radius of the event horizon of a non-rotating black hole, determined by the object's mass. For any mass compressed within its Schwarzschild radius, escape velocity exceeds the speed of light, forming a black hole.

A stream of charged particles (mostly protons and electrons) released from the Sun's upper atmosphere (corona) at speeds of 300-800 km/s. The solar wind creates the heliosphere and influences planetary magnetospheres and comet tails.

A catastrophic explosion of a star that can briefly outshine an entire galaxy, expelling most or all of the star's material at velocities up to 10% the speed of light. Supernovae are critical for dispersing heavy elements into the interstellar medium.

The dense, Earth-sized remnant of a low- to intermediate-mass star that has exhausted its nuclear fuel and shed its outer layers. White dwarfs are supported against gravitational collapse by electron degeneracy pressure and slowly cool over billions of years.