Astronomy Glossary

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The point at which a body orbiting the Earth's Sun is furthest from the Sun.

The point at which a body orbiting the Earth (such as the Moon or an artificial satellite) is furthest from the Earth.

Any pattern of stars recognizable in Earth's night sky. An asterism may form part of an official constellation or it may be composed of stars from more than one constellation.

asteroid belt
A region of space lying between Mars (1.5 AU) and Jupiter (5.2 AU), where the great majority of the asteroids are found. None of the belt asteroids have retrograde motion.

astronomical object
Also called a celestial object.
A type of naturally occurring physical entity, association, or structure that exists within the observable universe but is a more complex, less cohesively bound structure than an astronomical body, consisting perhaps of multiple bodies or even other objects with substructures, such as a planetary system, star cluster, nebula, or galaxy. Though the terms astronomical "object" and astronomical "body" are often used interchangeably, there are technical distinctions.

The scientific study of celestial objects and phenomena, the origins of those objects and phenomena, and their evolution.

The branch of astronomy that employs principles of physics and chemistry to determine the nature of astronomical objects and phenomena, examining properties such as luminosity, density, temperature, and chemical composition (rather than the positions or motions of objects in space, which is more specifically the emphasis of celestial mechanics).


black hole
A concentration of mass so compact that it creates a region of space from which not even light can escape. The outer boundary of this region is called the event horizon.


Cassini’s Division
A gap about 1800 km wide between the outermost rings of Saturn. It was discovered by Cassini in 1675. The period of a particle in Cassini’s division is about two-thirds that of Janus, one-half that of Mimas, one-third that of Enceladus, and one-quarter that of Tethys.

A phenomenon during which two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude as observed from a third body (usually the Earth), such that, from the observer's perspective, the objects appear to closely approach each other in the sky.

A region on the celestial sphere surrounding a specific and identifiable grouping of stars. The names of constellations are assigned by tradition and often have an associated folklore based in mythology, while the modern demarcation of their borders was established by the International Astronomical Union in 1930. Compare asterism.


In the equatorial coordinate system, the celestial equivalent of terrestrial latitude. Coordinates north of the celestial equator are measured in positive degrees from 0° to 90°, while coordinates to the south are measured in negative degrees. See also right ascension.

(a) Angular distance above (positive) or below (negative) the celestial equator. One of the co-ordinates, with right ascension, that defines the position of a heavenly body.
(b) Position on the sky in a north-south direction. Lines of declination are the celestial equivalent of latitude on Earth. Compare right ascension.
(c) Astronomical coordinate. Equivalent to latitude. The angle in degrees above or below the Celestial Equator, i.e. the projection onto the sky of the Earth's equator. Range of declination is from from zero to ± 90°.
(d) Angular distance on the celestial sphere north or south of the celestial equator. It is measured along the hour circle passing through the celestial object. Declination is usually given in combination with right ascension or hour angle.
(e) Angular distance north (+) or south (-) of the celestial equator to some object, measured in degrees, minutes, and seconds of arc along an hour circle passing through the object. Declination is analogous to latitude on the Earth's surface.

deep-sky object (DSO)
Any astronomical object that is not an individual star or an object within the Earth's Solar System. The classification is used mostly in amateur observational astronomy to distinguish faint objects in the night sky such as star clusters, nebulae, and galaxies.

dwarf galaxy
A galaxy at the faint end of the general luminosity function and generally exhibiting low surface brightness.


elliptical galaxy
A type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless appearance. They are one of three main morphological classes of galaxy, along with spiral and lenticular galaxies.

The angular separation between the Sun and an orbiting body, such as a planet, as it appears from Earth.


first light
The first use of a newly constructed telescope or other instrument to take an astronomical image.


A large, gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter, each of which orbits a center of mass. Galaxies may contain hundreds of billions of stars and are categorized according to their visual morphology as elliptical, spiral, or irregular. Most of the galaxies in the observable universe are between 1,000 and 3,000 parsecs (3,300 and 9,800 ly) in diameter though some, including the Milky Way, are much larger.

Galilean moons
A collective name for the four moons of Jupiter discovered by Galileo Galilei in 1610: Io, Europa, Ganymede, and Callisto.

gas giant
A giant planet composed mainly of hydrogen and helium gases rather than heavier elements, e.g. Jupiter and Saturn in the Solar System.

globular cluster
A tight, spherical conglomeration of many thousands of stars which are gravitationally bound to each other and which orbit a galactic core as a satellite. They differ from open clusters in having a much higher combined mass, with a typical lifespan extending for billions of years.


With reference to, or pertaining to, the geometric center of the Earth's Sun;[14] centered upon the Sun, e.g. a heliocentric orbit.


ice giant
A giant planet composed mainly of elements heavier than hydrogen or helium (such as oxygen, carbon, nitrogen, and sulfur), especially chemical volatiles with freezing points above 100 K (−173 °C), e.g. Uranus and Neptune in the Solar System.


Julian year
A unit of time defined as exactly 365.25 days of 86,400 SI seconds each. Because these are units of constant duration, the Julian year is also constant and does not vary with a specific calendar or with any of the other means of determining the length of a year, such as the tropical year. It is therefore widely used as the basis for defining the standard astronomical epoch and the light-year.


Kuiper belt
Also called the Edgeworth–Kuiper belt.
A circumstellar disc of small Solar System bodies such as asteroids, trojans, and centaurs in the outer Solar System, extending between 30 and 50 AU from the Sun. It is similar to the asteroid belt but far larger, and is home to several dwarf planets, including Pluto.


The distance that light (moving at about 186,000 miles per second) travels in one year, or about 6 trillion miles.


A numerical logarithmic scale indicating the brightness of an astronomical object, where the lower the value, the brighter the object. By convention, a first magnitude star is 100 times as bright as a sixth magnitude star. Magnitude 6 is considered the lower limit of objects that can be seen with the naked eye, although this can vary depending on sky conditions and eyesight.


natural satellite
Also moon.
Any astronomical body that orbits a planet, minor planet, or sometimes another small Solar System body.

Any astronomical object of indistinct nebulosity. In modern usage, the term typically refers to an interstellar cloud of dust, hydrogen, helium, and other ionized gases. Historically, it was also used to refer to extended sources of luminosity that could not be resolved into their individual components, such as star clusters and galaxies.


(a) Eclipse of a star by another celestial body.
(b) The cutoff of the light from a celestial body caused by its passage behind another object. (Strictly speaking, a Solar “eclipse” is a Solar occultation.)

open cluster
A gravitationally bound group of up to one thousand stars that formed together in the same molecular cloud.

The positioning of two celestial objects on opposite sides of the sky, from the perspective of an observer. This occurs, for example, when a planet makes its closest approach to the Earth, placing it in opposition to the Sun.

The gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. Though the smaller body is often said to orbit the larger body itself, both bodies actually follow approximately elliptical orbits around a common center of mass positioned at a focal point of each ellipse. The word "orbit" can variously refer to the elliptical trajectory itself or the act of following this trajectory, and can refer to a stable, regularly repeating trajectory as well as a non-repeating trajectory.

outer space
The vast, nearly empty expanse that exists beyond the Earth and between all celestial bodies, characterized generally by extremely low densities of particles, extremely low temperatures, and minimal gravity. Most of the volume of the Universe is intergalactic space, and even galaxies and star systems consist almost entirely of empty space.


(a) Less than full shadow (umbra).
(b) The portion of a shadow in which light from an extended source is partially but not completely cut off by an intervening body; the area of partial shadow surrounding the umbra.

The point at which a body orbiting the Earth (such as the Moon or an artificial satellite) is closest to the Earth.

The point at which a body orbiting the Earth's Sun is closest to the Sun.


A distant, point-like energy source originating from a powerful active galactic nucleus. Its luminosity is generated by the accretion of gas onto a supermassive black hole. Quasars emit radiation across the electromagnetic spectrum from radio waves to X-rays, and their ultraviolet and optical spectra are characterized by strong, broad emission lines.


red supergiant
A supergiant with spectral type M. Red supergiants are the largest stars in the universe: if put in place of the Sun, some would touch Saturn. The two brightest red supergiants in Earth's sky are Betelgeuse and Antares.

retrograde motion
Orbital or rotational motion of an object in the direction opposite the rotation of the object's primary. The direction of rotation is determined by an inertial frame of reference such as the fixed stars. Contrast prograde motion.

right ascension
In the equatorial coordinate system, the celestial equivalent of terrestrial longitude. It divides the celestial equator into 24 hours, each of 60 minutes.

Angular distance on the celestial sphere measured eastward along the celestial equator from the equinox to the hour circle passing through the celestial object. Right ascension is usually given in combination with declination.


satellite galaxy
A smaller companion galaxy that orbits within the gravitational potential of a more massive and luminous host galaxy; e.g. the Large Magellanic Cloud is a satellite galaxy of the Milky Way.

Describes the blurring of a stellar (point-like) image due to turbulence in the Earth’s atmosphere, both at high altitudes and within the telescope dome. Seeing estimates are often given in terms of the full-width in arcseconds of the image at the points where the intensity has fallen to half its peak value. The typical value at a good site is a little better than 1 arcsecond.

Solar System
The gravitationally bound planetary system of the Earth's Sun and all of the objects that orbit it, either directly or indirectly, including the eight true planets, five dwarf planets, and numerous small Solar System bodies such as asteroids, comets, and natural satellites.

A massive, luminous spheroid of plasma held together by its own gravity which, for at least a portion of its life, radiates energy into outer space due to the thermonuclear fusion of hydrogen into helium within its core. Astronomers can determine the mass, age, platemperature, chemical composition, and many other properties of a star by observing its motion through space, its luminosity, and its emission spectrum.

star cluster
large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely clustered groups of stars, generally containing fewer than a few hundred members, and are often very young.

a stellar explosion in which a star may be completely disrupted, leaving a compact stellar remnant such as a neutron star or black hole. At maximum light, the supernova can have luminosity about 108 or 109 times that of the Sun. The luminosity decays after the initial outburst, in certain classes of supernova, the decline being exponential with a half-life of about 80 days. In massive stars, the supernova occurs when the star has used up all its available nuclear fuel and it reaches a lower energy state through gravitational collapse to form a more compact star. In white dwarf stars in binary systems, accretion of mass onto the surface of a neutron star can be sufficient to take the star over the upper mass limit for stability as a white dwarf and it collapses to form a neutron star resulting in a supernova explosion.

supernova remnant
(a) SNR The expanding shell of gas ejected at a speed of about 10,000 km s-1 by a supernova explosion, observed as an expanding diffuse gaseous nebula, often with a shell-like structure. Supernova remnants are generally powerful radio sources.
(b) A gaseous nebula, the expanding shell ejected by a supernova, and deriving its energy (at least in some cases) from the conversion by the remanent neutron star of its rotational energy into a stream of high-energy particles being continually accelerated in the SNR. About 100 SNRs are known in our Galaxy. Supernova remnants are usually powerful radio sources.


The line that divides the illuminated side of a moon or planet from its dark side. The line moves as the object rotates with respect to its parent star.

Able to pass radiation without significant deviation or absorption. Note that a substance transparent to one radiation may be opaque to another. The divide between transparency and translucency is not well defined. Thus some people call a filter transparent (as it does not distort radiation): others would call it translucent (as it absorbs some of the radiation).


The portion of a shadow cone in which none of the light from an extended light source (ignoring refraction) can be observed.

The entirety of space and time and their contents, including galaxies, stars, planets, all other forms of matter and energy, and the physical laws and constants that describe them. When not otherwise qualified, "the Universe" usually refers to the entire Universe, whose spatial extent is unknown because it is not directly measurable; this is distinguished from the observable universe, whose size it is possible to measure.
One of many hypothetical parallel universes which exist as causally disconnected constituent parts of a larger multiverse, which itself comprises all of space and time and their contents.


variable star
Any star that is observed to vary in brightness. This variation may be periodic, with one or more cycles that last hours, days, months, or even years. Some stars vary in an irregular manner, while others undergo cataclysmic changes in brightness. Other forms of variability are intrinsic changes to the star's radial velocity or its profile of spectral lines.


white dwarf
A type of stellar remnant composed mostly of electron-degenerate matter. A white dwarf lacks the mass needed to continue the nuclear fusion process with its constituent atoms, so the object's energy output normally comes from radiative cooling. See nova and Type Ia supernova.


X-ray astronomy
an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and satellites. X-ray astronomy uses a type of space telescope that can see x-ray radiation which standard optical telescopes, such as the Mauna Kea Observatories, cannot.


yellow giant
A giant star with a spectral type of G. The nearest and brightest yellow giants are the two composing the double star Capella.

Yerkes System
A spectral classification system for stars.


The point in the sky that is directly overhead from the perspective of a particular location on the Earth.