Glossary of astronomy

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This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth; the field of astronomy features an extensive vocabulary and a significant amount of jargon.


Syrtis Major (center) is a prominent dark albedo feature on Mars
absolute magnitude
A measure of a star's absolute brightness. It is defined as the apparent magnitude the star would show if it were located at a distance of 10 parsecs, or 32.6 light-years.
accretion disk
A roughly circular mass of diffuse material in orbit around a central object, such as a star or black hole. The material is acquired from a source external to the central object, and friction causes it to spiral inward towards the object.
active galactic nucleus (AGN)
A compact region in the center of a galaxy displaying a much higher than normal luminosity over some part of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. A galaxy hosting an AGN is called an active galaxy.
albedo feature
A large area on the surface of a reflecting object that shows a significant contrast in brightness or darkness (albedo) with adjacent areas.
Am star
A chemically peculiar star belonging to the more general class of A-type stars. The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals. See metallicity.
anticenter shell
The point at which an orbiting body is furthest from the Earth's Sun.
The point of furthest excursion, or separation, between two orbiting objects.
apparent magnitude

Also called visual brightness (V).

A measure of the brightness of a celestial body as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower the value of its magnitude.
The closest approach of one celestial object to another, as viewed from a third body.
argument of periapsis

Also called the argument of perifocus or argument of pericenter.

artificial satellite
Any pattern of stars recognizable in Earth's night sky. It may form part of an official constellation or it may be composed of stars from more than one constellation.
asteroid belt
The circumstellar disc in the Solar System located roughly between the orbits of Mars and Jupiter that is occupied by numerous irregularly shaped small Solar System bodies ranging in size from dust particles to asteroids and minor planets. The asteroid belt is often called the main asteroid belt or main belt to distinguish it from other asteroid populations in other parts of the Solar System.
astrometric binary
A type of binary system where evidence for an unseen orbiting companion is revealed by its periodic gravitational perturbation of the visible component. See also spectroscopic binary.
astronomical body

Also called a celestial body.

A type of naturally occurring physical entity, association, or structure within the observable universe that is a single, tightly bound, contiguous structure, such as a star, planet, moon, or asteroid. Though the terms astronomical "body" and astronomical "object" are often used interchangeably, there are technical distinctions.
astronomical catalogue

Also spelled astronomical catalog.

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.
astronomical symbol
astronomical unit (AU)
A unit of length used primarily for measuring astronomical distances within the Solar System or between the Earth and distant stars. Originally conceived as the approximate average distance between the midpoints of the Earth and the Sun, the astronomical unit is now more rigidly defined as exactly 149,597,870.7 kilometres (92,956,000 miles; 4.8481×10−6 parsecs; 1.5813×10−5 light-years).
The scientific study of celestial objects and phenomena, the origins of those objects and phenomena, and their evolution.
autumnal equinox
The precise time of year on Earth when the Sun appears to cross the celestial equator, while generally trending southward at each zenith passage. It represents the moment when the North Pole of the Earth begins to tilt away from the Sun, and typically occurs on or near September 22 each year.
axial precession
axial tilt

Also called obliquity.

An angular measurement of an object's orientation along the horizon of the observer, relative to the direction of true north. When combined with the altitude above the horizon, it defines an object's current position in the spherical coordinate system.


The process by which the class of subatomic particles known as baryons were generated in the early Universe, including the means by which baryons outnumber antibaryons.
Big Bang
The prevailing cosmological model for the origin of the observable universe. It depicts a starting condition of extremely high density and temperature, followed by an ongoing expansion that led to the current conditions.
binary star
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.
break-up velocity

Also called critical velocity or critical rotation.

The surface velocity at which the centrifugal force generated by a rapidly spinning star matches the force of Newtonian gravity. Beyond this point, the star would begin to eject matter from its surface.[1]
brown dwarf
A substellar object that is too low in mass to sustain the nuclear fusion of hydrogen-1 in its core, with the latter being a characteristic of stars on the main sequence. Brown dwarfs can still generate energy from gravitational contraction and by the fusion of deuterium.


celestial equator
celestial event
celestial mechanics
The branch of astronomy that studies the motions of all types of astronomical objects, including stars, planets, and natural and artificial satellites, among others.
celestial pole
One of two coordinates in the Earth's sky at which a hypothetical indefinite extension of the Earth's axis of rotation "intersects" the celestial sphere. The celestial poles form the north and south poles of the equatorial coordinate system.
celestial sphere
An imaginary sphere that covers the Earth's entire sky and is stationary with respect to the background stars. It is used as a tool for spherical astronomy.
chromospheric activity index
A parameter indicating the magnetic activity in a star's chromosphere. One measure of this activity is log R′HK, where R′HK is the ratio of the equivalent width of a star's singly-ionized Calcium H and K lines, after correction for photospheric light, to the bolometric flux.[2] Schröder et al. (2009) divide solar-type stars into four groups depending on their activity index: very active (log R′HK above −4.2), active (−4.2 to −4.75), inactive (−4.75 to −5.1), and very inactive (below −5.1).[3]
circumstellar disc

Also spelled circumstellar disk.

clearing the neighbourhood
color index
A numeric value that is used to compare the brightness of a star measured from different frequency bands of the electromagnetic spectrum. Because the energy output of a star varies by frequency as a function of temperature, the color index can be used to indicate the star's temperature.
A relatively small, icy body that displays extended features when it approaches the Sun. The energy from the Sun vaporizes volatiles on a comet's surface, producing a visible coma around the cometary body. Sometimes a comet can produce a long tail radiating away from the Sun.
A property of two objects orbiting the same body whose orbital periods are in a rational proportion. For example, the orbital period of Saturn around the Sun is very nearly 5/2 the orbital period of Jupiter.
common proper motion
Used to indicate two or more stars that share the same motion through space, within the margin of observational error. That is, either they have nearly the same proper motion and radial velocity parameters, which may suggest that they are gravitationally bound or share a common origin,[4] or they are known to be gravitationally bound (in which case their proper motions may be rather different but average to be the same over time).
compact star
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.
An aura of plasma that surrounds cooler stars such as the Sun. It can be observed during a solar eclipse as a bright glow surrounding the lunar disk. The temperature of the corona is much higher than that of the stellar surface, and the mechanism that creates this heat remains subject to debate among astronomers.
coronal mass ejection (CME)
A significant release of plasma and the accompanying magnetic field from the Sun's corona, often following a solar flare or present during a solar prominence eruption.
cosmic dust
cosmic microwave background (CMB)

Also called the cosmic microwave background radiation (CMBR).

cosmic ray
The scientific study of the origin, evolution, and eventual fate of the Universe.
critical rotation
critical velocity
The velocity at the equator of a rotating body where the centrifugal force balances the Newtonian gravity. At this rotation rate, mass can be readily lost from the equator, forming a circumstellar disk.[5]


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 use coordinates in negative degrees. See also right ascension.
decretion disk
A circumstellar disk formed from gas ejected from the central star that now follows a near Keplerian orbit around it. This type of disk can be found around many Be stars.[6]
debris disk
A ring-shaped circumstellar disk of dust and debris orbiting its host star. It is created by collisions between planetesimals. A debris disk can be discerned from an infrared excess being emitted from the star system, as the orbiting debris re-radiates the star's energy into space as heat.
diurnal motion
double star
A pair of stars that appear near each other on the celestial sphere. This can happen because, by chance, the pair lie along nearly the same line of sight from the Earth. If the two are located in physical proximity to each other, they may form a co-moving pair or a binary star system.
dwarf planet
dwarf star
The category of ordinary main sequence stars like the Sun, in contrast to evolved giant stars like Betelgeuse and Antares. Confusingly, the term dwarf has also come to include stellar remnants known as white dwarfs as well as low mass, substellar objects known as brown dwarfs.


Sample evolutionary tracks for stars of different mass
early-type star
A hotter and more massive star, in contrast to late-type stars that are cooler and less massive. The term originated from historical stellar models that assumed stars began their early life at a high temperature then gradually cooled off as they aged, it can be used to refer to the higher temperature members of any particular population or category of stars, rather than just all stars in general.

Also called orbital eccentricity.

A parameter that determines how much an orbit deviates from a perfect circle. For an elliptical orbit, the eccentricity ranges from greater than zero to less than one.
ecliptic plane

Also called the plane of the ecliptic or simply the ecliptic.

The plane defined by the Earth's orbit around the Sun. Hence, the position of the Sun as viewed from the Earth defines the intersection of this plane with the celestial sphere. The ecliptic is widely used as a reference plane for describing the position of other Solar System bodies within various celestial coordinate systems. It differs from the celestial equator because of the axial tilt of the Earth.
effective temperature
(of a star or planet) The temperature of an ideal black body that would emit the same total amount of electromagnetic radiation.
elliptical galaxy
elliptical orbit
A list or table of the expected positions of astronomical objects or artificial satellites in the sky at various dates and times. Modern ephemerides are often provided by computer software.
A moment in time used as a reference point for some time-varying astronomical quantity, such as the celestial coordinates or orbital elements of an astronomical object, because such quantities are subject to perturbations and change over time. The primary use of astronomical quantities specified by epochs is to calculate other relevant parameters of motion in order to predict future positions and velocities. In modern usage, astronomical quantities are often specified as a polynomial function of a particular time interval, with a given epoch as the temporal point of origin.
equatorial coordinate system
escape velocity
evolutionary track
A curve on the Hertzsprung–Russell diagram that a solitary star of a particular mass and composition is expected to follow during the course of its evolution. This curve predicts the combination of temperature and luminosity that a star will have during part or all of its lifetime.[7]

Also called an extrasolar planet.

The absorption and scattering of electromagnetic radiation by matter (dust and gas) between an emitting astronomical object and the observer. Atmospheric extinction varies by the wavelength of the radiation, with the attenuation being greater for blue light than for red.



(pl. faculae)

A bright spot on a star's photosphere formed by concentrations of magnetic field lines. For the Sun in particular, they are most readily observed near the solar limb. An increase in faculae as a result of a stellar cycle increases the star's total irradiance.
field galaxy
Any galaxy that does not belong to a larger cluster of galaxies but is gravitationally alone.
field star
A randomly situated star that lies along the line of sight to a group of physically associated stars under study, such as a star cluster. These field stars can contaminate the results for a study and so they need to be identified.[8]
fixed stars
flare star
A class of variable star that undergoes sudden, dramatic increases in brightness due to magnetic activity on its surface. This change in brightness occurs across the electromagnetic spectrum from radio waves to X-rays. Most flare stars are faint red dwarfs.
Fulton gap
The apparent uncommonness of planets having a size between 1.5 and 2 times that of the Earth.


galactic anticenter
galactic coordinate system
galactic corona
galactic nucleus

Also called the galactic core or galactic center.

The region at the center of a galaxy, which is usually home to a very dense concentration of stars and gas. It almost always includes a supermassive black hole which, when active, can generate a much higher luminosity in a compact region than its surroundings. This excess luminosity is known as an active galactic nucleus, and the brightest such active galaxies are known as quasars.
galactic period

Also called the galactic year or cosmic year.

galactic tide
The tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way.
A large, gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter, each of which orbits a center of mass. Most of the galaxies in the observable universe are between 1,000 and 3,000 parsecs (3,300 and 9,800 ly) in diameter and are categorized according to their visual morphology as elliptical, spiral, or irregular.
galaxy cluster
galaxy group
Galilean moons
A collective name for the four moons of Jupiter that were discovered by Galileo Galilei in 1610. They consist of the moons Io, Europa, Ganymede, and Callisto, and were the first natural satellites ever discovered.
gamma-ray burst (GRB)
A cataclysmic event that generates a brief but intense outburst of gamma ray radiation which can be detected from billions of light-years away. The source of most GRBs is theorized to be supernova or hypernova explosions of high-mass stars. Short GRBs may also result from the collision of neutron stars.
gas giant
geometric albedo
The ratio of the brightness of an astronomical body at a phase angle of zero to an idealized flat, fully reflecting, diffusively scattering (Lambertian) disk with the same cross-section. It is a measure of how much of the incoming illumination is being scattered back toward an observer and has a value between zero and one.
giant planet
Any very large or massive planet, including gas giants and ice giants.
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.
gravitational collapse
gravitational lens
Any very large distribution of mass, such as a galactic cluster, which can bend passing light from a distant source by a noticeable degree. The effect, known as gravitational lensing, can make background objects appear to an observer to take on a ring or arc shape.
gravitational-wave astronomy


H II region
An ionized nebula powered by young, massive O-type stars. Ultraviolet photons from these hot stars ionize gas in the surrounding environment, and the nebular gas shines brightly in spectral lines of hydrogen and other elements; because O-type stars have relatively short lifetimes (typically a few million years), the presence of an H II region indicates that massive star formation has taken place recently at that location. H II regions are often found in the arms of spiral galaxies and in star-forming irregular galaxies.
Centered upon the Earth's Sun.
Hertzsprung–Russell diagram
A plot of luminosity versus effective temperature for a population of stars. Depending on the usage, the star's absolute magnitude may be substituted for luminosity, and its color index or spectral type for temperature. Depending on their mass and composition, single stars follow specific tracks across this chart over the course of their evolution. Hence, knowing a star's mass and metallicity allows its age to be estimated. Stars of similar types are also found grouped together in specific regions of the chart, including main sequence, red giant, and white dwarf stars.
Hill sphere

Also called the Hill radius.

The approximate region around an astronomical object where its gravitational attraction will dominate the motions of satellites. It is computed with respect to the next most gravitationally attractive object, such as the nearest star or the galactic core. Satellites moving outside this radius tend to be perturbed away from the main body.[9]
A system consisting of a large galaxy accompanied by multiple smaller satellite galaxies (often elliptical) as well as its galactic corona. The Milky Way and Andromeda systems are examples of hypergalaxies.[10]


ice giant
inferior planet
An archaic term that is sometimes used to refer to the planets Mercury and Venus. The name originated from the fact that these planets orbit closer to the Sun than the Earth and hence, in the geocentric cosmology of Ptolemy, both appeared to travel with the Sun across the sky. This is in contrast to the so-called superior planets, such as Mars, which appeared to move independently of the Sun.
International Astronomical Union (IAU)
interstellar medium (ISM)
The matter that exists in the space between the stars in a galaxy. This medium mainly consists of hydrogen and helium, but it is enhanced by traces of other elements contributed by matter expelled from stars.
interstellar reddening
An effect produced by the incremental absorption and scattering of electromagnetic energy from interstellar matter, known as extinction. This effect causes the more distant objects such as stars to appear redder and dimmer than expected, it is not to be confused with the separate phenomenon of redshift.
irregular galaxy
irregular moon
A curve on the Hertzsprung–Russell diagram that represents the evolutionary positions of stars having the same age but differing masses. This is in contrast to an evolutionary track, which is a plot of stars having the same mass but differing ages. In fact, multiple evolutionary tracks can be used to build isochrones by putting curves through equal-age points along the tracks; when the mass of a star can be determined, an isochrone can be used to estimate the star's age.


Jeans instability
A physical state in which an interstellar cloud of gas will begin to undergo collapse and form stars. A cloud can become unstable against collapse when it cools sufficiently or has perturbations of density, allowing gravity to overcome the gas pressure.


Kelvin–Helmholtz mechanism
Keplerian orbit

Also called a Kepler orbit.

Kuiper belt

Also called the Edgeworth–Kuiper belt.


Lagrangian point

Also called a Lagrange point, libration point, or L-point.

Any of a set of points near two large bodies in orbit at which a smaller object will maintain its position relative to the larger bodies. At other locations, a small object would eventually be pulled into its own orbit around one of the large bodies, but at the Lagrangian points the gravitational forces of the large bodies, the centripetal force of orbital motion, and (in certain scenarios) the Coriolis acceleration all align in a way that causes the small object to become "locked" in a stable or nearly stable relative position. For each combination of two orbital bodies, there are five such Lagrangian points, typically identified with the labels L1 to L5. The phenomenon is the basis for the stable orbits of trojan satellites and is commonly exploited by man-made satellites.
Laniakea Supercluster

Also called the Lenakaeia Supercluster, Local Supercluster, or Local SCI.

late-type star
A slight oscillating motion of the Moon as seen from the Earth. This tipping and tilting movement is a result of the elliptical orbit of the Moon, it can allow normally hidden parts of the Moon's far side to be visible along the limbs of the lunar disk.
limb darkening
An optical effect seen in stars (including the Sun), where the center part of the disk appears brighter than the edge or limb of the image.
longitude of the ascending node
The total amount of energy emitted per unit time by a star, galaxy, or other astronomical object. In SI units, luminosity is measured in joules per second or watts, and is often given in terms of astronomical magnitude. Luminosity is related to but distinct from visual brightness.


A mostly convex region formed when a plasma, such as the solar wind, interacts with the magnetic field of a body, such as a planet or star.
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 seeing conditions and eyesight.
main sequence
A category of stars which form a continuous and distinctive band on plots of stellar temperature versus luminosity. These stars are characterized by being in hydrostatic equilibrium and undergoing nuclear fusion of hydrogen-1 in their core region; the Sun is a main sequence star.
mean anomaly
A line running north–south across the sky and passing through the point directly overhead known as the zenith.
Messier object
One of a set of 110 "nebulous" astronomical objects, of which 103 were catalogued by French comet hunter Charles Messier between 1771 and 1781.

Also called a shooting star or falling star.

The visible passage of a glowing meteoroid, micrometeoroid, comet, or asteroid through the Earth's atmosphere, usually as a long streak of light produced when such an object is heated to incandescence by collisions with air molecules in the upper atmosphere and leaves an ionization trail as a result of its rapid motion and sometimes also the shedding of material in its wake.
A small rock or boulder that has entered a planetary atmosphere. If it survives to reach the ground, it is then termed a meteorite.
meteor shower
A series of meteors that seemingly radiate from a single area in the night sky. These are produced by debris left over from a larger body, such as a comet, and hence they follow roughly the same orbit. This makes many meteor showers predictable events, as they recur every year.
A measure of the abundance of elements other than hydrogen and helium within an astronomical object. Note that these "metals" include elements that are not traditionally considered metallic by chemical convention.
A stellar object such as a variable star that undergoes very small variations in luminosity. Detecting microvariability will typically require a sufficient number of observations to rule out random error as a source.[11]
Milky Way
minor planet
An object in direct orbit around the Sun that is neither a dominant planet nor originally classified as a comet. A moon is not a minor planet because it orbits another body instead of the Sun.
molecular cloud
An interstellar cloud in which the prevailing physical conditions allow molecules to form, including molecular hydrogen.
moment of inertia factor

Also called the normalized polar moment of inertia.

See natural satellite.
Morgan–Keenan classification system

Also called the MK classification.

morning width

Also called rise width.

The horizontal angular distance between the rise azimuth of a celestial body and the East direction.
moving group

Also called a stellar association.

A loose grouping of stars which travel together through space. Although the members were formed together in the same molecular cloud, they have since moved too far apart to be gravitationally bound as a cluster.
multi-messenger astronomy


natural satellite

Also called a moon.

Any astronomical body that orbits a planet, minor planet, or sometimes another small Solar System body.
near-Earth object (NEO)
A region of indistinct nebulosity. In modern terms, it means 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.
neutron star
A type of compact star that is composed almost entirely of neutrons, which are a type of subatomic particle with no electrical charge. Typically, neutron stars have a mass between about 1.35 and 2.0 times the mass of the Sun, but with a radius of only 12 km (7.5 mi), making them among the densest known objects in the universe.
night sky
The appearance of the Earth's sky at nighttime, when the Sun is below the horizon, and more specifically when clear weather and low levels of ambient light permit visibility of celestial objects such as stars, planets, and the Moon. The night sky remains a fundamental setting for both amateur and professional observational astronomy.
number density
The quantity of some specified particle or object class per unit volume. For atoms, molecules or subatomic particles, the volume is typically in cm−3 or m−3. With stars, cubic parsecs (pc−3) are often used.


OB association
A group of massive stars which are not gravitationally bound to each other, but move together through space in a loose association. The OB in the name is a reference to stars of stellar classifications O and B.
See axial tilt.
observable universe
Oort cloud

Also called the Öpik–Oort cloud.

A measure of the resistance of a medium to the radiative transmission of energy. Within a star, it is an important factor in determining whether convection occurs.
open cluster
A gravitationally bound group of up to one thousand stars that formed together in the same molecular cloud.
Occurs when two celestial objects are on opposite sides of the sky. 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 the 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.
orbit plot

Also called orbital plot.

A schematic diagram of a complete orbit. For a binary system, it is typically presented from the primary component's frame of reference.[12]
orbital elements
The set of parameters that uniquely define an orbit.
orbital period
The time a given astronomical object takes to complete one orbit around another object. For objects in the Solar System, the orbital period is often referred to as the sidereal period.
orbital resonance
orbital speed
outer space

Also simply called 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.


The parallax shift of a star at a distance of one parsec as seen from the Earth. Not to scale.
A unit of length defined as the distance at which a star would show a parallax shift of exactly one arcsecond as observed from Earth's orbit. It is equal to 3.2616 light-years or 206,265 astronomical units. The word "parsec" is a portmanteau of the words parallax and second.
The point of closest approach between two orbiting objects.
The point at which an orbiting body is closest to the Earth's Sun.
phase angle
The elongation or angle between an orbiting body and the Sun as viewed from a particular perspective such as the Earth. It determines the amount of a planet or moon's visible surface that lies in shadow. Inferior planets such as Venus generally have a low phase angle as seen from Earth, so they are often viewed as a crescent; superior planets such as Mars and Jupiter usually have a high phase angle, so little of the shadowed side is visible.
photometric system
A type of astronomical body orbiting a star or stellar remnant which is massive enough to be rounded by its own gravity (but not massive enough to achieve thermonuclear fusion) and has cleared its neighbouring region of all planetesimals.
planetary differentiation
The process of separating out different constituents of a planetary body, causing it to develop compositionally distinct layers (such as a metallic core).
planetary nebula
A type of emission nebula formed from a glowing shell of expanding plasma that has been ejected from a red giant star late in its life. The name derives from their resemblance to a planet. An example is the Ring Nebula.
planetary science

Also sometimes called planetology.

planetary system
planetary-mass object (PMO)

Also called a planemo or planetary body.

Any solid object (generally larger than 1 kilometre (0.62 mi) in diameter) that arises during the formation of a planet whose internal strength is dominated by self-gravity and whose orbital dynamics are not significantly affected by gas drag. The term is most commonly applied to small bodies thought to exist in protoplanetary disks and debris disks during the process of planet formation, but is also sometimes used to refer to various types of small Solar System bodies which are left over from the formation process. There is no precise distinction between a planetesimal and a protoplanet.
Any slow change in the orientation of an object's axis of rotation. For the Earth in particular, this phenomenon is referred to as the precession of the equinoxes. Apsidal precession refers to a steady change in the orientation of an orbit, such as the precession in the orbit of Mercury that was explained by the theory of general relativity.
precession of the equinoxes

Also called a gravitational primary, primary body, or central body.

The main physical body of a gravitationally bound, multi-object system. The primary constitutes most of the system's mass and is generally located near the system's barycenter.
projected separation
The minimum physical separation between two astronomical objects, as determined from their angular separation and estimated distance.[13] For planets and double stars, this distance is usually given in Astronomical Units; the actual separation of the two objects depends on the angle of the line between the two objects to the line-of-sight of the observer.
proper motion
The rate of angular motion of an object over an interval of time, usually years. For stars, this is typically given in milliarcseconds per year.
protoplanetary disk
A concentration of mass formed out of the contraction of a collapsing interstellar cloud. Once sufficient mass has fallen onto this central core, it becomes a pre-main-sequence star.
A highly magnetized rotating neutron star or white dwarf that emits a beam of electromagnetic radiation. This beam is observed only when it is pointing toward Earth, making the object appear to pulse.


quadratic field strength
A method of computing the mean strength of a varying stellar magnetic field. It is determined by taking the root mean square of a series of longitudinal magnetic field strength measurements taken at different time periods.[14]

Also called a quasi-stellar radio source

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.


radial velocity
The velocity of an object along the line of sight to the observer, which in astronomy is usually determined via Doppler spectroscopy. Positive values are used to indicate a receding object. An object such as a star can undergo changes in its radial velocity because of the gravitational perturbation of another body, or because of radial pulsations of its surface. The latter, for example, occurs with a Beta Cephei variable star.
radio source
red-giant branch
A conspicuous trail of enlarged red stars found on the Hertzsprung–Russell diagram for a typical globular cluster. It begins at the main sequence turnoff point and extends toward the higher luminosity and lower temperature range until reaching the red-giant tip; this branch consists of older stars that have evolved away from the main sequence but have not yet initiated helium fusion in their core region.
regular moon
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.
ring system
Roche limit
The distance from an astronomical object at which the tidal force matches an orbiting body's gravitational self-attraction. Inside this limit, the tidal forces will cause the orbiting body to disintegrate, usually to disperse and form a ring. Outside this limit, loose material will tend to coalesce.
Rosseland optical depth
An extinction coefficient of an atmosphere, which describes the net opacity to radiation at a given depth. See optical depth.[15]
rotation period
rotational modulation
A phenomenon which causes the luminosity of a star to vary as rotation carries star spots or other localized activity across the line of sight. Examples include RS CVn and BY Dra variables.[16]


Saber's beads
Broken arc of illuminations seen at the limb of very young or old lunar crescents. The visual similarity to the moments before and after a total solar eclipse was first noted by American astronomer Stephen Saber.
satellite galaxy
secular motion
Any change in movement that happens over a very long time period.[17] Examples include the perihelion precession of Mercury, the tidal acceleration of the Earth–Moon system, and precession of the Earth's axis.
semi-major axis
One half the maximum length of an ellipse. It is used to give a physical dimension to a two-body Keplerian orbit, such as for a binary star system. However, when the distance to the system is unknown, the semi-major axis may be given as an angle.
sidereal day
sidereal period
The orbital period of an object within the Solar System, such as the Earth's orbital period around the Sun. The name "sidereal" implies that the object returns to the same position relative to the fixed stars of the celestial sphere as observed from the Earth.
sidereal time
sidereal year
Everything that lies above the surface of the Earth, including the atmosphere and outer space. In the context of astronomy, the term "sky" is also used as another name for the celestial sphere. See also night sky.
small Solar System body (SSSB)
solar flare
solar prominence
Solar System
The gravitationally bound planetary system of the Earth's Sun and 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.
solar wind
One of two precise times of year when the Sun reaches either its most northerly or most southerly point in the sky as seen from Earth. The solstices occur on or near June 20 and December 21 each year; the "Summer Solstice", often used to refer to the June solstice because of its occurrence during the Northern Hemisphere's summer, is the annual date featuring the longest duration of daylight and the shortest duration of nighttime in the Northern Hemisphere. The reverse is true for the "Winter Solstice", which is often used to refer to the December solstice.
spectroscopic binary
A type of binary star system where the individual components have not been resolved with a telescope. Instead, the evidence for the binarity comes from shifts observed in the spectrum; this is caused by the Doppler effect as the radial velocity of the components change over the course of each orbit.
spherical astronomy

Also called positional astronomy.

spiral galaxy
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, temperature, chemical composition, and many other properties of a star by observing its motion through space, its luminosity, and its emission spectrum.
star catalogue

Also spelled star catalog.

star system
starburst galaxy
Any galaxy that has an anomalously high rate of star formation. The criteria for a starburst is a star formation rate that would normally consume the galaxy's available supply of unbound gas within a time period shorter than the age of the galaxy. Most starbursts occur as a result of galactic interactions, such as a merger.
Any set of stars visible in an arbitrarily sized field of view of a telescope, usually in the context of some region of interest within the celestial sphere.
stellar atmosphere

Also called the stellar envelope.

The outermost region of a star. Although it forms only a small portion of the star's mass, for some evolved stars the stellar envelope can form a significant fraction of the radius.
stellar classification

Also called spectral classification.

The categorization of stars based upon their spectra. The modern MK spectral classification scheme is a two-dimensional classification based on temperature and luminosity.
stellar designation
stellar evolution
stellar evolution model

Also simply called a stellar model.

A physics-based model of a star's stellar evolution over time based upon its mass and chemical composition.[18]
stellar magnetic field
stellar parallax
stellar remnant
substellar object
superior planet
An archaic term that is sometimes used to refer to planets that orbit further from the Sun than the Earth, such as Saturn. The name originated from the geocentric cosmology of Ptolemy. Contrast inferior planet.
supermassive black hole (SMBH)
A class of very large black hole which possesses a mass ranging from hundreds of thousands to many billion times the mass of the Sun. These are typically found at a galactic core, where they can have a profound effect upon the evolution of the surrounding galaxy.
surface gravity
synodic period
The time it takes for a body visible from another body (often the Earth) to complete a cycle with respect to the background stars visible in the second body's celestial sphere. Synodic period is most commonly used to indicate the elapsed time between a given body's consecutive appearances in the same location in the night sky as observed from Earth, but can in principle be calculated with respect to the sky as observed from any body. It is related to but distinct from the orbital period, a result of the fact that both the body being studied (e.g. Jupiter) and the body from which it is being observed (e.g. Earth) are independently orbiting a third body (the Sun); because of this, the time it takes for Jupiter to return to the same location in the Earth's sky is much shorter than the time it takes for Jupiter to completely orbit the Sun.
The straight-line configuration of three celestial bodies in a gravitational system.


telluric star
A star with nearly featureless continuum spectra that can be used to correct for the effect of telluric contamination of the Earth's atmosphere on the spectra of other stars. For example, water vapor in the atmosphere creates significant telluric absorption bands at wavelengths above 6800 Å; these features need to be corrected for in order to more accurately measure the spectrum.[19]
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.
thick disk population
thin disk population
The layer of the Milky Way galaxy where the spiral arms are found and where most of the star formation takes place. It is about 300–400 parsecs (980–1,300 light-years) deep and centered on the galactic plane. Stars belonging to this population generally follow orbits that lie close to this plane;[20] this is in contrast to members of the thick disk population and halo stars.
tidal braking

Also called tidal acceleration.

The transfer of momentum between an astronomical body and an orbiting satellite as the result of tidal forces. This can cause changes in the rotation periods for both bodies as well as modification of their mutual orbit. A satellite in a prograde orbit will gradually recede from the primary body, while slowing the rotation rate of both bodies.
tidal locking
The net result of continued tidal braking such that, over the course of an orbit, there is no net transfer of angular momentum between an astronomical body and its gravitational partner. When the orbital eccentricity is low, the result is that the satellite orbits with the same face always pointed toward its primary.[21] An example is the Moon, which is tidally locked with the Earth.
tidal stream
A stream of stars and gases which are stripped from gas clouds and star clusters because of interaction with the gravitational field of a galaxy such as the Milky Way.[22]
tilt erosion
The gradual reduction of the obliquity of an orbiting satellite due to tidal interactions.[23]
trans-Neptunian object (TNO)
An astronomical event during which an object passes visibly across the face of a much larger body. An example of this event is the transit of Venus across the face of the Sun, which was visible from Earth in 2004; because a transit results in a decrease in the net luminosity from the two objects, the transit method is used to detect extrasolar planets as they pass in front of their host stars. A transit by an object that appears roughly the same size or larger than the body it is transiting is called an occultation or eclipse.
Tully–Fisher relation
An empirical relationship between the mass or intrinsic luminosity of a spiral galaxy and its angular velocity or emission line width. It can be used to estimate the distance of the galaxy, and hence forms a rung on the Cosmic Distance Ladder.


UBV photometric system

Also called the Johnson system or Johnson–Morgan system.



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.
vernal equinox
The precise time of year on Earth when the Sun appears to cross the celestial equator, while generally trending northward at each zenith passage. It represents the moment when the North Pole of the Earth begins to tilt toward the Sun, and typically occurs on or near March 20 each year.
velocity dispersion
The statistical dispersion of velocities about the mean velocity for a group of objects, such as stars in a globular cluster or galaxies in a galactic cluster. This value can be used to derive the combined mass of the group by using the virial theorem.
Virgo Supercluster (Virgo SC)

Also called the Local Supercluster (LSC or LC).


weak-line star
A reference to the faintness of the spectral lines for a star compared to standard stars with the same stellar classification. Since most absorption lines are caused by elements other than hydrogen and helium—what astronomers refer to as "metals"—these are sometimes called metal weak stars.[24]
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.
Wilson–Bappu effect
A correlation between the width of the singly ionized calcium K-line (Ca II K) at 3933 Å and the absolute visual magnitude of the emitting late-type stars. This linear relation makes it useful for determining the distances of G, K, and M-type stars.[25]



An acronym of X-ray bright optically normal galaxy.

A seemingly normal galaxy that does not appear to have an active galactic nucleus, yet displays an anomalous level of excess X-ray emission.[26]


The point in the sky that is directly overhead from the perspective of a particular location on the Earth.
zero-age main sequence (ZAMS)
The sequence of positions along the Hertzsprung–Russell diagram achieved by newly formed, chemically homogeneous stars which have finished contracting and have reached hydrostatic equilibrium, with energy being derived solely from nuclear fusion.[27]
The area of the sky that extends approximately 8 degrees north or south (in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year as observed from Earth. The Sun, Moon, and visible planets appear to travel across a band of twelve Zodiac constellations within this belt as the Earth orbits the Sun.

See also[edit]


  1. ^ Maeder, Andre (2008), Physics, Formation and Evolution of Rotating Stars, Astronomy and Astrophysics Library, Springer Science & Business Media, ISBN 3540769498.
  2. ^ Noyes, R. W.; et al. (April 15, 1984), "Rotation, convection, and magnetic activity in lower main-sequence stars", Astrophysical Journal, Part 1, 279: 763–777, Bibcode:1984ApJ...279..763N, doi:10.1086/161945.
  3. ^ Schröder, C.; et al. (January 2009), "Ca II HK emission in rapidly rotating stars. Evidence for an onset of the solar-type dynamo", Astronomy and Astrophysics, 493 (3): 1099–1107, Bibcode:2009A&A...493.1099S, doi:10.1051/0004-6361:200810377.
  4. ^ Perryman, Michael (2009), Astronomical Applications of Astrometry: Ten Years of Exploitation of the Hipparcos Satellite Data, Cambridge University Press, p. 80, ISBN 0521514894.
  5. ^ Townsend, R. H. D.; et al. (May 2004), "Be-star rotation: how close to critical?", Monthly Notices of the Royal Astronomical Society, 350 (1): 189–195, arXiv:astro-ph/0312113, Bibcode:2004MNRAS.350..189T, doi:10.1111/j.1365-2966.2004.07627.x.
  6. ^ Silaj, J.; Jones, C. E.; Tycner, C.; Sigut, T. A. A.; Smith, A. D. (March 2010), "A Systematic Study of Hα Profiles of Be Stars", The Astrophysical Journal Supplement, 187 (1): 228–250, Bibcode:2010ApJS..187..228S, doi:10.1088/0067-0049/187/1/228.
  7. ^ Salaris, Maurizio; Cassisi, Santi (2005), Evolution of stars and stellar populations, John Wiley and Sons, p. 110, ISBN 0-470-09220-3, retrieved 2012-02-29.
  8. ^ Ridpath, Ian (2012), A Dictionary of Astronomy, OUP Oxford, p. 163, ISBN 0199609055, retrieved 2016-10-15.
  9. ^ Chebotarev, G. A. (April 1964), "Gravitational Spheres of the Major Planets, Moon and Sun", Soviet Astronomy, 7: 618, Bibcode:1964SvA.....7..618C.
  10. ^ Einasto, J. (1978), "Hypergalaxies", The large scale structure of the universe; Proceedings of the Symposium, Tallinn, Estonian SSR, September 12-16, 1977. (A79-13511 03-90), Dordrecht, D. Reidel Publishing Co., pp. 51–60, Bibcode:1978IAUS...79...51E.
  11. ^ Rufener, F.; Bartholdi, P. (June 1982), "List of 333 variable, microvariable or suspected variable stars detected in the Geneva photometry", Astronomy and Astrophysics Supplement Series, 48: 503–511, Bibcode:1982A&AS...48..503R.
  12. ^ Aitken, Robert G. (1935), The Binary Stars, New York: McGraw-Hill Book Company, pp. 70–124.
  13. ^ MacEvoy, Bruce; Tirion, Wil (2015), The Cambridge Double Star Atlas, Cambridge University Press, p. 4, ISBN 1107534208.
  14. ^ Bychkov, V. D.; et al. (April 2009), "Catalogue of averaged stellar effective magnetic fields - II. Re-discussion of chemically peculiar A and B stars", Monthly Notices of the Royal Astronomical Society, 394 (3): 1338–1350, Bibcode:2009MNRAS.394.1338B, doi:10.1111/j.1365-2966.2008.14227.x.
  15. ^ Schrijver, C. J.; Zwaan, C. (2008), Solar and Stellar Magnetic Activity, Cambridge Astrophysics, 34, Cambridge University Press, p. 25, ISBN 1139425420.
  16. ^ Rodono, M.; et al. (September 1986), "Rotational modulation and flares on RS CVn and BY Dra-type stars. I - Photometry and SPOT models for BY Dra, AU Mic, AR Lac, II Peg and V 711 Tau (= HR 1099)", Astronomy and Astrophysics, 165 (1–2): 135–156, Bibcode:1986A&A...165..135R.
  17. ^ Ridpath, Ian, ed. (2018), A Dictionary of Astronomy, Oxford Quick Reference Online (3 ed.), Oxford University Press, ISBN 0192542613.
  18. ^ Andersen, J. (2000), Livio, Mario (ed.), Unsolved Problems in Stellar Evolution, Space Telescope Science Institute Symposium, 12, Cambridge University Press, p. 97, ISBN 0521780918.
  19. ^ Husser, Tim-Oliver (2012), 3D-Spectroscopy of Dense Stellar Populations, Universitätsverlag Göttingen, pp. 54–59, ISBN 3863950925
  20. ^ "Components of the Milky Way", Galaxies, Center for Computational Physics in Hawaii, archived from the original on 2011-10-24, retrieved 2012-02-27
  21. ^ Barnes, Rory, ed. (2010), Formation and Evolution of Exoplanets, John Wiley & Sons, p. 248, ISBN 3527408967.
  22. ^ Sanders, Jason (2015), Dynamics of the Milky Way: Tidal Streams and Extended Distribution Functions for the Galactic Disc, Springer Theses, Springer, p. 9, ISBN 3319187724.
  23. ^ Heller, R.; et al. (April 2011), "Tidal obliquity evolution of potentially habitable planets", Astronomy & Astrophysics, 528: 16, arXiv:1101.2156, Bibcode:2011A&A...528A..27H, doi:10.1051/0004-6361/201015809, A27.
  24. ^ Jaschek, Carlos; Jaschek, Mercedes (1990), The Classification of Stars, Cambridge University Press, p. 257, ISBN 0-521-38996-8.
  25. ^ Pace, G.; et al. (April 2003), "The Wilson-Bappu effect: A tool to determine stellar distances", Astronomy and Astrophysics, 401: 997−1007, arXiv:astro-ph/0301637, Bibcode:2003A&A...401..997P, doi:10.1051/0004-6361:20030163.
  26. ^ Yuan, Feng; Narayan, Ramesh (September 2004), "On the Nature of X-Ray-Bright, Optically Normal Galaxies", The Astrophysical Journal, 612 (2): 724−728, arXiv:astro-ph/0401117, Bibcode:2004ApJ...612..724Y, doi:10.1086/422802.
  27. ^ Hansen, Carl J.; Kawaler, Steven D. (1999), Stellar Interiors: Physical Principles, Structure, and Evolution, Astronomy and Astrophysics Library, Springer Science & Business Media, p. 39, ISBN 038794138X.

External links[edit]