Glossary of astronomy

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.

A

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

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

aphelion

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

apoapsis

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.

appulse

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

asterism

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

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.

astrometry

astronomical body

Also called a celestial body.

astronomical catalogue

Also spelled astronomical catalog.

astronomical object

Also called a celestial object.

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).

astronomy

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

astrophysics

atmosphere

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.

azimuth

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.

B

barycenter

baryogenesis

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.

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.

C

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.

chromosphere

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. 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).

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.

comet

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.

commensurability

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, 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

conjunction

constellation

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.

corona

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)

cosmic dust

cosmic microwave background (CMB)

Also called the cosmic microwave background radiation (CMBR).

cosmic ray

cosmogony

cosmology

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.

D

declination

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.

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.

E

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.

eccentricity

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 used as a reference plane for describing the position of other Solar System bodies. 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

elongation

ephemeris

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.

epoch

equatorial coordinate system

equinox

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.

exoplanet

Also called an extrasolar planet.

extinction

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.

F

facula

(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.

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.

G

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.

galaxy

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

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.

heliocentric

Centered upon the Earth's Sun.

heliopause

heliosphere

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.

hypergalaxy

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 galaxy systems are examples of hypergalaxies.

I

ice giant

inclination

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.

ionosphere

irregular galaxy

irregular moon

isochrone

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.

J

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.

K

Kelvin–Helmholtz mechanism

Keplerian orbit

Also called a Kepler orbit.

Kuiper belt

Also called the Edgeworth–Kuiper belt.

L

Lagrangian point

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

Laniakea Supercluster

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

late-type star

libration

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 viewed along the limb.

light-year

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

luminosity

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.

M

magnetosphere

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.

magnitude

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

meridian

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.

meteor

The ionization trail produced by a meteoroid as it enters the Earth's atmosphere.

meteorite

meteoroid

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 reoccur every year.

metallicity

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.

microvariable

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.

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.

moon

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

N

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)

nebula

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.

neutrino

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

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.

O

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.

obliquity

See axial tilt.

observable universe

Oort cloud

Also called the Öpik–Oort cloud.

opacity

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.

opposition

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.

orbit

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.

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.

P

The parallax shift of a star at a distance of one parsec as seen from the Earth. Not to scale.

parsec

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.

periapsis

The point of closest approach between two orbiting objects.

perihelion

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

perturbation

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

photosphere

planet

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.

planetesimal

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.

planetoid

precession

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

primary

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. 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.

protoplanet

protoplanetary disk

protostar

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.

pulsar

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.

Q

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.

quasar

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.

R

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.

redshift

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.

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.

S

satellite galaxy

secular motion

Any change in movement that happens over a very long time period. 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

sidereal time

sidereal year

sky

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 System

solar wind

solstice

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.

spectroscopy

spherical astronomy

Also called positional astronomy.

spiral galaxy

star

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.

starfield

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.

stellar magnetic field

stellar parallax

stellar remnant

subsatellite

substellar object

Sun

supercluster

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.

supernova

surface gravity

synodic period

The time taken for a given object to make one complete orbit about another object, as calculated with respect to the background stars.

syzygy

The straight-line configuration of three celestial bodies in a gravitational system.

T

telescope

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.

terminator

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. 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. 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.

tilt erosion

The gradual reduction of the obliquity of an orbiting satellite due to tidal interactions.

trans-Neptunian object (TNO)

transit

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.

U

UBV photometric system

Also called the Johnson system or Johnson–Morgan system.

Universe

V

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).

W

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.

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.

X

XBONG

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.

Z

zenith

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.

zodiac

The apparent path of the Sun across the celestial sphere as observed from Earth's orbit. It crosses a band of twelve Zodiac constellations, through which the Moon and visible planets are seen to travel.

See also

• List of astronomical catalogues
• List of astronomy acronyms
• List of common astronomy symbols
• Modern constellations

References

External links

• "Astronomical Glossary", A Knowledgebase for Extragalactic Astronomy and Cosmology, NASA/IPAC, January 10, 2006, retrieved 2012-02-19
• Astronomy Terms, Sky & Telescope Media, retrieved 2018-03-09
• "ESO Astronomical Glossary", Public Outreach, European Southern Observatory, retrieved 2018-03-09
• "Glossary", HubbleSite – Reference Desk, Space Telescope Science Institute (STScI), retrieved 2018-03-09
• "Glossary of (comet and) astronomical terms", International Comet Quarterly, Harvard University, retrieved 2012-02-20

[[Category:Astronomy-related lists]