Glossary of Astronomical Terms

Glossary of Astronomical Terms

Glossary of Astronomical Terms


The point in the orbit of a planet, star, or other astronomical object when it is farthest from its primary star. (Antonym: periastron) Because the orbits of celestial bodies are never perfectly circular, they always have closest and farthest points. A term specific to the Solar System is used to name the farthest distance of any orbiting body from our Sun: aphelion.

Asteroid Belt

The debris-filled region of the Solar System located between the orbits of Mars and Jupiter in a band extending from about 2 AU to 4 AU. (See diagram.) The Asteroid Belt is understood to be a remnant of the swarm of planetesimals from which the Solar planets evolved. It contains mostly rocky bodies that range in size from planetoids like Ceres down to the hundreds of thousands of nameless boulders and “flying rockpiles” that comprise most of its population. Only about 30 asteroids have diameters of 200 km (124 miles) or more, and only Ceres presents a spherical appearance.

The lowercase term asteroid belt can be extended to describe postulated or observed features of exoplanetary systems, often with an implied limitation on orbital radius: asteroid belts can be understood as orbiting at or inside a host star’s ice line.

Astronomical Unit

A standard unit of measurement used by astronomers, equivalent to 93 million miles (149,598,000 km), the average distance of the Earth from the Sun. Abbreviation: AU


Common abbreviation for astronomical unit, equivalent to 93 million miles (149,598,000 km), the average distance of the Earth from the Sun.

Brown Dwarf

Sub-stellar objects of spectral classes L and T are called brown dwarfs because they emit infrared radiation but little or no visible light. Brown dwarfs can exist at a range of masses, from about 13 MJUP to about 80 MJUP.

  • L dwarfs have masses in the range of 65-80 MJUP and burn both lithium and deuterium.

  • T dwarfs have masses in the range of 13-65 MJUP and burn only deuterium.
All brown dwarfs have radii within 10% of Jupiter’s. As they age, they burn through their fusible material and steadily decrease in luminosity and temperature, so that their outward appearance becomes progressively less star-like and more planet-like.

Currently, the cut-off between lightweight brown dwarfs and heavyweight planets is poorly understood, with astronomers increasingly expressing skepticism that 13 MJUP represents a meaningful boundary. On the one hand, Isabelle Baraffe points out that even a gas giant planet of sufficient mass can experience deuterium fusion (Baraffe et al. 2008), while on the other, many observers suggest that T-dwarfs can have masses lower than 13 MJUP, perhaps as low as 5-10 MJUP (Luhman et al. 2007). See also the Web pages of I. Neill Reid and colleagues.

Dark Halo

An extensive structure of dark matter enclosing the Milky Way Galaxy. Similar halos envelop other spiral galaxies.

Debris Disk

The orbiting aggregation of planetesimals and collisional dust that succeeds a protoplanetary disk once its gaseous components have dispersed. Debris disks can be maintained for billions of years, as demonstrated by the examples of the Solar System (4.6 billion years) and Tau Ceti (10 billion years). As in the Solar System, Fomalhaut, and HD 69830, debris may be distributed in the form of relatively narrow rings. The debris fields of the Solar System are known as the Asteroid Belt and the Kuiper Belt.

Debris disks in other star systems are detected by the dust generated by cascading collisions among the rocky or icy objects that compose them. See also Debris Disk Systems.


The degree to which the orbit of an astronomical body deviates from a perfect circle. Eccentricity is expressed as a fraction between 0.00 (designating a circular orbit) and 0.99…. (designating an extremely elongated ellipse, such as the orbit of a comet).

Galilean Satellites

The four largest moons of the planet Jupiter. In order of distance from the planet, they are Io, Europa, Ganymede, and Callisto. They were discovered by Galileo in 1610, whence the adjective “Galilean.” See also Galilean Moons and Terrestrial Planets and Moons.

Gas Giant Planet

A planet composed primarily of hydrogen gas with a mass between about 0.2 MJUP and 13 MJUP (Ida & Lin 2005, Hartman et al. 2010). The upper limit for gas giants is poorly understood. Objects as massive as 23 MJUP have been detected in planet-like orbits, but opinion remains divided as to whether they are gas giants or brown dwarfs.

Studies of the Solar System’s two gas giants, Jupiter and Saturn, along with observations of dozens of transiting extrasolar gas giants, have established that this planetary species varies widely in composition and internal structure. With regard to mass, Jupiter seems to be a typical gas giant, while Saturn is just above the dividing line between gas giants and the second most massive type, ice giants.

Jupiter’s core is comprised of heavy elements, massing about 5 to 10 MEA. An additional 30 to 35 MEA in heavy elements are scattered through its deep hydrogen envelope. Its total mass is about 318 MEA, of which 10% to 15% consists of heavy elements. Saturn’s core is about 8 to 25 MEA, while its maximum mass in heavy elements is only 10 MEA. This presupposes a hydrogen envelope of 70 to 90 MEA, for a total mass of 95 MEA (10% heavy elements). Evidently Saturn has a more differentiated internal structure than Jupiter. (Alibert et al. 2005, Guillot 2005)

Extrasolar gas giants vary even more in composition. Transit studies suggest that the transiting Hot Jupiter HD 149026 b, with a mass of 0.359 MJUP, has a heavy element content of 70 MEA, about 60% of its total composition. At the other end of the scale is TrES-2 b, another transiting Hot Jupiter, with a mass of 1.2 MJUP and a heavy element content approaching zero. (Torres et al. 2008)

Giant Star

A star that has burned through all the hydrogen in its core and switched over to the fusion of helium. Such a star expands in radius by a factor of 10 to 200 or more, with a simultaneous increase in luminosity and heat output. Because the star’s color becomes redder during this phase, such objects are often known as red giants.

Transition to the giant phase occurs in stars with masses between about 0.7 MSOL and 8 MSOL (corresponding to spectral classes K, G, F, A, and B).

The ultimate evolution of M-type stars (those with masses of 0.5 MSOL or less) remains uncertain, and in fact none of the M stars formed since the emergence of the Milky Way Galaxy have had time to exhaust their hydrogen content. Stars more massive than 8 MSOL develop rapidly into so-called “supergiants” and most likely end their evolution in supernova explosions that result in contraction into black holes.

Stars spend a relatively brief portion of their evolutionary lifetimes in the giant phase, which is indicated in the system of spectral classification by appending the Roman numerals II or III to the spectral type. For example, the nearby red giant Aldebaran has a spectral type of K5 III. It probably originated as a blue-white star of spectral type A. Aldebaran has about 25 times the diameter and 150 times the luminosity of the Sun.

At the end of the giant phase, the star throws off much of its mass in a series of explosive contractions and shrinks into a white dwarf.

Habitable Zone

The region of a planetary system in which temperatures permit liquid water to exist on the surface of a terrestrial-mass planet with a suitable atmospheric pressure. This region may also be known as the liquid water zone. As yet there are no hard and fast rules for defining the habitable zone of any given star. For the Sun it obviously centers around the orbital radius of Earth (1 AU), but the exact boundaries remain a matter of disagreement. Our system’s habitable zone might be as wide as 0.7 AU-1.5 AU or as narrow as 0.9 AU-1.2 AU (Rivera & Haghighipour 2006). The habitable zones of other G-type stars will be similar in extent. For M-stars, which are much dimmer and cooler than the Sun, habitable zones probably range from 0.1 to 0.2 AU; for F-stars, which are brighter and hotter than the Sun, they center around 3 AU (Mandell et al. 2007).

Ice Giant Planet

A planet more massive than 10 MEA and less massive than 100 MEA, with a rocky core, a mantle of water ice, and a substantial hydrogen atmosphere. The Solar System contains two ice giants: Uranus, at 14.5 MEA, and Neptune, at 17.2 MEA. Planets in this mass range are being detected with increasing frequency in extrasolar systems.

In evolutionary theory, ice giants are explained as failed gas giants – that is, objects that accreted massive cores too late to capture large quantities of hydrogen gas from their dissipating protoplanetary disks (Ida & Lin 2005).

Ice Line

For any given star, the ice line is the distance from the central star beyond which free-floating molecules of water and other volatiles will condense into ice (Ida & Lin 2004, Mandell et al. 2007, Kennedy & Kenyon 2008a). This point is also known as the ice boundary or snow line.

In the present Solar System, most astronomers place the ice line at about 2.7 AU. This marks the boundary between the inner Solar System, which is dominated by rocky planets and asteroids, and the outer Solar System, which contains the four giant planets, their icy moons, and the debris fields of the Kuiper Belt. For M dwarf stars, which are much cooler than our Sun, the ice line will be correspondingly closer to the host star, at about 0.4 AU, while for A stars, which are much hotter, it will be much farther, at about 11 AU (Ida & Lin 2005).

During the era of planet formation, when stars still retain their gaseous protoplanetary disks, the location of the ice line depends on two parameters: the host star’s luminosity and the rate of viscous accretion of gases (Kennedy & Kenyon 2008a). Both parameters change over time, typically driving the ice line from larger to smaller radii. For example, the Solar System’s ice line probably began at about 5 AU and migrated inward to 2.7 AU over a period of half a million years (Kennedy & Kenyon 2008a).

The location of the ice line is a key variable in planetary evolution, since rocky planets like Earth and Mars must form within the ice line, while gas giants like Jupiter, Saturn, and most known extrasolar planets preferentially form just outside the ice line (Ida & Lin 2004).

Kuiper Belt

The ice- and debris-filled region of the Solar System located beyond the orbit of Neptune in a broad band extending from about 30 AU to 50 AU. It contains bodies of various sizes, the largest of which have masses and diameters approaching those of the Earth’s moon, or in other words more than 1600 kilometers (1000 mi). Among these so-called ice planetoids or ice dwarfs are the dwarf planets Pluto, Eris, Makemake, and Haumea. Most objects in the Kuiper Belt, however, are much smaller.

So far, more than 1200 objects with diameters of 100 kilometers (60 mi) or more have been discovered in the Kuiper Belt. A review by David Jewitt and colleagues (2008) estimated that this region contains about 70,000 additional objects in the same size range, with a total population of at least 100 million objects larger than 1 kilometer.

Main Sequence

The phase in a star’s evolution when it converts hydrogen to helium through nuclear fusion in its core. During this period the star’s mass, size, and luminosity remain consistent, providing a stable environment for the formation and evolution of planetary systems.


Abbreviations for “Earth mass” or “mass of Earth,” used as a unit of measurement for terrestrial planets, ice giant planets, and moons. The corresponding abbreviation for “Jupiter mass” is MJUP. Examples: Luna is 0.0123 MEARTH; Mars is 0.11 MEA; Neptune is 17.2 MEA; and Jupiter is 317.8 MEA.


The proportion of heavy elements (especially iron) in relation to hydrogen and helium in the chemical composition of a star. Metallicity can be expressed in logarithmic form and abbreviated [Fe/H]. Zero represents the metallicity of the Sun; positive numbers indicate higher metallicity, negative numbers indicate lower. The metal content of our Sun is somewhat enhanced compared to other stars in the local region of our Galaxy.

Stellar metallicity is correlated with the presence or absence of massive planets traveling on relatively short-period orbits. Surveys of F, G, and K stars within our Local Bubble suggest that about 25% of stars at the high end of metallicity have gas giant planets within 5 AU, compared to only about 3% of stars at the low end (Marcy et al. 2005).


Abbreviation for “Jupiter mass” or “mass of Jupiter,” used as a unit of measurement for giant planets and brown dwarfs. Jupiter is 317.8 times as massive as Earth.


Common abbreviation for “Solar mass” or “mass of the Sun,” used as a unit of measurement for stars. For example, the Sun is 1 MSOL, Sirius is 2.1 MSOL, and Gliese 581 is 0.31 MSOL.


A mathematical term referring to phenomena that occur at the boundary (i.e., separatrix) between two alternative modes or regimes. In exoplanetary studies, this term is used primarily by Rory Barnes, Richard Greenberg, and colleagues to describe the orbital behavior of adjacent planet pairs. In a 2007 abstract (BAAS, 39, 15.03) Greenberg and Barnes write: “Two-planet systems (whether librating or circulating) tend to lie near a separatrix between libration and circulation. Similarly, in systems of more than two planets, many adjacent orbits lie near a separatrix that divides modes of circulation. A defining characteristic of near-separatrix motion is that one eccentricity periodically reaches zero.”

Neutron Star

One of the possible outcomes of a supernova explosion, created when an extremely massive star of spectral class O or B throws off mass and collapses into a tiny remnant. Neutron stars have masses in excess of the Sun’s but diameters of only a few dozen kilometers, meaning that they are far denser even than white dwarf stars. They rotate extremely fast, often with periods of a tiny fraction of 1 second.


A standard unit of measurement used by astronomers, equivalent to 3.26 light years and abbreviated pc. A kiloparsec is 1000 parsecs.


The point in the orbit of a planet, star, or other astronomical object when it is closest to its primary star. (Antonym: apastron) Because orbits of celestial bodies are never perfectly circular, they always have closest and farthest points. A term specific to the Solar System is used to name the closest approach of any orbiting body to our Sun: perihelion.


A rocky or icy object whose diameter is a kilometer or more, orbiting in the protoplanetary disk around a young star. Planetesimals accrete from grains of metals, silicates, and ices, and they can in turn combine to form protoplanets of various masses. Planetesimals that are left over after planetary formation comprise debris disks such as the Asteroid Belt and Kuiper Belt of the Solar System.

Protoplanetary Disk

A dusty, gaseous disk surrounding a young star during the early phase of planet formation. The disk’s gaseous components dissipate within 3 to 10 million years (Haisch et al. 2001). If any sufficiently massive planets form within this period, a portion of the gas accretes onto their growing cores. The rest of the gas is either evaporated by proximity to the star or blown out by stellar radiation. Once a protoplanetary disk’s primordial gas has dispersed, any fine dust remaining will also be ejected by radiation. Planetary evolution may still continue in the absence of gas and dust by means of collisions and ejections among surviving planetesimals.

In astronomical literature, the Solar System’s protoplanetary disk is often called the Solar nebula. However, this term is not applicable to extrasolar systems. The artificial term proplyd, a contraction of “PROtoPLanetarY Disk,” has been coined to describe the gaseous shells surrounding newborn stars in regions of active star-formation such as the Orion Nebula. For some reason, however, “proplyd” is never used to describe the protoplanetary disks of extrasolar planetary systems.

Radial Velocity

The speed at which an astronomical object travels in a radial direction – that is, along the line of sight of the observer. The radial velocity of a star can be measured by a spectroscope (see spectrum), which converts starlight into a characteristic pattern of absorption lines. Over time, absorption lines may shift toward the blue end of the spectrum, indicating movement toward the observer, or toward the red end, indicating movement away.

Radial velocity can be tracked over time, with variations plotted in a radial velocity curve. Regular patterns of variation can then be analyzed as the possible signature of a planet in orbit around the star, producing very slight changes in the star’s radial velocity as the planet’s gravitational influence perturbs the central star’s motion.

Semimajor Axis

Roughly speaking, the average distance of an astronomical body (e.g., star, planet, moon) from the object around which it orbits (e.g., black hole, star, planet). The semimajor axis of Earth’s orbit around the Sun is about 93 million miles. This value has become the standard unit of measurement used by astronomers to describe planetary orbits in general – i.e., the astronomical unit or AU.

Orbits of astronomical objects are never perfect circles. Thus every orbit has one point where the object is closest to its primary (periapsis) and an opposite point where it is farthest away (apoapsis). A line drawn through the primary to connect the periapsis and the apoapsis defines the orbit’s major axis. One-half of the major axis is the semimajor axis.

Spectral Class

Nine spectral classes are currently used to characterize stars and substellar objects. Each spectral class denotes a typical range of mass and luminosity. Listed in order of decreasing mass, the spectral classes are O, B, A, F, G, K, M, L, T. For classes O through M, each spectral class represents a successively more numerous population of stars — in other words, stellar populations vary inversely according to mass. However, this mass/number relationship does not extend to the dimmest classes, L and T, which represent brown dwarfs and are evidently far less numerous than stars of spectral class M.

Each spectral class spans a characteristic mass range, with some overlap between adjacent classes:

O = 20+ MSOL . . . . . . . B = 3 to 20 MSOL . . . . . . A = 1.6 to 3.0 MSOL

F = 1.1 to 1.5 MSOL . . . G = 0.85 to 1.2 MSOL . . . .K = 0.6 to 0.95 MSOL

M = 0.08 to 0.5 MSOL . . L = 65 to 85 MJUP . . . . . . T = 10 to 65 MJUP


The light from a star when it is refracted into its constituent wavelengths by a spectrograph. The resulting spectrum exhibits patterns of vertical lines known as absorption lines, which correspond to the wavelengths that have been absorbed by individual molecules in the star’s composition. Each type of star or spectral class has a distinctive spectrum, as determined by these patterns. See these examples. (Plural: spectra)

Subgiant Star

A star that is approaching the end of its lifetime on the main sequence and has begun to expand in size and increase in temperature. In the system of spectral classification, the subgiant phase is indicated by appending the Roman numeral IV to the star’s spectral class. For example, the nearby exoplanetary host star Gamma Cephei is classified as K1 IV. Subgiants eventually progress to the giant phase of evolution.

Terrestrial Planet

In current astronomical usage this term has two meanings, one referring to mass and the other to composition. These meanings may or may not overlap.

In the broader sense, a terrestrial planet is an object less massive than an ice giant and more massive than a dwarf planet, following an independent orbit around a star. The defining mass range extends from about 0.01 MEA, similar to the mass of Earth’s Moon, to 10 MEA, a bit less than the mass of Uranus. The Solar System’s four inner planets — Mercury, Venus, Earth, and Mars — fit this definition.

In the narrower sense, a terrestrial planet is an object composed primarily of refractory elements, typically silicate rocks and metals. The formation of such objects is limited to the area inside a star’s ice line, where refractory elements dominate and volatiles cannot survive in free orbit. Our system’s four inner planets also fit this definition, but low-mass extrasolar planets may not.

Investigations of the principal moons of the Solar System’s gas giant planets (Ganymede, Titan, Callisto, Io, Europa) have established that most of them contain large proportions of water ice, up to about 50% of their total composition. Yet all of them are far less massive than Mercury. At the same time, the discovery of increasing numbers of so-called “Super Earths” in nearby extrasolar systems implies that icy worlds like Ganymede can grow quite heavy, up to the mass limit of 10 MEA. Such objects are variously known as ice planets, ocean planets, or steam planets, depending on their inferred temperatures. Their formation is limited to the area outside a star’s ice line. Nevertheless, planet-planet interactions or Type I migration may deliver such objects to small orbits, similar to those of the Solar System’s terrestrial planets.

White Dwarf

A small, extremely dense, extremely hot object that represents the last phase in the evolution of a significant proportion of stars. Stars with masses of 8 MSOL to 0.7 MSOL (corresponding to spectral classes B, A, F, G, and K) eventually burn through all their hydrogen and expand into red giants. The process of hydrogen exhaustion may take about a billion years for a bright A-type star (e.g., Vega) and 10 billion years or more for a dimmer G-type star (e.g., the Sun).

After a much shorter period as a red giant, the star undergoes a phase of explosive contractions in which it expels a significant portion of its mass. The ejected material takes the form of a planetary nebula. Well-known planetary nebulae include the Cat’s Eye Nebula and Abell 39. These tenuous structures persist only for a period of millennia before dispersing.

The surviving stellar core is now appropriately known as a white dwarf, since it radiates white light and yet is no larger than the planet Earth. The maximum possible mass for a white dwarf star is 1.4 MSOL; this cut-off point is known as the Chandrasekhar limit, beyond which a contracting star will necessarily collapse into a black hole. Among nearby white dwarfs, the companion of Sirius has a mass of about 1 MSOL; the companion of the exoplanetary host star Gliese 86 has a mass of about 0.54 MSOL.

The extreme density of white dwarf stars has sometimes been expressed with the following analogy: a portion of a white dwarf the size of a sugar cube would weigh (in Earth’s gravity) as much as a hippopotamus. White dwarf stars are common in the immediate Solar neighborhood and throughout the Milky Way.

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All text is copyright Raymond Harris 2006-2010. Credits for each image are listed in the accompanying caption.