Astronomical Terms

Updated November 29, 2022 | Infoplease Staff

The Milky Way, the galaxy containing our solar system, is about 100,000 light-years in diameter and about 10,000 light-years thick.

Aphelion: see Orbit.

Apogee: see Orbit.

Black hole: the theoretical end-product of the total gravitational collapse of a massive star or group of stars. Crushed even smaller than the incredibly dense neutron star, the black hole may become so dense that not even light can escape its gravitational field. In 1996, astronomers found strong evidence for a massive black hole at the center of the Milky Way. Recent evidence suggests that black holes are so common that they probably exist at the core of nearly all galaxies.

Conjunction: the alignment of two celestial objects at the same celestial longitude. Conjunction of the Moon and planets is often determined with reference to the Sun. For example, Saturn is said to be in conjunction with the Sun when Saturn and Earth are aligned on opposite sides of the Sun.

Mercury and Venus, the two planets with orbits within Earth's orbit, have two positions of conjunction. Mercury, for example, is said to be in inferior conjunction when the Sun and Earth are aligned on opposite sides of Mercury. Mercury is in superior conjunction when Mercury and Earth are aligned on opposite sides of the Sun.

Dwarf planet: see Planet.

Elongation: the angular distance between two points in the sky as measured from a third point. The elongation of Mercury, for example, is the angular distance between Mercury and the Sun as measured from Earth. Planets whose orbits are outside Earth's can have elongations between 0° and 180°. (When a planet's elongation is 0°, it is at conjunction; when it is 180°, it is at opposition.) Because Mercury and Venus are within Earth's orbit, their greatest elongations measured from Earth are 28° and 47°, respectively.

Galaxy: gas and millions of stars held together by gravity. All that you can see in the sky (with a very few exceptions) belongs to our galaxy—a system of roughly 200 billion stars. The exceptions you can see are other galaxies. Our own galaxy, the rim of which we see as the “Milky Way,” is about 100,000 light-years in diameter and about 10,000 light-years in thickness. Its shape is roughly that of a thick lens; more precisely, it is a spiral nebula, a term first used for other galaxies when they were discovered and before it was realized that these were separate and distinct galaxies. Astronomers have estimated that the universe could contain 40 to 50 billion galaxies. In 2004, the Hubble Space Telescope and observers at the Keck Observatory in Hawaii discovered a new galaxy 13 billion light-years from Earth.

Neutron star: an extremely dense star with a powerful gravitational pull. Some neutron stars pulse radio waves into space as they spin; these are known as pulsars.

Occultation: the eclipse of one celestial object by another. For example, a star is occulted when the Moon passes between it and Earth.

Opposition: the alignment of two celestial objects when their longitude differs by 180°. Opposition of the Moon and planets is often determined with reference to the Sun. For example, Saturn is said to be at opposition when Saturn and the Sun are aligned on opposite sides of Earth. Only the planets whose orbits lie outside Earth's can be in opposition to the Sun.

Orbit: the path traveled by an object in space. The term comes from the Latin orbis, which means “circle” or “disk,” and orbita, “orbit.” Theoretically, there are four mathematical figures, or models, of possible orbits: two are open (hyperbola and parabola) and two are closed (ellipse and circle), but in reality all closed orbits are ellipses. Ellipses can be nearly circular, as are the orbits of most planets, or very elongated, as are the orbits of most comets, but the orbit revolves around a fixed, or focal, point. In our solar system, the Sun's gravitational pull keeps the planets in their elliptical orbits; the planets hold their moons in place similarly. For planets, the point of the orbit closest to the Sun is the perihelion, and the point farthest from the Sun is the aphelion. For orbits around Earth, the point of closest proximity is the perigee; the farthest point is the apogee. See also Retrograde.

Perigee: see Orbit.

Perihelion: see Orbit.

Planet: the International Astronomical Union (IAU) issued the definition for planet (from the Greek planetes, “wanderers”) at their General Assembly in August 2006. A planet is a body that (a) is in orbit around the Sun, (b) is massive enough that its self-gravity gives it a nearly-spherical shape, and (c) has cleared the neighborhood around its orbit. A body that fulfills the first two criteria but not the third is a dwarf planet, provided that it (d) is not a satellite.

While the exact definition of “clearing the neighborhood” was not established at press time, the eight planets from Mercury through Neptune have either assimilated or repulsed most other objects in their orbits, and each has more mass than the combined total of everything else in its area. The same cannot be said for Pluto, which has now been reclassified as a dwarf planet. There are currently eight planets and three dwarf planets recognized in the solar system, and more dwarf planets are expected to be admitted.

In 1994, Dr. Alexander Wolszcan, an astronomer at Pennsylvania State University, presented convincing evidence of the first known planets to exist outside our solar system. These particular extrasolar planets circle a pulsar, or exploded star, in the constellation Virgo.

In 1995, several of these extrasolar planets were discovered orbiting stars similar to our Sun. Swiss astronomers found the first extrasolar planet (HD 209458b, nicknamed “Osiris”) to circle a normal Sun-like star. As of May 2006, 170 such planets have been discovered.

In Feb. 2004, using the Hubble Space Telescope, a team of scientists at the Institut d'Astrophysique de Paris announced that they had discovered oxygen and carbon in the atmosphere of “Osiris.”

In Aug. 2004, NASA and the National Science Foundation announced the discovery of two new planets, the smallest yet found, about the size of Neptune. The discovery opens up the possibility of smaller, Earth-sized extrasolar planets.

In April 2005, a team of American and European astronomers reported that the first image of an extrasolar planet had been made. The planet is orbiting a brown dwarf near the constellation Hydra, 230 million light-years from Earth.

Pulsar: a celestial object, believed to be a rapidly spinning neutron star, that emits intense bursts of radio waves at regular intervals.

Quasar: “quasi-stellar” object. Originally thought to be peculiar stars in our own galaxy, quasars are now believed to be the most remote objects in the universe.

Quasars emit tremendous amounts of light and microwave radiation. Although they are not much bigger than Earth's solar system, quasars pour out 100 to 1,000 times as much light as an entire galaxy containing a hundred billion stars. It is believed that quasars are powered by massive black holes that suck up billions of stars.

Retrograde: describes the clockwise orbit or rotation of a planet or other celestial object, which is in the direction opposite to Earth and most celestial bodies. As viewed from a position in space north of the solar system (from some great distance above Earth's North Pole), all the planets revolve counterclockwise around the Sun, and all but Venus, Uranus, and Pluto rotate counterclockwise on their own axes. These three planets have retrograde motion.

Sometimes retrograde is also used to describe apparent backward motion as viewed from Earth. This motion happens when two objects rotate at different speeds around another fixed object. For example, the planet Mars appears to be retrograde when Earth overtakes and passes by it as they both move around the Sun.

Satellite (or moon): an object in orbit around a planet. Until the discovery of Jupiter's four main moons by Galileo Galilei, celestial objects in orbit around a planet were called moons. However, upon Galilei's discovery, Johannes Kepler (in a letter to Galileo) suggested satellite (from the Latin satelles, which means “attendant”) as a general term for such objects. The word satellite is used interchangeably with moon, and astronomers speak and write about the moons of Neptune, Saturn, etc. The term satellite is also used to describe man-made devices of any size that are launched into orbit.

Small Solar System Objects: at the 2006 IAU General Assembly, solar system bodies not defined as planets, dwarf planets, or satellites were placed in this category. These include most asteroids, most Trans-Neptunian Objects, comets, and other small bodies.

Star: a celestial object consisting of intensely hot gases held together by gravity. Stars derive their energy from nuclear reactions going on in their interiors, generating heat and light. Stars are very large. Our Sun has a diameter of 865,400 mi—a comparatively small star.

A dwarf star is a small star that is of relatively low mass and average or below-average luminosity. The Sun is a yellow dwarf, which is in its main sequence, or prime of life. This means that nuclear reactions of hydrogen maintain its size and temperature. By contrast, a white dwarf is a star at the end of its life, with low luminosity, small size, and very high density.

A red giant is a star nearing the end of its life. When a star begins to lose hydrogen and burn helium instead, it gradually collapses, and its outer region begins to expand and cool. The light we see from these stars is red because of their cooler temperature. There are also red super giants, which are even more massive.

A brown dwarf lacks the mass to generate nuclear fusion like a true star, but it is also too massive and hot to be a planet. A brown dwarf usually cools into a dark, practically invisible object. The existence of brown dwarfs, also called failed stars, was confirmed in Nov. 1995 when astronomers at Palomar Observatory in California took the first photograph of this mysterious object.

Supernova: a celestial phenomenon in which a star explodes, releasing a great burst of light. There are two basic types of supernova. Type Ia happens when a white dwarf star draws large amounts of matter from a nearby star until it can no longer support itself and collapses. The second more well-known kind of supernova, type IIa, is the result of the collapse of a massive star. (Massive is a classification for a star that is at least eight times the size of our Sun.) Once the star's nuclear fuel is exhausted, if its core is heavy enough, the star will collapse in on itself, releasing a huge amount of energy (the supernova), which may be brighter than the star's host galaxy.

On Feb. 24, 1987, Canadian astronomer Ian Shelter at the Las Campanas Observatory in Chile discovered a supernova—an exploding star—from a photograph taken on Feb. 23 of the Large Magellanic Cloud, a galaxy some 160,000 light-years away from Earth. Astronomers believe that the dying star was Sanduleak –69°202, a 10-million-year-old blue supergiant.

Supernova 1987A was the closest and best-studied supernova in almost 400 years. One was previously observed by Johannes Kepler in 1604, four years before the telescope was invented.