Search Results

Planets beyond our solar system. For millennia, humankind has known about most of the major planets in our solar system because these worlds are visible to the naked eye. Exoplanets, however, were not indisputably known until 1995, when astronomers…

A space telescope mission dedicated to detecting exoplanets. The tenth mission launched under the National Aeronautics and Space Administration's (NASA's) Discovery Program, Kepler was the agency’s first mission dedicated to the search for planets…

The planet fourth outward from the Sun and the planet most like Earth in terms of geology, climate, and suitability for life. Mars is nicknamed the "Red Planet" for its reddish hue (Fig. 1). Besides Earth, Mars is the most-studied world in the solar…

A natural object, not human-made, in orbit around at least one more massive object, called a primary, when this primary is not a star. An object orbiting a star, a pair of stars, a galaxy, or a black hole will not be referred to as a natural…

Groups of stars held together by mutual gravitational attraction. There are two basic morphological types: open clusters and globular clusters. Typical densities of field stars near the Sun are 0.1 star per cubic parsec. (One parsec equals 1.9 ×…

A galaxy that is observed to be undergoing an unusually high rate of formation of stars. It is often defined as a galaxy that, if it continues to form stars at the currently observed rate, will exhaust its entire supply of star-forming material, the…

A magnetic field, far stronger than the Earth's magnetic field, which is possessed by many stars. Magnetic fields are important throughout the life cycle of a star. Initially, magnetic fields regulate how quickly interstellar clouds collapse into…

One of the categories into which stars may be classified, based on their place in the evolution of the galaxy that they occupy. The stellar component of the Milky Way Galaxy consists of three populations: the thin disk, the thick disk, and the halo.…

The spinning of stars, due to their angular momentum. Stars do not necessarily rotate as solid bodies, and their angular momentum may be distributed nonuniformly, depending on radius or latitude. However, it is nearly impossible to resolve the…

An evolving star of luminosity class IV. Such a star is brighter than the main-sequence dwarfs and fainter than the true giants in its spectral class, lying between the two on the Hertzsprung-Russell diagram. The classic subgiants fall in a small…

An instrument for telling time by the Sun. It is composed of a style that casts a shadow and a dial plate, which is the surface upon which hour lines are marked and upon which the shadow falls. The style lies parallel to Earth's axis. The…

A member of a class of evolved stars that occupy the top of the Hertzsprung-Russell diagram to the right of the main sequence. The absolute visual magnitudes (MV ) of supergiants range approximately between −4 and −10, and they are the largest and…

A system of convective cells, called supergranules, with diameters of about 30,000 km (20,000 mi) and lifetimes of 1 or 2 days, which cover the visible disk of the Sun, the photosphere. Smaller convection cells called granules, which are about 1500…

Proper motion of an astronomical object apparently exceeding the velocity of light, c. This phenomenon is relatively common in the nuclei of quasars, many of which exhibit systematic changes in their images over periods of months to years. In some…

A double star system in the late stage of stellar evolution. A symbiotic star is a binary system and not a single star. The symbiotic phase represents a brief span in the life of the binary. Symbiotic stars are rare objects, and their distances are…

The alignment of three celestial objects within a solar system. Syzygy is most often used to refer to the alignment of the Sun, Earth, and Moon at the time of new or full moon. Although syzygy is strongly associated with these two lunar phases in…

A young low-mass star characterized by variability, the presence of hydrogen emission lines, and often other signatures of circumstellar gas and dust, including bright x-ray, ultraviolet, infrared, and radio emissions. T Tauri stars are named after…

The Bull, a large zodiacal northern constellation prominent in the evening winter sky (see illustration). Taurus is usually said to be the form assumed by Zeus in Greek mythology in order to carry off the princess Europa. Zodiac Modern boundaries…

The largest moon of Saturn and the only satellite in the solar system with a substantial atmosphere, as well as the only object, besides Earth, known to host stable bodies of liquid on its surface. Titan, 5150 km (3200 mi) in diameter, is the second…

The passage of one astronomical object across another. The term "transit" implies that the foreground object is smaller than the background object (Fig. 1). A related term, "eclipse," implies that the foreground object hides a background object that…

Asteroids located near the equilateral Lagrangian stability points of a Sun-planet system (Fig. 1). As shown by J. L. Lagrange in 1772, these are two of the five stable points in the circular, restricted, three-body system, the other three points…

The Great Bear, a circumpolar constellation for observers at mid-northern latitudes or higher latitudes (see illustration). Part of this constellation is the prominent asterism (star pattern) known as the Big Dipper, whose bowl of four stars is…

The Little Bear, a circumpolar constellation from northern latitudes (see illustration). Part of this constellation is the prominent asterism (star pattern) known as the Little Dipper, whose stars are fainter than the corresponding stars of the Big…

A star with a detectable change in brightness that is often accompanied by other physical changes. Every star varies in brightness sometime during its life, usually in the early stages (while it is forming) or in the late stages (close to its…

The fifth brightest of all stars and the third brightest in the northern sky. It will be the north polar star in about 12,000 years. In moving through the Milky Way Galaxy, the Sun is generally heading toward the position now occupied by Vega. At a…

The Virgin, perhaps Astraea, in Greek mythology the goddess of justice, whose scales (the constellation Libra) are adjacent. Also, she could be Ceres, goddess of agriculture, or her daughter Proserpina who, because she ate from a forbidden…

The nearest large cluster of galaxies and one of the best studied. It dominates the distribution of bright galaxies in the sky, forming a conspicuous clump in the distribution of easily visible elliptical and spiral galaxies. The clump lies mostly…

A theorem in classical mechanics which relates the kinetic energy of a system to the virial of Clausius, as defined below. The theorem can be generalized to quantum mechanics and has widespread application. It connects the average kinetic and…

The apparent change in direction of a source of light caused by an observer's component of motion perpendicular to the impinging rays.To visualize the effect, first imagine a stationary telescope (illus.a) aimed at a luminous source such as a star,…

The closest spiral galaxy to the Milky Way Galaxy. The name of the Andromeda Galaxy (Fig. 1) derives from its appearance in the constellation of Andromeda. The galaxy is also known as M31 and NGC 224, based on its entries in two widely used…

A gravitationally bound, condensed body whose mass is too small to sustain nuclear fusion reactions in its core. Brown dwarfs are often likened to "failed stars." The approximate mass range for brown dwarfs is between 13 and 80 times the mass of…

An algorithm that gives a name to each day. These day names are generally distinct, but not always. For example, the day of the week is a calendar, in a trivial sense, with infinitely many days having the same day name. The design of calendars has…

A force whose line of action is always directed toward a fixed point. The central force may attract or repel. The point toward or from which the force acts is called the center of force. If the central force attracts a material particle, the path of…

Light emitted by a high-speed charged particle when the particle passes through a transparent, nonconducting, solid material at a speed greater than the speed of light in the material. The blue glow observed in the water of a nuclear reactor, close…

A quantitative measure of a star's color. Even a casual look at the night sky reveals the stars to be colored. From spectral analysis and the laws of radiation, it has been known since early in the twentieth century that such color is closely…

The Southern Cross, a small southern constellation. Because of the absence of a star at the center of the cross, this constellation really looks like a kite (see illustration). To its side, the Coalsack is a giant dark silhouette of dust against the…

A unit of time equal to the period of rotation of Earth. Different sorts of day are distinguished, according to how the period of rotation is reckoned with respect to one or another direction in space. A day is normally defined as 86,400 SI (Système…

A star whose state of evolution resembles that of the Sun. The term “dwarf star” derives from the early work of Ejnar Hertzsprung and Henry Norris Russell, who distinguished two kinds of stars: large ones, called giants (or supergiants), and smaller…

A source of data pertaining to features of various objects, usually astronomical, that depend on time. The ephemeris can be a printed or computer-readable table such as on a compact disk, or it can be a set of numerical data accessible by computer…

The annual, cyclic variation between mean solar time shown on uniformly running clocks and apparent solar time displayed on sundials.In the course of the Sun's daily east-to-west transit of the sky, the Sun crosses the meridian, an imaginary line…

Dark absorption features in the solar spectrum. They are named in honor of J. Fraunhofer, who first studied them in 1814 (see illustration). They are found from the ultraviolet at approximately 180 nanometers (nm) to the infrared at 20 micrometers…

A nearby neutron star that emits pulsed x-rays and gamma rays, steady optical radiation, and possibly radio and optical pulsations. Since the 1970s, it has been studied at a level of detail very unusual for a neutron star. As a result, not only are…

A small, opaque nebula seen in silhouette against a rich star field or a bright nebula. Globules were first cataloged in the 1920s. In 1947, B. J. Bok called attention to their potential significance for star formation, and since then they have been…

A technique akin to terrestrial seismology (geoseismology) that studies the Sun's otherwise invisible gaseous interior by the use of waves that propagate throughout the interior and are observed at the Sun's surface. The characteristics of these…

Measurement of the time dimension. In practice, horology is the search for a steady or repetitive action, and the design of an instrument to perform that action and to indicate (read out) a measure of the action. Until early in the twentieth…

A photoelectric device for intensifying faint astronomical images. In these devices a photoemissive surface, called the photocathode, emits electrons through the photoelectric effect. In most tubes the photocathode is semitransparent and is…

Low-density dust or gas that fills the space in a planetary system around or between the planets. Most interplanetary matter in the inner solar system is dust created by collisions among asteroids or released by comets as they are heated by the Sun.…

The three laws of planetary motion discovered by Johannes Kepler during the early years of the seventeenth century. Kepler, a German astronomer (Fig. 1), published the first two laws in 1609 in a book entitled Astronomia nova, and the third law in…

The Lion, a northern zodiacal constellation (see illustration). Its head and front legs are marked by stars tracing out a backward question mark (without a bottom dot), also known as the Sickle. The bright star Regulus is at the bottom of the…

The Scales, a southern zodiacal constellation (see illustration). It has no bright stars. Traditional pictures show a set of scales (a balance) with pans held by chains. Perhaps its name comes from the nearby constellation Virgo, who was the goddess…