The third brightest star in the sky, apparent magnitude −0.3, and the brightest in the southern constellation Centaurus. It is the closest star to the Sun at a distance of 1.35 parsecs (2.59 × 10¹³ mi or 4.16 × 10¹³ km), and its light takes more than 4 years to reach the Earth. It has an unusually large proper motion across the sky of 3.7 seconds of arc per year. See also: Centaurus

A prominently located constellation in the northern sky (see illustration), named for the daughter of Cassiopeia in Greek mythology: When Cassiopeia bragged that her daughter Andromeda was more beautiful than Poseidon's daughters, the Nereids, Poseidon created Cetus, the sea monster. (In some versions of the myth, Cassiopeia boasted of her own beauty.) The situation required Andromeda's sacrifice. However, Perseus saved Andromeda by showing Cetus the head of Medusa, turning Cetus to stone. See also: Cassiopeia; Perseus

α Scorpii, a cool supergiant star of spectral type M1Ib, whose red color stands out in the midsummer sky. With an effective temperature of approximately 6000°F (3600 K), Antares resembles Betelgeuse, the brightest of the red supergiants, and would fill the solar system beyond the orbit of Mars if it replaced the Sun. Antares has a distance of 185 parsecs (600 light-years) from the Sun, and its angular diameter of about 0.045 arc-second has been measured by interferometric and lunar occultation methods. Red supergiants of this type originate as stars with mass at least 20 times that of the Sun. Such stars quickly evolve through successive stages of thermonuclear fusion of heavier and heavier elements in their cores with lighter elements undergoing fusion in concentric shells surrounding the hot core. Eventually, the core builds up with iron, the most tightly bound of all atomic nuclei. The fusion of iron nuclei absorbs rather than releases energy, and the supergiant star implodes in a type II supernova explosion. This fate is likely for Antares in less than a million years. Prior to this dramatic event, Antares will have shed up to 50% of its mass through a stellar wind of material blown away from the star into the surrounding interstellar medium. See also: Betelgeuse; Scorpius; Spectral type; Stellar evolution; Supergiant star; Supernova

The Water Bearer, in Greek mythology, is a zodiacal constellation, meaning that the path of the Sun and planets passes through it. (The Sun actually passes through 13 of the modern constellations each year, not the traditional 12.) It is almost entirely south of the celestial equator; none of its stars is especially bright (see illustration). Early constellation drawings show Aquarius as a man or boy pouring water from a bucket. The stream of water is oriented so as to flow into Piscis Austrinus, the constellation of the Southern Fish, pointing at the bright star Fomalhaut, which can be taken to correspond to the mouth of the fish. The constellation may have gotten its name from the Sun's position in it during the often-rainy month of February, which often led to floods in Babylonia and Egypt. The modern boundaries of the 88 constellations, including this one, were defined by the International Astronomical Union in 1928. See also: Constellation; Zodiac

The brightest star in the northern sky, apparent magnitude −0.05, also known as α Boötis. It is a yellow giant star of spectral type K1.5, one of the nearest giants to the Earth at a distance of 11.25 parsecs (2.16 × 10¹⁴ mi or 3.47 × 10¹⁴ km). Unlike the Sun, which is currently converting hydrogen into helium in its core, Arcturus has already exhausted its central hydrogen and has evolved away from the main sequence. It is approximately 25 times larger in diameter than the Sun, and more than 100 times more luminous. Its effective temperature is estimated to be 7700°F (4300 K). See also: Boötes; Spectral type; Stellar evolution

The Ram, a northern zodiacal constellation, meaning that the path of the Sun and planets passes through it (see illustration). In Greek mythology, the golden fleece of this ram was pursued by Jason and the Argonauts, whose ship is memorialized in four current-day constellations in the southern sky. In Johannes Bayer's 1603 star atlas, the first three lettered stars are located in the ram's horns. See also: Zodiac

A cool and highly luminous star, prominently located in the right shoulder of the constellation Orion and noticeably red in color. Betelgeuse, or α Orionis, is a supergiant star about 130 parsecs (430 light-years) from the Sun. Its spectral type of M2 indicates an effective temperature of approximately 3500 K (5800°F). This temperature would result in a low overall luminosity were it not for the enormous diameter of the star, about 1100 times that of the Sun. Replacing the Sun, Betelgeuse would fill the solar system to beyond the orbit of Mars. Betelgeuse is a supergiant star with approximately 150,000 times the Sun's luminosity. Betelgeuse varies in brightness with a period of 6.4 years. Its mean density is extremely low, less than one-millionth the density of water, and the low surface gravity of the rarefied outer layers results in a continual loss of matter, presently at the rate of 1 solar mass every 250,000 years. Observations at infrared and radio wavelengths show a complex system of dust and gas shells extending out as far as a thousand stellar radii. If this material were visible to the eye, Betelgeuse would appear as large as Mars through a telescope. These shells appear to originate from occasional outbursts in which material is ejected and eventually merges with the interstellar medium. See also: Spectral type; Stellar evolution; Supergiant star; Variable star

Two stars held together by their mutual gravitational attraction in a permanent (or long-term) association. The stellar universe is hierarchical. Stars exist singly and in binary pairs, multiple systems, and clusters. On large scales, roughly 10⁵ light-years, astronomical objects congregate into galaxies. In fact, most stars are in binary systems. The Sun, with its collection of planets, is an exception. Stars in binaries revolve around their common center of mass (which itself moves through space). This star-star gravitational interaction makes possible the measurement of stellar masses and other basic properties. Stellar evolution in some binary systems can lead to spectacularly energetic activity. See also: Galaxy, external; Milky Way Galaxy; Solar system; Star clusters

A star that is a member of a stellar association and is located at an unusual position on the association's color-magnitude diagram, above the turnoff from the main sequence.

The Herdsman, a constellation perhaps linked in mythology with the invention of the plow, though no specific Greek myth is associated with the name. The herdsman can be thought of holding Canes Venatici, the Hunting Dogs, and using them to drive Ursa Major, the Great Bear. The kite-shaped outline often given to the brightest stars of this northern constellation (see illustration) is linked to the kite's string by Arcturus, a red giant and the fourth brightest star in the sky (the second brightest in the northern sky). The arc of the Big Dipper's handle leads to Arcturus: “arc to Arcturus.” See also: Arcturus; Ursa Major