Arcturus, << ahrk TUR uhs, >> also called Alpha Bootis, ranks as the fifth brightest star in the sky. As seen from Earth, only the sun and three other stars shine more brightly. Arcturus is the brightest star in the constellation Bootes (pronounced boh OH teez), the Herdsman. Because of the apparent motion of the stars, Arcturus seems to follow the constellation Ursa Major, the Great Bear. The name Arcturus comes from Greek words meaning bear watcher. Arcturus appears in the northern sky (see Star (Brightness of stars) ).
Stargazers remember how to locate the star using the phrase Follow the arc to Arcturus. The stars in the handle of the Big Dipper form the beginning of the arc. Tracing the arc away from the dipper’s bowl, Arcturus appears as an orangish star about 1/4 of the way across the sky.
Arcturus is a red giant. A red giant forms when a star like our sun uses up all the hydrogen fuel in its core. The star then begins to fuse (combine) hydrogen nuclei in a shell surrounding the core. The energy released by this fusion causes the star’s outer layers to expand and grow cooler.
Arcturus’s diameter measures about 30 times that of the sun. Its reddish-orange glow indicates a surface temperature of about 4290 K, making it only about two-thirds as hot as the sun. One kelvin (K) equals one Celsius degree above absolute zero (–273.15 °C). Because Arcturus is so large, it gives off about 100 times as much light as the sun does.
Arcturus moves relatively quickly in relation to nearby stars. Astronomers can detect a change in Arcturus’s position over the course of just a few years. Its chemical composition also differs from that of most of the stars in our part of the galaxy. Because of its peculiar motion and chemical composition, many astronomers think that Arcturus may not be related to the nearby stars. Instead, Arcturus may remain from an older generation of stars, or it may come from another galaxy that merged with our own Milky Way.
Arcturus lies about 37 light-years from Earth (see Light-year ). One light-year is the distance light travels in a vacuum in a year, about 9.46 trillion kilometers.
