ALKAID (Eta Ursae Majoris). Though the name may not be so well known as those of the first magnitude stars, the star itself certainly is, as Alkaid is the end star in the handle of the Big Dipper, the great asterism that makes most of the grand constellation of Ursa Major, the Greater Bear. Just fainter than Dubhe (the front bowl star of the Dipper), second magnitude (1.86) Alkaid is the third brightest star in the constellation and places number 38 in the list of the brightest stars. Though Johannes Bayer in his Uranometria generally listed stars by Greek letter name in order of brightness within a constellation, he almost as commonly used position: the stars of the Dipper are named from west to east, rendering Alkaid Eta Ursae Majoris rather than Gamma. Different cultures see the sky differently as well. Alkaid's Arabic name means "the leader," and appears to refer to the "daughters" (the handle of the Dipper) that stand by a funeral bier made of the Dipper's bowl. Alkaid is also known as Benetnasch, which also refers to the daughters. The star lies 104 light years away (give or take just 1). As a hot, blue-white, class B (B3) hydrogen-fusing dwarf, Alkaid is a bit odd in its position well off the Milky Way, being sort of "above us" in the Galactic disk. Measures of surface temperature range from 15,700 to 17,900, probably the result of "gravity darkening," in which a rapidly rotating star has a higher temperature at its pole than at its equator. And with a projected equatorial rotation speed of at least 166 kilometers per second, Alkaid probably qualifies. The mean temperature of 16,800 Kelvin is decidedly low for the spectral class. Consistently, the star is also slightly too "red" (that is, not as blue in color as one would expect) as well, both for reasons unknown. From distance and temperature (which allows an estimate of the large amount of ultraviolet radiation), we find a luminosity of 580 times that of the Sun. The resulting radius of 2.9 times solar is smaller than the 3.2 solar radii measured through interferometry. From the rotation speed, the rotation period is under 21 hours. Theory gives Alkaid a mass of five Suns. The star appears to be very young, no more than about 15 million years old, well under the total hydrogen-fusing lifetime of 95 million years, after which it will become a luminous giant. Were Alkaid our Sun, we would have to be 25 times farther away to survive, almost to the orbit of Neptune. Alkaid is one of the two renegades of the Dipper. The five middle stars are all moving through space together as part of a loosely bound cluster. Alkaid and Dubhe, however, are moving in their own directions, ultimately dooming the Dipper's shape. Alkaid is just below the temperature limit at which stars produce strong X-rays as a result of shock waves in their winds, and is therefore only a weak source of X-rays, if there are any at all. Well under the limit beyond which stars explode as supernovae, Alkaid will slough off its outer layers and become a white dwarf with a mass of about 0.85 times that of the Sun.

Written byJim Kaler 5/8/98; revised 7/18/14. Return to STARS.