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.
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