ELNATH (Beta Tauri). Equally well called El Nath, Elnath lies at
the tip of the great northern horn of Taurus, the Bull. "El Nath" (actually al nath) from
Arabic means the "butting one," the name transferred over from
stars in Aries and obviously appropriate
for Taurus as well (the ancient names in Aries no longer used).
Bright second magnitude (1.65), Elnath is one of the sky's brighter
stars, ranking 25th overall and number three within second magnitude,
right behind Castor in Gemini and Bellatrix in Orion. Number two in Taurus (behind Aldebaran), it received the Beta
designation from Bayer. Elnath is peculiar, however, as it is one
of the sky's rare linking stars. Formally now in Taurus, the star
is also a part of the classic outline of Auriga and is simultaneously Gamma Aurigae (though it
ranks number two in Auriga as well, after Capella). (The other linking star, Alpheratz or Alpha Andromedae, is also
Delta Pegasi.) "Gamma Aurigae" is never used, however. Elnath
also has broader positional significance, as it is in the Milky Way
only three degrees to the west of the anticenter of the Galaxy, the
point directly opposite the Galactic Center in Sagittarius. In
this broad direction lie vast dark cold clouds of interstellar gas
and dust in which stars are being born. Physically, Elnath is a
hot (13,600 Kelvin) class B giant star. From its distance of 130
light years, we find a luminosity (corrected for the ultraviolet
radiation from the hot surface) almost 700 times that of the Sun. The radius derived from temperature and
luminosity, 4.6 times that of the Sun, agrees well that found from
the original measurement of angular diameter, though not with a
more recent measure, which makes the star 35 percent larger,
showing the difficulty of such observations. Like so many stars of
its class, Elnath is "chemically peculiar" and appears to be a
"mercury-manganese" star, the manganese abundance 25 times normal,
the calcium and magnesium abundance reduced and only an eighth
solar. These peculiarities are caused by the combined action of
gravity and radiation, which make the atoms of some elements drift
downward and out of sight and others rise. The best bright
example, is, by rather amazing coincidence, the other "linking
star," Alpheratz! Elnath has a mass of around 4.5 times that of
the Sun. It is beginning to die, and if it has not already
completely shut down the fusion of hydrogen in its core, it will
before too long. Within only a couple million years, it will
expand to become a cooler orange giant star.