ALDEBARAN (Alpha Tauri). Aldebaran is by far the brightest, and
therefore the Alpha, star of the
constellation Taurus. The ancient
name, from Arabic, means "the Follower," as the star seems to
follow the Pleiades, or Seven Sisters
star cluster, across the
sky. Aldebaran, 67 light years away, is positioned in front of the
sprawling Hyades star cluster (in
mythology, half-sisters to the Pleiades) that make the head of Taurus the Bull, but is not a part of
it, the cluster (at 150 light years) over twice as far away.
Nevertheless, it makes a fine guide to it. In most renderings of
the constellation, Aldebaran makes the
celestial Bull's eye. As part of a constellation of the Zodiac, Aldebaran is close to the
Sun's path, the Sun passing to the north of it about June 1, the
star also regularly covered, or occulted, by the Moon. This class K (K5) giant star, of first magnitude
(0.85) and 14th brightest in the sky, is a low-level irregular
variable star that fluctuates erratically and to the eye
unnoticeably by about two-tenths of a magnitude. Aldebaran's
surface temperature of 4010 degrees Kelvin (compared to the Sun's 5780 degree temperature) gives it a
distinct orangy color not all that dissimilar to that of Mars,
which commonly passes it. Allowance for infrared radiation reveals
the star to have a fairly high luminosity 425 times that of the
Sun, which leads to a radius of 43 times solar. A projected
equatorial rotation velocity of 5.2 kilometers per second gives the
star a rotation period that could be as long as 400 days. The
duration of disappearance in lunar occultations, as well as direct
measure of angular diameter through interferometry, give an angular
diameter of 0.01996 second of arc (the apparent size of a US nickel
seen at a distance of about 50 kilometers), the star a leader in
the number of such measures. That combined with distance gives a
physical diameter of 44 times solar, in excellent agreement with
the one found from luminosity and temperature. From the theory of
stellar structure and evolution, Aldebaran carries a mass of around
1.7 times that of the Sun. Most class K giants are quietly fusing
their internal helium cores into carbon and oxygen. Aldebaran, on
the other hand, seems to be in a preliminary state in which a
still-dead helium core is contracting and heating, causing the star
as a whole to continue to expand and brighten. Consistent with its
large size and luminosity, it's also known for a strong wind,
through which it's beginning to lose mass, which surrounds it out
to about 100 Astronomical Units. Within only another few million
years, the star will top out at around 800 solar luminosities as it
fires its helium, then shrinks and dims some to become one of the
usual K-giant crowd. If Aldebaran were in our Solar System, it
would extend halfway to the planet Mercury and would appear 20
degrees across in our sky, its great luminosity making life on
Earth quite impossible. For a time, Aldebaran was thought to
harbor its own planet (discovered through
shifts in the star's velocity), but it has never been confirmed.
Such would be counter to the usual finding that stars with planets tend
toward high metal abundances, as Aldebaran's iron content (relative to
hydrogen) is about half that of the Sun's.
Written by Jim Kaler 1/30/98; revised
10/30/99, 11/14/08, 05/22/09. Return to STARS.