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.