Aldebaran: Red Giant at the Eye of the Bull

To observers on Earth, the great bull of Taurus has a fiery red eye. This is Aldebaran, an old red giant star which dwarfs our Sun. Let’s have a closer look at the facts and fiction about this aging star.

Finding the Bullseye! (Image credit: Armagh Planetarium)

If you go out after dark tonight you cannot fail to miss the great constellation of Orion (assuming you are in the northern hemisphere). Find the three stars of his belt and follow the line they make up the sky and you will meet Aldebaran, the brightest star in Orion’s neighbour, Taurus the Bull.

Often represented as the bullòÀÙs eye, Aldebaran (or Alpha Tauri) lies 65 light years from Earth, right in front of the Hyades cluster (which is about 70 light years further from us). A K5 III red giant about 350 times as luminous as our Sun, it appears as a red-orange magnitude 0.9 object in our sky. Aldebaran has advanced further through its lifecycle than the Sun, having exhausted its supply of hydrogen it is now generating energy in its core by fusing atoms of helium into carbon. This process is not be running steadily so Aldebaran is a variable star, fluctuating randomly through about 0.2 of a magnitude. This effect is not visible to the naked eye.

Two stars compared (Image credit: via wikimedia.org)

The star’s surface temperature is about 4000K, whereas the SunòÀÙs surface is hotter at about 5800K. It is about seventy per cent more massive and forty times as wide as the Sun. To put this size into perspective, were Aldebaran to replace the Sun as centre of the Solar System, AldebaranòÀÙs surface would extend half way to Mercury. Its size and relative closeness to us has made it possible for astronomers to measure both AldebaranòÀÙs size directly (rather than inferring it from its luminosity and spectral class) and its rotation rate. The star rotates at the amazingly slow rate of once every two years.

In 1997 astronomers suggested that Aldebaran is orbited by another body about twelve times as massive as Jupiter with a period of two years. This object would lie in the fuzzy boundary between giant planets and brown dwarf stars. It is presumably no coincidence that the orbital period of this companion, the existence of which is yet to be confirmed, matches the rotation rate of the star.

The name Aldebaran is derived from the Arabic for òÀØthe FolloweròÀÙ as the ancient Muslim astronomers saw it as trailing behind the Pleiades star cluster across the sky. Aldebaran was also the name of a truly awesome spacecraft concept proposed in the 1960s by engineer Dandridge Cole which I’ve covered elsewhere on this blog.

Aldebaran has made a few notable appearances in SF. A powerful space- bound race basks in the staròÀÙs radiance in Peter F. HamiltonòÀÙs Fallen Dragon. ˆàIn Joe HaldemanòÀÙs classic The Forever War (almost forty years old, but even more relevant today), the alien Taurans are first encountered near Aldebaran, but since òÀÜAldebaranian” is a little hard to handle, they named the enemy “Tauran.” In Narabedla Ltd, a novel by that criminally overlooked author Frederik Pohl, the contemporary protagonist investigates disappearances linked to the mysterious eponymous corporation, unfortunately he doesnòÀÙt spell its name backwards until it is too late! Guess where he ends up!

(Article by Colin Johnston, Science Communicator)