Explore Aldebaran, the Fiery Orange-Red Eye of a Starry Bull

the star aldebaran and the Hyades star cluster.
The Hyades star cluster with the bright orange-red star Aldebaran (upper left) in the picture. The Hyades is a cluster that lies farther away from Aldebaran, which is in the same line of sight. NASA/ESA/STScI

Behind every star in the sky is a fascinating origin tale. Just as the Sun does, they shine by burning fuel in their cores and giving off light. And, like the Sun, many have their planets. All were born in a cloud of gas and dust millions or billions of years ago. And, eventually, all stars grow old and evolve. That's what's happening to Aldebaran, a star that is practically a neighbor to our own star, the Sun, at 65 light-years distance.

You've probably seen Aldebaran in the constellation Taurus (which is visible to us at night from about October through March each year). It's the reddish-orange star at the top of the V-shaped face of the Bull. Observers in ancient times saw it as many things. The name "Aldebaran" is from the Arabic word for "follower", and it does seem to follow along as the Pleiades star cluster rises higher in the sky late in the year. For the Greeks and Romans it was the eye or the heart of the bull. In India, it represented an astronomical "house", and portrayed it a deity's daughter. Others around the world have associated it with the season to come, or even as an aid to the Pleiades (who, in some cultures, were seven women in the sky). 

Observing Aldebaran

The star itself is fairly easy to spot, particularly beginning in the evening skies of October each year. It also presents a remarkable experience for skygazers patient enough to wait for it: an occultation. Aldebaran lies close to the ecliptic, which is the imaginary line along which the planets and Moon appear to move as seen from Earth. Occasionally, the Moon will slide between Earth and Aldebaran, essentially "occulting" it. The event is visible from northern hemisphere locations in the early autumn. Observers with a keen interest in watching it happen through a telescope can see a detailed view of the lunar surface as the star slips slowly behind the Moon and then reappear a short time later.

Why Is it in a Vee of Stars?

Aldebaran looks like it's part of a cluster of stars called the Hyades. This is a V-shaped moving association of stars that lies much farther away from us than Aldebaran does, at a distance of about 153 light-years. Aldebaran happens to lie in the line of sight between Earth and the cluster, so it appears to be part of the cluster. The Hyades themselves are fairly young stars, about 600 million years old. They're moving together through the galaxy and in a billion years or so, the stars will have evolved and grown older and scattered apart from each other. Aldebaran will have moved from its position, too, so future observers will no longer see an angry red eye at the top of a vee-shaped swarm of stars.

What is Aldebaran's Status?

Technically speaking Aldebaran is a star that has stopped fusing hydrogen in its core (all stars do this at some point in their lives) and is now fusing it in a shell of plasma surrounding the core. The core itself is made of helium and collapsed in on itself, sending the temperature and pressure soaring. That heats up the outer layers, causing them to swell. Aldebaran has "puffed out" so much that it's now nearly 45 times the size of the Sun, and is now a red giant. It varies slightly in its brightness, and is slowly blowing its mass out to space.

Aldebaran's Future

In the very distant future, Aldebaran may experience something called a "helium flash" in its future. This will happen if the core (which is made of helium atoms) gets so densely packed that helium starts trying to fuse to make carbon. The temperature of the core has to be at least 100,000,000 degrees before this will happen, and when it gets that hot, almost all the helium will fuse at once, in a flash. After that, Aldebaran will start to cool and shrink, losing its red giant status. The outer layers of the atmosphere will puff away, forming a glowing cloud of gas that astronomers refer to as a "planetary nebula". This won't happen any time soon, but when it does, Aldebaran will, for a short time, glow even more brightly than it does now. Then, it will dim down, and fade slowly away.