Aldebaran
What is Aldebaran?
Stars· Distance 65 ly · size 44× Sun. The orange giant that marks the red eye of Taurus, the Bull. It seems to sit inside the V-shaped Hyades cluster, but it is actually less than half as far away as the cluster.
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Distance and spectral type
Aldebaran is about 67 light-years away, a orange bright giant (spectral type K5III), in the constellation Gemini. Its apparent magnitude is 0.9, visible to the naked eye.
Stellar figures
| Distance | 65 ly |
|---|---|
| Magnitude | 0.86 — fourteenth brightest |
| Type | K5 red giant |
| Constellation | Taurus |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Behind the name
Arabic "the follower" — it trails the Pleiades across the sky. Pioneer 10 is drifting toward it.
Aldebaran's planet: announced, then doubted
Distant planets are usually too faint to see directly. Instead, astronomers look for a star that wobbles back and forth as a planet tugs on it. A wobble like this was first noticed in Aldebaran in 1993. Then in 2015 a study announced a planet about six times the mass of Jupiter, circling Aldebaran every 629 days. It was named Aldebaran b.
In 2019, however, a study with more data looked again. It concluded that the wobble was more likely caused by the star itself pulsing than by a planet. A giant star like Aldebaran slowly swells and shrinks at its surface, and that motion can mimic the signal of a planet. Later studies have also treated Aldebaran b as doubtful or ruled out. The case shows how hard it is to find planets around old, swollen stars.
The star in depth
Aldebaran is the brightest star in Taurus, the Bull. At magnitude 0.86 it is the fourteenth-brightest star in the night sky, 65 light-years away. Its name is Arabic for 'the follower', because it rises after the Pleiades star cluster. In pictures of Taurus it marks the Bull's red eye.
Aldebaran is an old star. Its mass is about 1.2 times the Sun's, not so different from our own star. But it has used up the hydrogen in its core and swollen until it is 44 times as wide as the Sun. A star like this is called a giant. Its surface is about 3,900 K, cooler than the Sun's, so it glows orange. Because it is so large, it gives off about 440 times as much light as the Sun. The Sun is expected to swell into a giant like this in the far future.
In the sky, Aldebaran seems to sit in the middle of the Hyades, a cluster of stars arranged in a V. But the Hyades are 153 light-years away. Aldebaran is less than half that distance, and simply lines up with the cluster by chance. Aldebaran also lies along the Moon's path across the sky, so now and then the Moon passes in front of it, hiding it for a while.
From December to February, follow the three stars of Orion's Belt up and to the right in the southern evening sky to reach Aldebaran; go further and you reach the Pleiades. The Pioneer 10 probe, launched in 1972, is heading in Aldebaran's general direction, but it will not make its closest approach for about two million years. In the Sky view, choose a January night and select Taurus to see Aldebaran glowing red at the tip of the V.
Frequently asked
- How far is Aldebaran from Earth?
- The distance of Aldebaran is 65 ly.
- How bright is Aldebaran (apparent magnitude)?
- The magnitude of Aldebaran is 0.86 — fourteenth brightest.
- What kind of object is Aldebaran?
- Aldebaran is a K5 red giant.
- Which constellation is Aldebaran in?
- Aldebaran lies in the constellation Gemini.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.