Aldebaran
Brightest star in Taurus, a red giant with a faint binary companion.
Aldebaran, whose name comes from the Arabic for "the follower," is a red giant star in the zodiac constellation Taurus. It carries the Bayer designation Alpha Tauri, often shortened to α Tau. Its brightness shifts between apparent magnitudes 0.75 and 0.95, which makes it the brightest star in Taurus and usually the fourteenth-brightest in the entire night sky. The star sits about 67 light-years from Earth. Although it appears along the same line of sight as the Hyades star cluster, Aldebaran is not part of that group and is far older. As a red giant, Aldebaran has a radius roughly 45 times that of the Sun, but its mass is about the same. This is because the star is more than six billion years old and has entered the final phase of its life. It has moved off the main sequence on the Hertzsprung–Russell diagram after using up the hydrogen in its core. Its surface temperature is cooler than the Sun’s, at about 3,900 K, yet it shines over 400 times more brightly because of its enormous surface area. The star rotates slowly, taking 520 days to complete one spin. Aldebaran forms a binary system with a companion called Alpha Tauri B. The two are separated by at least 680 astronomical units—680 times the distance from Earth to the Sun. The companion is much fainter, with an apparent magnitude of 13.21, and cannot be seen without a telescope. The traditional name Aldebaran comes from the Arabic *al Dabarān*, meaning "the follower," because it seems to trail the Pleiades. In 2016, the International Astronomical Union’s Working Group on Star Names officially approved Aldebaran as the star’s proper name. It also has the Flamsteed designation 87 Tauri, the Bright Star Catalogue number 1457, the HD number 29139, and the Hipparcos catalogue number 21421. It is a variable star listed in the General Catalogue of Variable Stars, but it is listed using its Bayer designation and does not have a separate variable star designation. Aldebaran and several nearby stars appear in double star catalogues: it is WDS 04359+1631 in the Washington Double Star Catalog, ADS 3321 in the Aitken catalogue, H IV 66 in the Herschel catalogue, Σ II 2 in the Struve catalogue, and β 550 in the Burnham catalogue. Finding Aldebaran is straightforward because it is bright and sits near a well-known asterism.
- Apparent magnitude
- 0.75 to 0.95
- Distance
- approximately 67 light-years
- Radius
- about 45 times that of the Sun
- Mass
- about equal to the Sun
- Surface temperature
- 3,900 K
- Luminosity
- over 400 times that of the Sun
- Rotation period
- 520 days
Lore & Background
Aldebaran is the brightest star in the constellation Taurus, given the Bayer designation α Tauri (Alpha Tauri) by Johann Bayer. The traditional name Aldebaran derives from the Arabic al Dabarān (الدبران), meaning 'the follower', because it seems to follow the Pleiades. In 2016, the International Astronomical Union Working Group on Star Names (WGSN) approved the proper name Aldebaran for this star. Aldebaran additionally has the Flamsteed designation 87 Tauri, as the 87th star in the constellation of approximately 7th magnitude or brighter, ordered by right ascension. It also has the Bright Star Catalogue number 1457, the HD number 29139, and the Hipparcos catalogue number 21421, mostly seen in scientific publications. It is a variable star listed in the General Catalogue of Variable Stars, but it is listed using its Bayer designation and does not have a separate variable star designation.
Reader's Guide
Aldebaran is one of the easiest stars to find in the night sky, partly due to its brightness and partly due to being near one of the more noticeable asterisms in the sky. Following the three stars of Orion's belt in the direction opposite to Sirius, the first bright star encountered is Aldebaran. It is best seen at midnight between late November and early December. The star is, by chance, in the line of sight between the Earth and the Hyades, so it has the appearance of being the brightest member of the open cluster, but the cluster that forms the bull's-head-shaped asterism is more than twice as far away, at about 150 light years. Aldebaran is 5.47 degrees south of the ecliptic and so can be occulted by the Moon. Such occultations occur when the Moon's ascending node is near the autumnal equinox. A series of 49 occultations occurred starting on 29 January 2015 and ending at 3 September 2018. Each event was visible from points in the northern hemisphere or close to the equator. People further south, in Australia or Southern Africa for example, can never observe occultations of Aldebaran because of parallax. The change in position of the Moon relative to the stars due to the effect of parallax means that Aldebaran is too far south of the ecliptic for occultations to be observed. A reasonably accurate estimate for the diameter of Aldebaran was obtained during the occultation of 22 September 1978. In the 2020s, Aldebaran is in conjunction in ecliptic longitude with the sun around May 30 of each year. With a near-infrared J band magnitude of −2.1, only Betelgeuse (−2.9), R Doradus (−2.6), and Arcturus (−2.2) are brighter at that wavelength.
Did You Know?
- Aldebaran varies in brightness from an apparent visual magnitude of 0.75 down to 0.95.
- The star is about 45 times the Sun's radius but has about the same mass.
- Aldebaran has a faint binary companion, Alpha Tauri B, with an apparent magnitude of 13.21.
- English astronomer William Herschel discovered a faint companion to Aldebaran in 1782.
More in Binary and Multiple Stars, Part 2 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced