Aldebaran /ælˈdɛbərən/, designated α Tauri (Latinized to Alpha Tauri, abbreviated Alpha Tau, α Tau), is a giant star measured to be about 65 light-years from the Sun in the zodiac constellation Taurus. It is the brightest star in Taurus and generally the fourteenth-brightest star in the night sky, though it varies slowly in brightness between magnitude 0.75 and 0.95. Aldebaran is believed to host a planet several times the mass of Jupiter, named Aldebaran b.
Aldebaran is a giant star, cooler than the Sun with a surface temperature of 3,900 K, but its radius is about 44 times the Sun's, so it is over 400 times as luminous. It spins slowly and takes 520 days to complete a rotation.
Aldebaran is the brightest star in the constellation Taurus and so has the Bayer designation α Tauri, Latinised as Alpha Tauri. It 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.
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.[23] A series of 49 occultations occurred starting on 29 January 2015 and ending at 3 September 2018.[24] Each event was visible from points in the northern hemisphere or close to the equator; people in e.g. Australia or South Africa can never observe an Aldebaran occultation since it is too far south of the ecliptic. A reasonably accurate estimate for the diameter of Aldebaran was obtained during the occultation of 22 September 1978.[25] Aldebaran is in conjunction with the Sun around June 1 of each year.[26]
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.
Physical characteristics
Size comparison between Aldebaran and the Sun
Aldebaran is listed as the spectral standard for type K5+ III stars.[6] Its spectrum shows that it is a giant star that has evolved off the main sequence band of the Hertzsprung-Russell diagram after exhausting the hydrogen at its core. The collapse of the centre of the star into a degenerate helium core has ignited a shell of hydrogen outside the core and Aldebaran is now on the red giant branch (RGB).[5]
Measurements by the Hipparcos satellite and other sources put Aldebaran around 65.3 light-years (20.0 parsecs) away.[10] Asteroseismology has determined that it is about 16% more massive than the Sun,[11] yet it shines with 518 times the Sun's luminosity due to the expanded radius. The angular diameter of Aldebaran has been measured many times. The value adopted as part of the Gaia benchmark calibration is 20.580±0.030 mas.[13] It is 44 times the diameter of the Sun, approximately 61 million kilometres.[36]
Aldebaran is a slightly variable star, assigned to the slow irregular type LB. The General Catalogue of Variable Stars indicates variation between apparent magnitude 0.75 and 0.95 from historical reports.[4] Modern studies show a smaller amplitude, with some showing almost no variation.[37] Hipparcos photometry shows an amplitude of only about 0.02 magnitudes and a possible period around 18 days.[38] Intensive ground-based photometry showed variations of up to 0.03 magnitudes and a possible period around 91 days.[37] Analysis of observations over a much longer period still find a total amplitude likely to be less than 0.1 magnitudes, and the variation is considered to be irregular.[39]
The photosphere shows abundances of carbon, oxygen, and nitrogen that suggest the giant has gone through its first dredge-up stage-a normal step in the evolution of a star into a red giant during which material from deep within the star is brought up to the surface by convection.[40] With its slow rotation, Aldebaran lacks a dynamo needed to generate a corona and hence is not a source of hard X-ray emission. However, small scale magnetic fields may still be present in the lower atmosphere, resulting from convection turbulence near the surface.
Beyond the chromosphere of Aldebaran is an extended molecular outer atmosphere (MOLsphere) where the temperature is cool enough for molecules of gas to form. This region lies at about 2.5 times the radius of the star and has a temperature of about 1,500 K. The spectrum reveals lines of carbon monoxide, water, and titanium oxide.[40] Outside the MOLSphere, the stellar wind continues to expand until it reaches the termination shock boundary with the hot, ionized interstellar medium that dominates the Local Bubble, forming a roughly spherical astrosphere with a radius of around 1,000 AU, centered on Aldebaran.