Aldebaran (α Tau) is an orange giant star located 65.3 light-years away in the constellation Taurus. The bright star marks the eye of the celestial Bull.
Aldebaranby admin2025-02-022026-03-27Aldebaran, Alpha Tauri (α Tau), is an orange giant star located 65.3 light-years away in the constellation Taurus. With an apparent magnitude that varies from 0.75 to 0.95, it is the brightest star in Taurus and the 14th brightest star in the sky.Aldebaran lies in the same line of sight as the Hyades cluster and marks the eye of the celestial Bull. The evolved star hosts a giant exoplanet, Aldebaran b, whose existence was confirmed in 2015.Table of Contents ToggleStar typeSizePlanetFactsNameLocationConstellationStar typeAldebaran is a giant star of the spectral type K5+ III. The aging giant is currently on the red giant branch (RGB), in the hydrogen shell burning phase. It has exhausted the hydrogen supply in its core and, as it evolved away from the main sequence, it started to expand to its current size.Aldebaran has a mass of 1.16 solar masses and a radius 45.1 times that of the Sun. With a surface temperature of 3,900 K, it shines with a luminosity 439 times that of the Sun. A lot of the star’s energy output is in the invisible infrared.The red giant star is a relatively slow spinner. With a projected rotational velocity of around 3.5 km/s, it takes 520 days to complete a rotation. It has a metallicity around 30% lower than the Sun’s and an estimated age of 6.4 billion years.<img fetchpriority="high" decoding="async" aria-describedby="caption-attachment-7684" class="size-full wp-image-7684" src="https://www.star-facts.com/wp-content/uploads/2019/10/Aldebaran-Alpha-Tauri-1.webp" alt="aldebaran star,alpha tauri,bull's eye star" width="700" height="468" srcset="https://www.star-facts.com/wp-content/uploads/2019/10/Aldebaran-Alpha-Tauri-1.webp 700w, https://www.star-facts.com/wp-content/uploads/2019/10/Aldebaran-Alpha-Tauri-1-300x201.webp 300w" sizes="(max-width: 700px) 100vw, 700px" />Aldebaran (Alpha Tauri), image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)Aldebaran is shedding mass at a rate of (1–1.6) × 10−11 M⊙ yr−1, or one Earth mass every 300,000 years, through a strong stellar wind.The star’s MOLsphere – the molecular layer beyond the chromosphere – stretches across a distance 2.5 times the star’s radius and has a temperature of 1,500 K. Lines of water, carbon monoxide and titanium oxide have been detected in its spectrum.Aldebaran’s stellar wind expands beyond the MOLsphere, until it reaches the boundary with the interstellar medium, where it slows down to subsonic speed. It forms an astrosphere that extends for about 1,000 astronomical units from the star.Alpha Tauri is classified as a slow irregular variable (LB) star. Its brightness varies from magnitude 0.75 to 0.95 according to historical records given in the General Catalogue of Variable Stars (GCVS). However, modern observations have shown that the variations have a smaller amplitude. Some of the studies have even shown almost no variation.The epoch photometry of the Hipparcos catalogue indicates that the star’s brightness varies by only 0.02 magnitudes over a possible period of about 18 days. Ground-based photometry found an amplitude of up to 0.03 magnitudes with a possible period of about 91 days. Long-term observations have found irregular variations by less than 0.1 magnitudes.Aldebaran has several faint visual companions. The first one was discovered by the German-born British astronomer William Herschel in 1782. It is an 11th magnitude star separated by 117 arcseconds from Aldebaran.In 1888, a full century later, the American astronomer Sherburne Wesley Burnham observed the star and found that it was a close binary system. Burnham found another companion, a 14th magnitude star, at a separation of 31 arcseconds.The companion discovered by Herschel does not have a similar proper motion to Aldebaran. However, the second star has almost the same parallax and proper motion as its bright neighbour, indicating that the two stars are physically related and form a wide binary system. Since the companion appears so close to the bright Aldebaran, it is difficult to unambiguously confirm that the two stars are related.Aldebaran’s line-of-sight companions have been designated Alpha Tauri B, C, D, E, and F in order of discovery. Alpha Tauri B has the stellar classification M2.5 and an apparent magnitude of 13.60. Alpha Tauri C is the brightest of the companions, with a visual magnitude of 11.30.Alpha Tauri D, E and F have apparent magnitudes of 13.70, 12.00 and 13.60. Alpha Tauri C and Alpha Tauri D are gravitationally bound to each other and form a binary system. They are located at a much greater distance from us than Aldebaran and belong to the Hyades cluster.None of the five visual companions have been confirmed as physical companions to Aldebaran.SizeAldebaran has a radius of 45.1 solar radii. The value comes from a 2015 study of long-period radial velocity variations in the giant. It is based on Aldebaran’s angular diameter used for the Gaia benchmark calibration, 20.580 ± 0.030 milliarcseconds.The star...