Star Facts: Aldebaran

Aldebaran Star

Aldebaran (Alpha Tauri) is the brightest star in the constellation Taurus, and the 14th most luminous star in the entire night sky. Although it is relatively close to Earth, the Pioneer 10 space probe currently moving in the general direction of Aldebaran will only make its closest approach to the star in about 2 million years from now.

Quick Facts

• Constellation: Taurus
• Star Type: Orange-Red giant (K5 III)
• Age: 7 billion years
• Distance: 65.3 light years
• Apparent magnitude (V): 0.75 to 0.95
• Apparent magnitude (J): -2.10
• Radius: 44.2 sol
• Mass: 1.7 sol
• Luminosity: 425 sol
• Surface Temperature: 3,910K
• Radial velocity: 54.26 km/s
• Rotation: 643 days
• Coordinates: RA: 4h 35m 55s, dec: 16°30’35”
• Other Designations: 87 Tauri, Alpha Tauri, BD+16°629, GJ 171.1, GJ 9159


Aldebaran the Red Eye of TaurusAround 5000 years ago in ancient Persia, Aldebaran was one of the Four Royal Stars considered to be guardians of the heavens, and represented the ‘Watcher of the East’, with the others being Regulus (South), Antares (West), and Fomalhaut (North). The star was likewise worshiped by the Babylonians, who called the constellation Gud.Anna (“Bull of Heaven“), while to the ancient Egyptians it represented the bull-god Orissi. In Ancient Greece, Aldebaran marked the southern eye of the Bull, with the star’s name derived from the Arabic word ‘Al Dabaran’, meaning the ‘Follower’, in reference to its apparent tracking motion of the Pleiades star cluster. Interestingly, for the Seris people of north-western Mexico, Aldebaran is “[the] star that goes ahead” and provides light for the Pleiades, which is known as “[the] seven women giving birth”.


Even novice observers should not find it difficult to spot Aldebaran in the Taurus constellation, the star being located, by pure chance, exactly in the line of sight between Earth and the Hyades open cluster. This gives it the appearance of being the most luminous star in the V-shaped asterism that marks out the “head” of the Bull, but recent measurements have shown that the five brightest stars which make up the Hyades asterism lie 151 light years away compared to the 65 light years distant Aldebaran. Therefore, Aldebaran in no way interact with the Hyades, is not considered to be part of the asterism, and its exact origins remain unknown, since it cannot be linked to any structure, cloud, or group of stars.


Aldebaran in Taurus Constellation
Image Credit: Duncan Moran

One way of finding Aldebaran in the northern hemisphere is to follow the line formed by the three stars in Orion’s Belt from left to right to the first bright red star that you come across. Southern observers need to follow the stars in Orion’s Belt from right to left, though.


Taurus is a northern hemisphere constellation that can be seen by observers located between +90° and -65° of latitude, with the best time to view Aldebaran, the constellation’s brightest star, being autumn/winter in the northern hemisphere, and spring/summer from southern locations. Although Aldebaran is too far south of the ecliptic to be occulted by the planets, due to precession, in the distant past both Mercury and Venus had occulted the star, with the next occultation of Aldebaran by a planet, in this case Venus, set to happen on August 27th of 5366 AD. Aldebaran can be occulted by the Moon, though, but this phenomena cannot be observed from the southern hemisphere.

Physical Properties

Aldebarans’ K5 III classification denotes it as an orange-red giant that has evolved off the main sequence, having consumed its hydrogen fuel. The resulting collapse of the core has ignited a remaining shell of hydrogen surrounding it, which has subsequently overcome the inward pushing force of the gravity of the star’s outer layers, thereby causing it to swell up to around 44.2 solar masses. A further consequence is that the expanded bulk of the star now shines at a visual luminosity 153 times brighter than the Sun, and an absolute luminosity about 425 times brighter. In terms of infrared (IR) radiation in the J-band, Aldebaran shines at magnitude -2.1, making it the fifth brightest star in this frequency, with only Betelgeuse (-2.9), Antares (-2.7), R Doradus (-2.6), and Arcturus (-2.2) being more luminous.


The stars’ photosphere displays carbon, oxygen, and nitrogen abundances, which suggests the first dredge-up stage in the life cycle of Aldebaran, a normal step in the evolutionary process of red giant stars that involves “dredging” up material from the star’s deep interior by means of convection currents, which then gets mixed into the star’s outer layers. Being a very slow rotator the processes that create a corona do not occur on Aldebaran, which means that the star does not emit hard X-ray radiation. Although a fierce dynamo effect is lacking, weak magnetic fields and together with a strong solar wind is causing the star to lose mass, and in a few million years it will become a planet-sized, white dwarf star.

Extended Influence

Stretching out from the star to a distance of between 1.2 and 2.8 times the radius of the star, is a MOLsphere (molecular outer atmosphere) in which temperatures are low enough (1,000K – 2,000K) for various gaseous molecules to form. Spectra of this region have revealed the presence of carbon monoxide, water, and titanium oxide, however, past this region, and out to a distance of about 1 AU, the temperature of the slow solar wind declines to only about 7,500K. Nonetheless, the stars’ solar wind continues outward up to the termination shock boundary, which is where it collides with the (hot) ionized interstellar medium that predominates in the Local Bubble, a roughly spherical astrosphere that stretches outward from Aldebaran to a distance of about 1,000 AU.

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