Alpha Tauri - Aldebaran

Aldebaran, or Alpha Tauri, is the brightest star in Taurus, the Bull, and at magnitude 0.87 is the 13th brightest star in the night sky. It appears in the V-shaped star group known as the Hyades. The Hyades represents the head of the Taurus, and Aldebaran marks the red eye of the bull.

Aldebaran is one of the easiest stars to find in the night sky, partly due to its brightness and partly due to its position. If one follows the three stars of Orion's belt in the opposite direction of Sirius, the first bright star found is Aldebaran. This star is close enough to the ecliptic to be occulted by the Moon. Such occultations occur when the moon's ascending node is near the autumnal equinox, as will be the case around 2015.

NASA's Pioneer 10 spacecraft, which flew by Jupiter in 1973, will reach and pass by Aldebaran in about two million years.

Name and Mythology

Aldebaran's name in Arabic means "follower", because it rises after the Pleiades and follows them across the sky. The Romans called it Parilicium or Cor Tauri ("Heart of the Bull"). It is known as Bixiuwu, the Fifth Star of the Net, in Chinese. In Persia it was known as Satvis and Kugard. Aldebaran is identified as the lunar mansion Rohini in Hindu astronomy, and as one of the twenty-seven daughters of Daksha and the wife of the god Chandra.

For the Dakota branch of the Native American Sioux tribe, Aldebaran took on a heroic aspect. The young star was the child of the sun and the lady Blue Star. One day, he desired to hunt the white buffalo (the Pleiades). After he pulled up a sapling to make a spear, a hole was made in the ground, and he could see all the people of Earth below. The white buffalo took this chance to push him through. He was found by an old woman, and was to be known as Old Woman's Grandson. On Earth, he killed many strange monsters that had been troubling the Native Americans; one monster of which was a serpent that caused drought. The young hero killed the serpent, releasing a great stream of water that became the Mississippi River. In time, Old Woman's Grandson remembered the white buffalo, and returned him to hunting the white buffalo in the sky to fulfill his destiny.

For the Seris of northwestern Mexico, this star provides light for the seven women giving birth (the Pleiades), and has three different names: Hant Caalajc Ipapjo, Queeto, and Azoj Yeen oo Caap ("star that goes ahead"). The lunar month corresponding to October is called Queeto yaao, "Aldebaran's path".

In Inuit astronomy, Aldebaran is called "Spirit of a Polar Bear".

Properties and Evolution

Aldebaran is a binary star system located 65 light years from Earth, consisting of an orange giant and a faint red dwarf companion. In 1997, a possible substellar companion was reported, with a mass at least 11 times that of Jupiter with an orbital period of around 2 years; however, this has not been confirmed.

Aldebaran A is an orange giant star of spectral class K5 III, with a surface temperature just under 4000 K. It is not as large or as cool as the famous red supergiants Antares and Betelgeuse, but is still 44 times the Sun's diameter (based on an interferometric estimates of its angular diameter at 0.02"). Aldebaran contains about 2.5 solar masses, and has 425 times the Sun's luminosity, including infrared radiation. Magnesium-II emissions indicate a cooler shell of gas and dust surrounding the star, with infrared emissions extending out to 10". Aldebaran is estimated to have a metallicity (abundance of elements heavier than hydrogen) of 47% - 100% of the Sun's.

Aldebaran was once a main-sequence star of spectral class A, fusing hydrogen into helium at its core. Having since evolved out of the main sequence, Aldebaran has fully shifted to fusing helium into carbon and oxygen at its core. This helium-burning, orange-red giant stage is relatively brief, lasting tens to hundreds of million years. Eventually, the star will lose much of its current mass, from an intensified stellar wind that puffs out its outer envelope into interstellar space as a planetary nebula. The result will be a planet-sized, white dwarf core that gradually cools and fades. Given that Aldebaran's surrounding shell of gas and dust contains the higher element magnesium, the star may have already shed a substantial share of its mass.

Though Aldebaran appears in the Hyades, it is not a member of the cluster; it lies at about half the cluster's distance, and has a different space motion.

Variability and Companion

Aldebaran is a slow, pulsating irregular variable that fluctuates by only 0.2 magnitude. This is not very perceptible by the naked eye.

Aldebaran has a distant, red dwarf companion (α Tau B) that is currently separated by about 607 AU. (semi-major axis of 30.4" at 65.1 ly). α Tau B is a red main sequence dwarf star of spectral class M2 V. It may have 15% of the Sun's mass, 36% of its diameter, and 1/700th of its luminosity.

There is also an optical binary companion system of K5 and M2 dwarf/subdwarf stars at a separation of about 122". These were once referred to as α Tau C and D. The stars in this system are separated by around 34 AUs, and they have a combined visual luminosity of about 1.1% of the Sun's.

Planet or Brown Dwarf?

In 1997, a team of astronomers announced the tentative discovery of a giant planet companion "b" to Aldebaran A. The discovery has yet to be confirmed, and some astronomers suspect that the radial velocity variations may actually reflect a long-term oscillation in Aldebaran A itself. In fact, it is extremely difficult to confirm the detection of this substellar object around a highly evolved star like Aldebaran, because giant stars often pulsate and produce radial velocity patterns similar to substellar companions. Astronomers would find it very difficult to detect an Earth-sized planet around Aldebaran using present methods.

Planet b appears to move around Aldebaran A with an average separation of 1.35 AUs with a period of almost 654 days, or about 1.8 years, in a mildly eccentric orbit (e=0.182). The object has at least 11 times the mass of Jupiter, but has about the same diameter. However, because the method of detection relies on radial velocities using the Doppler technique only determines the object's minimum mass, companion b may actually be a brown dwarf. Indeed, Aldebaran A was once suspected as being a spectroscopic binary star.