Aldebaran 2

In 1997, a team of astronomers announced the tentative discovery of a giant planet companion "b" to Aldebaran A (more details below). The discovery has yet to be confirmed, and some astronomers suspect that the radial velocity variations may actually reflect a long-term oscillation in Star A itself. (See an animation of the orbit of this possible substellar object around Aldebaran, with a table of basic orbital and physical characteristics.)

Alpha Tauri A

Aldebaran A is a orange-red giant star of spectral and luminosity type K5 III. The star may have a mass between one and 2.5 of Sol's (Hatzes and Cocharn, 1993) but around 40 to 52 times its diameter -- extending halfway to the planet Mercury in the Solar System so that it would be 20 degrees across in Earth's skies. According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 1457, two determinations of diameter were estimated (0.022" and 0.0237"), and Magnesium-II emissions indicate a cooler shell of gas and dust surrounding Star A (with an infrared emission determination of the envelope diameter to be 10"). It also has around 150 times Sol's luminosity (based on a Johnson Vmag of 0.87 at a HIPPARCOS distance estimate of 65.12 ly), but this may increase to around 170 times Sol's. Aldebaran A is estimated to have between 47 to 100 percent as enriched than Sol with elements heavier than hydrogen ("metallicity") based on its abundance of iron (Cayrel de Strobel et al, 1991, page 282). Before the star evolved out of the main sequence, it may have been a spectral type A dwarf (Hatzes and Cochran, 1993).

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See a discussion of
the "main sequence"
as part of stellar
evolution and death.

Aldebaran A is believed to have evolved into helium-burning "clump" giant. Given that its surrounding shell of gas and dust contains the higher element Magnesium, the star may have already shed a substantial share of its mass after progressing up the giant branch farther than, say, Pollux. According to the SIMBAD Astronomical Database, Aldebaran A is a slow irregular, pulsating variable star that fluctuates by only two-tenths of a magnitude, which is not very perceptible by Humans with the naked eye (more discussion at: Wasatonic and Guinan, 1997). It has been designated in the Catalogue of Suspected Variable stars as CSV 6116. Useful catalogue numbers and designations for the star include: Alp or Alf Tau A, 87 Tau A, HR 1457*, Gl 171.1 A, GJ 9159 A, Hip 21421, HD 29139, BD+16 629 A, SAO 94027, FK5 168, Wo 9159 A, LTT 11462, and ADS 3321 A.

Aldebaran has a relatively distant, red dwarf companion B that is currently separated by about 607 AUs (semi-major axis of 30.4" at 65.1 ly). There is also an optical binary companion system of K5 and M2 dwarf/subdwarf stars -- at about 122", according to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 1457 -- that were once referred to as Stars "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 percent of Sol's (Kyle M. Cudworth, 1985; and R.F. Griffin, 1985).

If Aldebaran A's substellar companion "b" does actually exist with its apparent inner orbit, then it is highly unlikely that an Earth-type planet could have formed in Star A's water zone before it left the main sequence. Even if such an orbit were possible, any Earth-type planets that orbited Aldebaran A during its youth would have been burnt to a cinder by now, and possibly fallen into the star from frictional drag with the giant star's gaseous envelope. Astronomers would find it very difficult to detect an Earth-sized planet around this star using present methods.

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See a discussion of
"core helium burning"
as part of stellar
evolution and death.

As a star that has evolved out of the "main sequence," Aldebaran A has fully shifted from the fusion of hydrogen to helium at its core to the fusion of helium to carbon and oxygen, with trace activity of other nuclear processes. This helium-burning, orange-red giant stage is relatively brief, lasting tens to hundreds of million years (e.g., lasting around 700 million years for a star of one Solar mass).

Eventually, the star will lose much of its current mass, from an intensified stellar wind that eventually puffs out its outer gas envelopes of hydrogen and helium (and lesser amounts of higher elements such as carbon and oxygen) into interstellar space as a planetary nebula. The result will be a planet-sized, white dwarf core that gradually cools and fades in brightness from the shutdown of thermonuclear fusion. (Nearby white dwarfs include solitary Van Maanen's Star and the dim companions of Sirius, Procyon, and 40 (Omicron2) Eridani.)

H. Bond (STSci), R. Ciardullo (PSU), WFPC2, HST, NASA -- larger image
(White dwarfs are remnant stellar cores that have cast off their outer gas
layers, like planetary nebula NGC 2440.)

Planet or Brown Dwarf "b"?

In 1997, a team of astronomers (including Artie P. Hatzes and William D. Cochran) announced the discovery of a giant planet companion "b" to Aldebaran (Hatzes and Cochran, 1998, in postscript; and 1993). The discovery has yet to be confirmed, and some astronomers suspect that the radial velocity variations may actually reflect a long-term oscillation in Star 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.

© John Whatmough -- larger image
(Artwork from Extrasolar Visions, used with permission)

Planetary candidate "b" with moons and dust ring around
old and swollen Aldebaran A, as imagined by Whatmough.

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. Its orbit is eccentric (e= 0.182 +/- 0.065). The object has at least 11 times the mass of Jupiter, with a similar diameter (about 11 times that of Earth). 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 spectrocopic binary star. (See an animation of the orbit of this substellar object around Aldebaran A, with a table of basic orbital and physical characteristics.)

Alpha Tauri B

NASA -- larger image

Aldebaran B is a dim red dwarf star, like
Gliese 623 A (M2.5 V) and B (M5.8Ve) at lower right.