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Dupree, Ronald Gilliland,
NASA and ESA
(Larger ultraviolet image -- more).
A highly evolved, orange-red giant star, Aldebaran A
is still much smaller than the red supergiant
Betelgeuse, at left. (See a Digitized Sky Survey
image of Aldebaran A from NASA's NStars Database.)
Aldebaran, or Alpha Tauri, is a binary star system located around 65.1 light-years (ly) from Sol. The star lies in the central part (4:35:55.2+16:30:33.5 for Star A and 4:35:57.0+16:30:22 for Star B, ICRS 2000.0) of Constellation Taurus (see chart and photo), the Bull -- northwest of Constellation Orion and on the left or eastern tip of the Hyades star cluster. Its proper name, Aldebaran, is derived from the Arabic "Al Dabaran" for the Follower because the star appears to track the night time movement of the Pleiades ("Seven Sisters") star cluster. This first magnitude star is the 13th brightest in Earth's night sky. According to Robert Burnham, Jr. (1931-93), Aldebaran A exhibits an annual proper motion of 0.21" around PA 160° with an radial velocity of about 33 miles (53 km) per second in recession. The system is a member of the Hyades group.
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.)
Aldebaran A is a orange-red giant star of spectral and luminosity type K5 III. The star may have a mass similar to Sol's 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).
It is notable that Aldebaran A has an inferred mass near that of Sol but is more evolved as a 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.
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.)
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.
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.)
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.
Star "B" is a red red sequence dwarf star of spectral and luminosity type M2 V. It may have 15 percent of Sol's mass, 36 percent of its diameter, and 14/10,000th of its luminosity. The orbit of an Earth-like planet (with liquid water) around Star B would be centered around 0.036AU -- well inside the orbit of Mercury in the Solar System -- with an orbital period of 6.5 days. Useful catalogue numbers and designations for the star include: Gl 171.1 B, GJ 9159 B, Wo 9159 B, BD+16 629 B, and ADS 3321 B.
Brown Dwarfs or Planets?
When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.
University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover a nearby brown dwarf, Gliese 229 b, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 80 (or so) Jupiters. While these objects cannot fuse "ordinary" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter masses would be defined as planets.
Other prominent astronomers, such as San Francisco State University astronomer Geoffrey W. Marcy who also has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. Similarly, there may also be many different processes that lead to the creation of brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.
The following table includes all star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of Aldebaran.
|Star System||Spectra &|
|VA 366||M0 V||4.4|
|Ross 388||M3 V||9.1|
|* plus bright stars *||. . .|
|BD+16 527||G0-5 V||12|
|BD+05 614||G0 IV||14|
|BD+05 613||G5 IV||14|
|39 Tauri 2?||G5 V |
|104 Tauri 2?||G4 V |
|BD+27 688||K3-5 III||16|
|Wo 9125||G0 V||18|
|LTT 17742||G V||18|
|BD+23 548||F6 V||19|
|Wo 9127||G3 V||18|
Try Professor Jim Kaler's Stars site for other information about Aldebaran at the University of Illinois' Department of Astronomy. In addition, John Whatmough has developed illustrated web pages on this system at Extrasolar Visions.
Up-to-date technical summaries on this star can be found at: Jean Schneider's Extra-Solar Planets Encyclopaedia; the Astronomiches Rechen-Institut at Heidelberg's ARICNS for Star A and Star B, and NASA's NStar Database. Additional information may be available at Roger Wilcox's Internet Stellar Database.
Constellation Taurus represents the front half of the transformed Greek God Zeus, whose hind quarters are underwater because he is quite busy carrying Europa (the daughter of the King of Tyre who was lured to ride him) across the sea to Crete where King Asterius ends marries her when Zeus realizes that he can not marry her himself. Aldebaran is viewed by some as the glowing eye of the Bull, with white star Beta Tauri (Elnath) as its pushing horn and the Hyades star cluster forming its head. Since the Sun passes through this constellation from mid-May through mid-June in the Northern Hemisphere, Taurus is best viewed in early winter. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Taurus. For another illustration, see David Haworth's Taurus.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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