Aldebaran

For other uses, see Aldebaran (disambiguation). Not to be confused with Alderaan or Alderaan (astronomy).

Aldebaran

The position of Aldebaran in the Taurus constellation.
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Taurus
Right ascension	04h 35m 55.239s
Declination	+16° 30′ 33.49″
Apparent magnitude (V)	0.86 (0.75-0.95)
Characteristics
Spectral type	K5III
Apparent magnitude (J)	-2.10
U−B color index	1.90
B−V color index	1.54
Variable type	LB
Astrometry
Radial velocity (Rv)	+54.26±0.03 km/s
Proper motion (μ)	RA: 62.78±0.89 mas/yr Dec.: −189.35±0.58 mas/yr
Parallax (π)	49.97 ± 0.75 mas
Distance	65.3 ± 1.0 ly (20.0 ± 0.3 pc)
Absolute magnitude (MV)	−0.641±0.034
Details
Mass	1.5±0.3 M☉
Radius	44.2±0.9 R☉
Luminosity	518±32 L☉
Surface gravity (log g)	1.59 cgs
Temperature	3,910 K
Metallicity [Fe/H]	–0.34 dex
Rotation	643 days
Other designations
87 Tauri, Alpha Tauri, BD +16°629, GJ 171.1, GJ 9159, HD 29139, HIP 21421, HR 1457, SAO 94027.
Database references
SIMBAD	data
ARICNS	data

Aldebaran /ælˈdɛbərən/ (α Tau, α Tauri, Alpha Tauri) is an orange giant star located about 65 light years away in the zodiac constellation of Taurus. With an average apparent magnitude of 0.87 it is the brightest star in the constellation and is one of the brightest stars in the nighttime sky. The name Aldebaran is Arabic (الدبران al-dabarān) and means "the Follower", presumably because it rises near and soon after the Pleiades.

In 1993 a substellar companion was reported. Subsequent observations did not confirm this claim, but a paper accepted for publication in Astronomy and Astrophysics in 2015 shows evidence for a planetary companion.

The planetary exploration probe Pioneer 10 is currently heading in the general direction of the star and should make its closest approach in about two million years.

Observation history

On March 11, of 509 CE, a lunar occultation of Aldebaran was observed in Athens, Greece. English astronomer Edmund Halley studied the timing of this event, and in 1718 concluded that Aldebaran must have changed position since that time, moving several minutes of arc further to the north. This, as well as observations of the changing positions of stars Sirius and Arcturus, led to the discovery of proper motion. Based on present day observations, the position of Aldebaran has shifted 7′ in the last 2000 years; roughly a quarter the diameter of the full Moon.

English astronomer William Herschel discovered a faint companion to Aldebaran in 1782; an 11th magnitude star at an angular separation of 117″. This star was shown to be itself a close double star by S. W. Burnham in 1888, and he discovered an additional 14th magnitude companion at an angular separation of 31″. Follow on measurements of proper motion showed that Herschel's companion was diverging from Aldebaran, and hence they were not physically connected. However, the companion discovered by Burnham had almost exactly the same proper motion as Aldebaran, suggesting that the two formed a wide binary star system.

Working at his private observatory in Tulse Hill, England, in 1864 William Huggins performed the first studies of the spectrum of Aldebaran, where he was able to identify the lines of nine elements, including iron, sodium, calcium, and magnesium. In 1886, Edward C. Pickering at the Harvard College Observatory used a photographic plate to capture fifty absorption lines in the spectrum of Aldebaran. This became part of the Draper Catalogue, published in 1890. By 1887, the photographic technique had improved to the point that it was possible to measure a star's radial velocity from the amount of Doppler shift in the spectrum. By this means, the recession velocity of Aldebaran was estimated as 30 miles per second (48 km/s), using measurements performed at Potsdam Observatory by Hermann C. Vogel and his assistant Julius Scheiner.

The angular diameter of this star was measured for the first time in 1921 using an interferometer attached to the Hooker Telescope at the Mount Wilson Observatory. The result was 0.0237″, which was in close agreement with the estimated values of the time.

Physical properties

Size comparison between Aldebaran and the Sun.

Aldebaran is classified as a type K5III star, which indicates it is an orange giant star that has moved off the main sequence line of the Hertzsprung–Russell diagram after exhausting the hydrogen at its core. Compression from gravity increased the temperature and pressure at the core until it passed through a helium flash stage, when it began the generation of energy through the triple-alpha fusion process of helium. Additional energy is being produced through hydrogen fusion along a shell surrounding the helium core. As a result of the increase in the core temperature, the star has expanded to a diameter of 44.2 times the diameter of the Sun, approximately 61 million kilometres (see 10 gigametres for similar sizes).

Measurements by the Hipparcos satellite and other sources put Aldebaran around 65.3 light-years (20.0 parsecs) away. Stellar models predict it only has about 50% more mass than the Sun, yet it shines with 425 times the Sun's luminosity due to the expanded radius. Aldebaran is a slightly variable star, of the slow irregular variable type LB. It varies by about 0.2 in apparent magnitude from 0.75 to 0.95. With a near-infrared J band magnitude of −2.1, only Betelgeuse (−2.9), R Doradus (−2.6), and Arcturus (−2.2) are brighter.

The photosphere shows abundances of carbon, oxygen, and nitrogen that suggest the giant has gone through its first dredge-up stage—a normal step in the evolution of a star into a red giant during which material from deep within the star is brought up to the surface by convection. With its slow rotation, Aldebaran lacks a dynamo needed to generate a corona and hence is not a source of hard X-ray emission. However, small scale magnetic fields may still be present in the lower atmosphere, resulting from convection turbulence near the surface. (The measured strength of the magnetic field on Aldebaran is 0.22 G.) Any resulting soft X-ray emissions from this region may be attenuated by the chromosphere, although ultraviolet emission has been detected in the spectrum. The star is currently losing mass at a rate of (1–1.6) × 10⁻¹¹ M_⊙ yr⁻¹ with a velocity of 30 km s⁻¹. This stellar wind may be generated by the weak magnetic fields in the lower atmosphere.

Beyond the chromosphere of Aldebaran is an extended molecular outer atmosphere (MOLsphere) where the temperature is cool enough for molecules of gas to form. This region lies between 1.2 and 2.8 times the radius of the star, with temperatures of 1,000−2,000 K. The spectrum reveals lines of carbon monoxide, water, and titanium oxide. Past this radius, the modest outflow of the stellar wind itself declines in temperature to about 7,500 K at a distance of 1 Astronomical Unit (AU)−the distance of the Earth from the Sun. The wind continues to expand until it reaches the termination shock boundary with the hot, ionized interstellar medium that dominates the Local Bubble, forming a roughly spherical astrosphere with a radius of around 1,000 AU, centered on Aldebaran.

Visibility

In this predawn occultation, Aldebaran has just reappeared on the dark limb of the waning crescent Moon (July 1997 still frame captured from video).

Aldebaran is one of the easiest stars to find in the night sky, partly due to its brightness and partly due to its spatial relation to one of the more noticeable asterisms in the sky. If one follows the three stars of Orion's belt from left to right (in the Northern Hemisphere) or right to left (in the Southern), the first bright star found by continuing that line is Aldebaran.

Since the star is located (by chance) in the line of sight between the Earth and the Hyades, it has the appearance of being the brightest member of the more scattered Hyades open star cluster that makes up the bull's-head-shaped asterism; however, the star cluster is actually more than twice as far away, at about 150 light years.

Aldebaran 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. A number of such events occur every revolution of the Moon starting at 29 Jan 2015 and ending at 3 Sep 2018. Each event is visible from a different location on Earth, but always in the northern hemisphere (or southern close to the equator). That means that people from Australia or South Africa never can observe an Aldebaran occultation. This is due to the fact that Aldebaran is slightly too far south of the ecliptic. A reasonably accurate estimate for the diameter of Aldebaran was obtained during the September 22, 1978 occultation. Aldebaran is in conjunction with the Sun around June 1 of each year.

Double star

WDS 04359+1631 Table Entry

α Tau	Apparent Magnitude	Angular Separation	Position Angle	Year
B	13.60	31.60″	113°	2007
C	11.30	129.50″	32°	2011
D	13.70	—	—	—
E	12.00	36.10″	323°	2000
F	13.60	255.70″	121°	2000

Five faint stars are positioned so that they appear close to Aldebaran. These double stars were given alphabetic secondary star designations more or less in the order of their discovery, with the letter A reserved for the primary star. Some of the characteristics of these components, including their position relative to Aldebaran, are listed in the table at right.

Some surveys have indicated that Alpha Tauri B may have about the same proper motion and parallax as Aldebaran and thus may be a physical binary system. However these measurements are difficult to make because the dim B component appears so close to the bright primary star. The resulting margin of error is too large to positively establish (or exclude) a physical relationship between the two stars. So far neither the B component, nor anything else, has been unambiguously shown to be physically associated with Aldebaran.

Alpha Tauri CD is a binary system with the C and D component stars gravitationally bound to and co-orbiting each other. These co-orbiting stars have been shown to be located far beyond Aldebaran and are members of the Hyades star cluster. As with the rest of the stars in the cluster they do not physically interact with Aldebaran in any way.

Claims of a planetary system

Main article: Aldebaran b

In 1993, radial velocity measurements of Aldebaran, Arcturus and Pollux showed that Aldebaran exhibited a long-period radial velocity oscillation, which could be interpreted as a substellar companion. The measurements for Aldebaran implied a companion with a minimum mass 11.4 times that of Jupiter in a 643-day orbit at a separation of 2.0 AU (300 Gm) in a mildly eccentric orbit. However, all three stars surveyed showed similar oscillations yielding similar companion masses, and the authors concluded that the variation was likely to be intrinsic to the star rather than due to the gravitational effect of a companion. Subsequent observations have not confirmed any substellar companions in orbit around Aldebaran.

A paper accepted for publication in the journal Astronomy and Astrophysics in 2015 says that analysis of observations shows evidence for both a planetary companion and stellar activity.

Etymology

From Arabic الدبران (al-dabarān), meaning "the follower" (of the Pleiades).

Names in other languages

In Persia it was known as Tascheter.

The Romans called it Palilicium.

In the Middle Ages it was sometimes called Cor Tauri (the Heart of the Bull/Taurus).

John Gower refers to it as Aldeboran.

In Chinese it is known as 畢宿五 (Bìxiùwŭ, the Fifth Star of the Net).

In Hindu astronomy it is identified as the lunar mansion Rohini ("the red one") and as one of the twenty-seven daughters of Daksha and the wife of the god Chandra (moon).

Mythology

This easily seen and striking star in its suggestive asterism is a popular subject for ancient and modern myths.

• Mexican culture: For the Seris of northwestern Mexico, this star is providing light for the seven women giving birth (Pleiades). It has three names: Hant Caalajc Ipápjö, Queeto, and Azoj Yeen oo Caap ("star that goes ahead"). The lunar month corresponding to October is called Queeto yaao "Aldebaran's path".
• Aboriginal culture: In the Clarence River of northeastern New South Wales, this star is the Ancestor Karambal, who stole another man's wife. The woman's husband tracked him down and burned the tree in which he was hiding. It is believed that he rose to the sky as smoke and became the star Aldebaran.
• In Hindu mythology Aldebaran is known by the name Rohini and is the fourth of the 27 daughters of Daksha given to God Chandra(moon) as wives. The Moon God enamoured by the beauty of Rohini started spending all his time with her neglecting the others. (This amounts to moon not going round the constellations of zodiac and remaining in one place!). Daksha got angry and cursed him to suffer by slow desiccation. After pleading to the lord Shiva the curse was held at bay and the moon was rejuvenated by shiva's grace for 15 days and desiccated for another 15 days thus waning and waxing to make a lunar month.

See also

• Aldebaran in astrology
• Aldebaran in fiction
• Iota Draconis
• Pollux

External links

Media related to at Wikimedia Commons

• Aldebaran at Constellation Guide