Aldebaran

Aldebaran, also designated α Tauri (Latinized to Alpha Tauri, abbreviated Alpha Tau, α Tau), is a red giant star located about 65 light-years from the Sun in the zodiac constellation Taurus. It is the brightest star in Taurus and generally the fourteenth-brightest star in the night sky, though it varies slowly in brightness between magnitude 0.75 and 0.95.

Observation data Epoch J2000.0      Equinox J2000.0 Constellation The position of Aldebaran in the Taurus constellation. Taurus /ælˈdɛbərən/[1][2] 04h 35m 55.23907s[3] +16° 30′ 33.4885″[3] 0.86[4] Red giant branch[5] K5+ III[6] −2.095[7] +1.92[4] +1.44[4] LB[8] Radial velocity (Rv) +54.26±0.03[9] km/s Proper motion (μ) RA: 63.45±0.84[3] mas/yr Dec.: −188.94±0.65[3] mas/yr Parallax (π) 49.97 ± 0.75[10] mas Distance 65.3 ± 1.0 ly (20.0 ± 0.3 pc) Absolute magnitude (MV) −0.641±0.034[10] Mass 1.16±0.07[11] M☉ Radius 44.13±0.84[12] R☉ Luminosity 518±32[12] L☉ Surface gravity (log g) 1.59[12] cgs Temperature 3,910[12] K Metallicity [Fe/H] −0.15±0.2[13] dex Rotation 520 days[14] Rotational velocity (v sin i) 2.7[15] km/s Age 6.4+1.4−1.1[11] Gyr 87 Tauri, α Tauri, BD+16°629, GJ 171.1, GJ 9159, HD 29139, HIP 21421, HR 1457, SAO 94027 SIMBAD data ARICNS data

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.

Aldebaran hosts a planet several times the size of Jupiter, named Aldebaran b.

Nomenclature

α Tauri is the star's Bayer designation. The name Aldebaran is derived from the Arabic for "the Follower" (الدبران),[16][17] because it seems to follow the Pleiades.[18][19]

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[20] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[21] included a table of the first two batches of names approved by the WGSN, which included Aldebaran for this star. It is now so entered in the IAU Catalog of Star Names.[22]

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 provides 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".[26]
• 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.[27]

Observational history

On 11 March AD 509, a lunar occultation of Aldebaran was observed in Athens, Greece.[28] 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.[29][30] 5,000 years ago the vernal equinox was close to Aldebaran.[31]

English astronomer William Herschel discovered a faint companion to Aldebaran in 1782;[32] 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.[33]

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.[34]

Aldebaran was observed using an interferometer attached to the Hooker Telescope at the Mount Wilson Observatory in 1921 in order to measure its angular diameter, but it was not resolved in these observations.[35]

Physical characteristics

Size comparison between Aldebaran and the Sun

Aldebaran is listed as the spectral standard for type K5+ III star,[6] which indicates it is a giant star that has evolved off the main sequence band of the Hertzsprung–Russell diagram after exhausting the hydrogen at its core. The collapse of the centre of the star into a degenerate helium core has ignited a shell of hydrogen outside the core and Aldebaran is now on the red giant branch (RGB).[5]

The effective temperature of Aldebaran's photosphere is 3,910 K. It has a surface gravity of 1.59 cgs, typical for a giant star, but around 25 times lower than the Earth's and 700 times lower than the sun's. Its metallicity is about 30% lower than the sun's.

Measurements by the Hipparcos satellite and other sources put Aldebaran around 65.3 light-years (20.0 parsecs) away.[10] Asteroseismology has determined that it is about 16% more massive than the Sun,[11] yet it shines with 518 times the Sun's luminosity due to the expanded radius. It has expanded to 44 times the diameter of the Sun,[12] approximately 61 million kilometres. Aldebaran is a slightly variable star, of the slow irregular type LB. It varies by about 0.2 magnitudes between apparent magnitude 0.75 and 0.95.[8] With a near-infrared J band magnitude of −2.1, only Betelgeuse (−2.9), R Doradus (−2.6), and Arcturus (−2.2) are brighter at that wavelength.[7]

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.[36] 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 Gauss.[37] Any resulting soft X-ray emissions from this region may be attenuated by the chromosphere, although ultraviolet emission has been detected in the spectrum.[38] The star is currently losing mass at a rate of (1–1.6) × 10−11 M yr−1 (about one Earth mass in 300,000 years) with a velocity of 30 km s−1.[36] This stellar wind may be generated by the weak magnetic fields in the lower atmosphere.[38]

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.[36] 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.[39]

Observation

Occultation of Aldebaran by the Moon. Aldebaran is the red dot to the right, barely visible in the thumbnail.

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 5.47 degrees south of the ecliptic and can be occulted by the Moon. Such occultations occur when the Moon's ascending node is near the autumnal equinox. A series of 49 occultations occurred starting on 29 January 2015 and ending at 3 September 2018.[40] Each event was visible from points in the northern hemisphere or close to the equator; people in e.g. Australia or South Africa can never observe an Aldebaran occultation since it is too far south of the ecliptic. A reasonably accurate estimate for the diameter of Aldebaran was obtained during the occultation of 22 September 1978.[41] Aldebaran is in conjunction with the Sun around June 1 of each year.[42]

Visual companions

Five faint stars are positioned so that they appear close to Aldebaran. These double star components were given upper-case Latin letter 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.

WDS 04359+1631 Catalogue Entry[43]
α Tau Apparent
Magnitude
Angular
Separation
(″)
Position
Angle
(°)
Year Parallax (mas)
B 13.60 31.60 113 2007 47.3417±0.1055[44]
C 11.30 129.50 32 2011 19.1267±0.4274[45]
D 13.70
E 12.00 36.10 323 2000
F 13.60 255.70 121 2000 0.1626±0.0369[46]

Some surveys, for example Gaia Data Release 2,[44] 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.[47] A spectral type of M2.5 has been published for Alpha Tauri B.[48]

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.[32]

Planetary system

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.[49]

In 2015 a study showed stable long-term evidence for both a planetary companion and stellar activity.[14] An asteroseismic analysis of the residuals to the planet fit has determined that Aldebaran b has a minimum mass of 5.8±0.7 Jupiter masses.[11]

View from this star

If the Sun were to be observed from this star, it would be located at the antipodal point of Aldebaran's coordinates, at  16h 35m 55s, –16° 30′ 33″ in the constellation Ophiuchus. Assuming a distance of 20 pc and negligible extinction, it would be a faint 6.3 magnitude star, dimmer than Uranus at maximum brightness from Earth at 5.38.[50]

In modern culture

The name Aldebaran or Alpha Tauri has been adopted many times, including

The star also appears in works of fiction such as Far From the Madding Crowd and Down and Out in Paris and London. It is frequently seen in science fiction, including the Lensman series and Fallen Dragon. As the brightest star in a Zodiac constellation, it is also given great significance within astrology.

The planetary exploration probe Pioneer 10 is no longer powered or in contact with Earth, but its trajectory is taking it in the general direction of Aldebaran. It is expected to make its closest approach in about two million years.[51]

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