Aldebaran

Aldebaran, Alpha Tauri (α Tau), is an orange giant star in the constellation Taurus. It marks the eye of the celestial Bull. With an apparent magnitude that varies from 0.75 to 0.95, it is the brightest star in Taurus and the 14th brightest star in the sky. It is usually slightly fainter than Altair in the constellation Aquila and Acrux in Crux, but just outshines Antares, Spica and Pollux, the luminaries of Scorpius, Virgo and Gemini.  Aldebaran lies at a distance of 65.3 light years from Earth and is in the same line of sight as the bright Hyades open cluster. The star hosts a giant exoplanet, Aldebaran b, whose existence was confirmed in 2015.

Star type

Aldebaran is a giant star of the spectral type K5+ III. It has exhausted the hydrogen supply in its core and, as it evolved off the main sequence, it started to expand to its current size. Now it is on the red giant branch (RGB), in the hydrogen shell burning phase.

The star has a mass of 1.16 solar masses and has grown to a size of 44.13 solar radii. With a surface temperature of 3,900 K, it shines with a luminosity 439 times that of the Sun, with a lot of its energy output being in the invisible infrared. It rotates very slowly, with a projected rotational velocity of 3.5 km/s, taking 520 days to complete a rotation. Its estimated age is 6.4 billion years and its metallicity around 30% lower than the Sun’s.

Aldebaran (Alpha Tauri), image: Wikisky

Aldebaran is shedding mass at a rate of (1–1.6) × 10⁻¹¹ M_⊙ yr⁻¹, or one Earth mass every 300,000 years, through a stellar wind.

It is classified as a slow irregular variable (LB) star. Its brightness varies from magnitude 0.75 to 0.95 according to historical records given in the General Catalogue of Variable Stars (GCVS). However, modern observations have shown that the variations have a smaller amplitude. Some of the studies have even shown almost no variation.  The epoch photometry of the Hipparcos catalogue indicates that the star’s brightness varies by only 0.02 magnitudes over a possible period of about 18 days. Ground-based photometry found an amplitude of up to 0.03 magnitudes with a possible period of about 91 days. Long-term observations have found irregular variations by less than 0.1 magnitudes.

Aldebaran, image: NASA, ESA, and STScI

The star’s MOLsphere – the molecular layer beyond the chromosphere – stretches across a distance 2.5 times the star’s radius and has a temperature of 1,500 K. Lines of water, carbon monoxide and titanium oxide have been detected in its spectrum. Aldebaran’s stellar wind expands beyond the MOLsphere until it reaches the boundary with the interstellar medium, where it slows down to subsonic speed. It forms an astrosphere that extends for about 1,000 astronomical units from the star.

Aldebaran has several faint visual companions. The first one was discovered by the German-born British astronomer William Herschel in 1782. It is an 11th magnitude star separated by 117’’ from Aldebaran. In 1888, a full century later, American astronomer Sherburne Wesley Burnham observed the star and found that it was a close binary system. Burnham found another companion, a 14th magnitude star, at a separation of 31’’. The companion discovered by Herschel does not have a similar proper motion to Aldebaran, but the second star has almost the same parallax and proper motion as Aldebaran, indicating that the two stars are physically related and form a wide binary system. However, as the companion appears so close to the bright Aldebaran, it is difficult to unambiguously confirm that the two stars are related.

This image shows the Hyades star cluster, the nearest cluster to us. The Hyades cluster is very well studied due to its location, but previous searches for planets have produced only one. A new study led by Jay Farihi of the University of Cambridge, UK, has now found the atmospheres of two burnt-out stars in this cluster — known as white dwarfs — to be “polluted” by rocky debris circling the star. Seeing evidence of asteroids points to the possibility of Earth-sized planets in the same system, as asteroids are the building blocks of major planets. Planet-forming processes are inefficient, and spawn many times more small bodies than large bodies — but once rocky embryos the size of asteroids are built, planets are sure to follow. Image: NASA, ESA, and STScI, 2013

Aldebaran’s line-of-sight companions have been designated Alpha Tauri B, C, D, E, and F in the order of discovery. Alpha Tauri B has the stellar classification M2.5 and an apparent magnitude of 13.60. Alpha Tauri C is the brightest of the companions, with a visual magnitude of 11.30. Alpha Tauri D, E and F have apparent magnitudes of 13.70, 12.00 and 13.60. Alpha Tauri C and Alpha Tauri D are gravitationally bound to each other and form a binary system. They are located at a much greater distance from us than Aldebaran and belong to the Hyades cluster. None of the five companions have been confirmed as physical companions to Aldebaran.

Size

Aldebaran is 44.13 times larger than the Sun, with a diameter of about 61.4 million kilometres. While it also larger than the fellow class K giants Pollux (8.8 solar radii), Arcturus (25.4 R_☉) and Unukalhai (12 R_☉), it does not rival the supergiants Antares (680 – 800 R_☉), Rigel (78.9 R_☉) and Betelgeuse (887 R_☉), and it certainly does not come even close to the red supergiant UY Scuti (1,708 R_☉), currently the largest star known.

Size comparison of planets and stars, image: Wikimedia Commons/ Dave Jarvis, Jcpag2012, JoeyPknowsalotaboutthat (CC BY-SA 4.0)

Planet

The presence of a planet orbiting Aldebaran was proposed in 1993, but its existence was only hypothetical until 2015.

In 1993, a study measuring long-period radial velocity variations in three class K giants – Aldebaran, Arcturus and Pollux – found periods of 643 days for Aldebaran, 233 days for Arcturus and 558 days for Pollux. These periods are too long to be a result of radial pulsations and can only be explained by surface features coupled with rotation, non-radial pulsations, or the presence of a planet. However, the study concluded that the cause was likely intrinsic to the stars. Based on its findings, a hypothetical planet orbiting Aldebaran would have a mass of at least 11.4 Jupiter masses and orbit the star with a period of 643 days from a distance of 2 astronomical units.

In 2015, a team investigated the nature of the radial velocity variations, analysing precise measurements taken over a period of 30 years, and found evidence for both rotation modulation by stellar surface structure and an orbiting planet with a mass of at least 6.47 Jupiter masses and an orbital semi-major axis of about 1.46 astronomical units.

In 2018, an asteroseismic study determined a mass of at least 5.8 ± 0.7 Jupiter masses for Aldebaran b, also concluding that, while the planet is now likely very hot – the estimated temperature is 1,500 K due to its proximity to the giant host star – it probably had a similar temperature to that of the Earth before Aldebaran evolved away from the main sequence, and was possibly habitable billions of years ago. The planet orbits the star from a distance of 1.46 astronomical units.

In 2019, the planet’s existence was cast into doubt by an analysis of new radial velocity data collected by the Lick Observatory in California, which did not support the presence of the planet. The study suggested that two planets with at least several Jupiter masses would fit the data more closely, but would not explain the radial velocity variations observed in the star, concluding that the variations were likely caused by an intrinsic property of the star and not gravitational interaction with a planetary companion.

Facts

Aldebaran is one of the two stars in Taurus included on the list of the 58 navigational stars. The constellation’s other navigational star is Elnath, Beta Tauri.

Aldebaran was the brightest star in the sky for about 200,000 years between the years 420,000 and 210,000 BCE. The star came within 21.5 light years of the solar system and, at its maximum, was slightly brighter than Sirius is today (mag. -1.46), shining at magnitude -1.54.

Aldebaran is the fourth brightest star in the sky in near-infrared wavelengths. With a J-band magnitude of -2.1, it is only outshone by Betelgeuse (-2.99), R Doradus (-2.6) and Arcturus (-2.2).

Aldebaran is one of the bright stars whose movement across the sky led to the discovery of proper motion. In 1718, the English astronomer and mathematician Edmund Halley studied the records of the lunar occultation of the star that occurred on March 11, 509 CE and was seen from Athens, Greece. He found that the star had moved several arcminutes to the north since that time. Halley compared his measurements with the data Ptolemy provided in his Almagest in the 2nd century CE and noted that Sirius and Arcturus has also moved significantly, realizing that what were previously believed to be “fixed” stars were not so fixed after all.

Aldebaran is used as the spectral standard for its class, K5+ III, in The Perkins Catalog of Revised MK Types for the Cooler Stars (1989).

The first person to study Aldebaran’s spectrum was the English astronomer and astronomical spectroscopy pioneer Sir William Huggins, who observed the star from his private observatory in Tulse Hill, London in 1864. Huggins was able to identify nine elements in Aldebaran’s spectral lines, including iron, sodium, calcium and magnesium.

American astronomer and physicist Edward Charles Pickering, who is credited for the discovery of the first spectroscopic binary star systems, captured 50 absorption lines in Aldebaran’s spectrum using a photographic plate at Harvard College Observatory, where he was director, in 1886. His findings were published in the Draper Catalogue of Stellar Spectra in 1890.

Aldebaran is one of the most studied stars in the sky. Its substantial record of observations and studies led to it being used to calibrate the instruments of the Hubble Space Telescope. Aldebaran is also one of the 33 stars used as benchmarks to calibrate the stellar parameters during the Gaia space observatory mission (2013 – 2022), launched to take precise astrometric measurements and create the largest 3D space catalogue of the Milky Way to date.

The surface gravity of Aldebaran is 1.59 cgs, about 25 times lower than our planet’s and 700 times lower than the Sun’s, but not unusual for a giant star.

Aldebaran lies only 5.47 degrees south of the ecliptic, the Sun’s apparent path across the sky, and can be occulted by the Moon. An occultation that occurred on September 22, 1978 allowed astronomers to get a fairly accurate estimate for the star’s diameter. Between January 29, 2015 and September 3, 2018, 49 occultations of the star could be seen from locations near the equator and in the northern hemisphere. The Sun comes close to Aldebaran around June 1 every year.

About 5,000 years ago, Aldebaran was close to the vernal equinox, the instant when the Sun crosses the celestial equator, marking the beginning of spring in the northern hemisphere. Due to the Earth’s axial precession, the star has since moved away.

Aldebaran’s recession velocity was first calculated as 48 km/s by the German astrophysicist Hermann Carl Vogel and his assistant, the astronomer Julius Scheiner, at the Astrophysical Observatory Potsdam in Germany in the late 1880s.

In 1921, American astronomer Francis G. Pease used a 20-foot interferometer and the 100-inch Hooker refractor at the Mount Wilson Observatory in California to measure the angular diameters of Aldebaran, Arcturus, and other stars, but was unable to resolve the star.

Aldebaran’s angular diameter has been measured many times since. The most recent value is 20.580 ± 0.030 milliarcseconds, provided for the Gaia benchmark calibration in 2015.

Aldebaran is the brightest of all stars in zodiac constellations. It narrowly outshines Antares in Scorpius and is also brighter than first magnitude stars Spica in Virgo, Pollux in Gemini and Regulus in Leo.

In medieval astrology, Aldebaran was one of the 15 Behenian fixed stars, believed to be a source of special astrological power, and it was associated with the planets Venus and Mars.  Each star was connected with a plant and a gemstone – in Aldebaran’s case, milk thistle and ruby – and these would be used in rituals to bring out the star’s influence.

The space Probe Pioneer 10, launched in 1972, is headed in the general direction of the star. It would take more than two million years to make its closest approach to Aldebaran if the star had zero relative velocity.

Aldebaran has been used or referenced in countless works of fiction. Some of the best known uses include H. P. Lovecraft’s The Cthulhu Mythos (1921- ), E. E. “Doc” Smith’s novels in the Lensman series (1934 –1948), Leigh Brackett’s The Starmen (1952), Alfred Bester’s The Stars My Destination (1956), Stanisław Lem’s short story collection The Invasion from Aldebaran (1959), Ursula K. Le Guin’s novel The Lathe of Heaven (1971), Joe Haldeman’s The Forever War (1974), Douglas Adams’ The Hitchhiker’s Guide to the Galaxy (1979), Yoshiki Tanaka’s novel series Legend of the Galactic Heroes (1982-1987), Frederik Pohl’s novel Narabedla Ltd. (1988), Kim Stanley Robinson’s Blue Mars (1996), Peter F. Hamilton’s Fallen Dragon (2001), and Keith Hamilton’s Johnny Mackintosh: Star Blaze (2010).

On television, the star was memorably referenced in multiple episodes of Star Trek, among others in the episodes “Where No Man Has Gone Before” (1966) and “Amok Time” (1967) of Star Trek: The Original Series, and in the episode “Hide and Q” (1987) of Star Trek: The Next Generation.

Name

The name Aldebaran (pronunciation: /ælˈdɛbərən/) comes from the Arabic al Dabarān, meaning “the follower.” It refers to the star following the bright Pleiades cluster across the sky. The name is derived from the phrase Nā᾽ir al Dabarān, “the bright one of the follower,” which was the star’s original name.

The name was officially approved by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on June 30, 2016.

The Chinese know Aldebaran as 畢宿五 (Bì Xiù wǔ), or the Fifth Star of Net. The Chinese Net asterism is formed by Aldebaran with Ain (Epsilon Tauri), Prima Hyadum (Gamma Tauri), Secunda Hyadum (Delta¹ Tauri), Delta³ Tauri, Chamukuy (Theta¹ Tauri), 71 Tauri and Lambda Tauri.

In Hindu astronomy, the star is associated with the Rohini (“the red one” or “red deer”) lunar mansion. In Hindu mythology, Rohini was one of the daughters of Daksha, the son of Lord Brahma who created the nakshatras (lunar mansions), and wives of Chandra, the Moon God, who married all her sisters at her father’s request even though he only wanted to marry her.

In ancient Greece, the star was known as Λαμπαδίας (Lampadias), which means “torch bearer” or “torch-like.”

In Babylonian astronomy, Aldebaran marked an asterism known as Pidnu-sha-Shame, the Furrow of Heaven.

The ancient Persians regarded it as one of the four Royal Stars of Persia. The four stars – Aldebaran, Regulus, Fomalhaut and Antares – were seen as the guardians of the sky. In Persian astronomy, the sky was divided into four districts and each was guarded by one of the stars. Aldebaran was the Watcher of the East.

The Seri people of Mexico have several names for Aldebaran: Queeto, Hant Caalajc Ipápjö, and Azoj Yeen oo Caap. The last one means “the star that goes ahead.” In local lore, the star gives light for seven women giving birth, represented by the Pleiades cluster. The month that corresponds to October is known as Queeto yaao, meaning “Aldebaran’s path.”

The Aboriginal people of the Clarence River in New South Wales saw the star as the Ancestor Karambal. In local lore, Karambal was a man who fell in love with Meamei, one of the Pleiades, but when he carried her off to be his wife, her sisters forced him to release her. After this episode, he stole the wife of Bullabogabun, a great warrior, who followed the lovers’ tracks and quickly overtook them. Karambal left the warrior’s wife and climbed a tall tree to hide. Bullabogabun burned the tree down and Karambal was lifted up into the sky by flames and became Aldebaran, where he still pursues the Pleiades through the sky.

Pleiades, Hyades and Aldebaran, image: Giuseppe Donatiello (CC0 1.0)

Location

Aldebaran is very easy to find and identify because it is one of the brightest stars in the sky and also because it has one of the best known asterisms – the Orion’s Belt – pointing at it. Aldebaran is the brightest star in the region between the Belt and the Pleiades. A line extended from the Belt stars, Alnitak, Alnilam and Mintaka, in the direction of the Pleiades leads directly to the star. In the opposite direction, the three stars point toward Sirius, the brightest star in the sky.

Aldebaran location, image: Wikisky

Aldebaran is one of the six first-magnitude stars that form the Winter Hexagon. Also known as the Winter Circle, the asterism dominates the evening sky during the northern hemisphere winter, containing the Winter Triangle and most of the constellation Orion within its borders. The stars that form the Winter Hexagon are Aldebaran, Capella in the constellation Auriga, Pollux in Gemini, Procyon in Canis Minor, Sirius in Canis Major, and Rigel in Orion.

The Winter Triangle and the Winter Hexagon, image: Wikisky

Aldebaran lies in the same line of sight as the Hyades, the nearest open star cluster to Earth. The V-shaped cluster outlines the head of the celestial Bull and Aldebaran appears as its brightest member even though it is not. The star lies only 65.3 light years away, while the cluster is located at more than twice the distance (153 light years).

Aldebaran, the Hyades and the Pleiades, image: Wikisky

Constellation

Aldebaran is located in the zodiac constellation Taurus. Representing the celestial bull, Taurus is best known for its two bright, nearby open star clusters – the Pleiades (Messier 45) and the Hyades – as well as for several other notable deep sky objects. These include the Crab Nebula (Messier 1), a historic supernova remnant, the supernova remnant Sh2-240, also known as the Spaghetti Nebula, Hind’s Variable Nebula (NGC 1555), a young reflection nebula illuminated by the variable pre-main sequence star T Tauri, the Crystal Ball Nebula (NGC 1514), a planetary nebula that made William Herschel reconsider his beliefs about the starry nature of these objects, and the colliding galaxies NGC 1410 and NGC 1409.

Taurus constellation map by IAU and Sky&Telescope magazine

The best time of year to observe the stars and deep sky objects of Taurus is during the month of January, when the constellation rises high in the evening sky.

The 10 brightest stars in Taurus are Aldebaran (Alpha Tau, mag. 0.86), Elnath (Beta Tau, mag. 1.65), Alcyone (Eta Tau, mag. 2.87), Tianguan (Zeta Tau, mag. 2.97), Chamukuy (Theta² Tauri, mag. 3.40), Lambda Tauri (mag. 3.47), Ain (Epsilon Tau, mag. 3.53), Omicron Tauri (mag. 3.61), Atlas (27 Tau, mag. 3.63), and Prima Hyadum (Gamma Tau, mag. 3.654).

Aldebaran – Alpha Tauri