Alioth

Alioth, Epsilon Ursae Majoris (ε UMa), is the brightest of the seven stars of the Big Dipper and the brightest star in the constellation Ursa Major. It has an apparent magnitude of 1.77 and lies at a distance of about 82.6 light years from Earth. It is the star in the Big Dipper’s handle closest to the bowl.

Alioth is the 31st brightest star in the sky. It shares the 31st place with Alnitak, one of the three stars of Orion’s Belt. The two stars are only slightly fainter than Miaplacidus in the constellation Carina, Alnilam in Orion and Alnair in Grus, and they just outshine Mirfak in Perseus, Dubhe in Ursa Major and Regor in Vela.

Star type

Alioth has the stellar classification A1III-IVp kB9, indicating a white or blue-white star, either a giant or subgiant, with a peculiar (“p”) spectrum. The “kB9” suffix in the star’s spectral class refers to the presence of the calcium K line, representative of the spectral type B9, but the star’s spectrum is mostly consistent with the class A.

Alioth (Epsilon Ursae Majoris), image: Wikisky

Alioth has a mass 2.91 times that of the Sun and a radius 4.14 times solar. With a surface temperature of 9,020 K, it shines with a luminosity 102 times that of the Sun. The star’s estimated age is around 300 million years.

Alioth is the brightest peculiar class A star in the sky. Its spectrum is consistent with an Alpha² Canum Venaticorum variable star. These are chemically peculiar stars still on the main sequence, belonging to the spectral class B8p to A7p, with strong magnetic fields and strontium, silicon or chromium spectral lines. Their spectral lines and magnetic fields vary, as does their brightness, and the periods of their variations are believed to correspond to their rotation periods.

Alioth’s magnetic field separates different elements in the star’s hydrogen supply and the star’s axis of rotation is positioned at an angle to its magnetic axis and believed to be spinning different elements into different regions that come in and out of our line of sight as the star rotates. These elements react differently as they spin in and out of view and, as a result, we see the star’s spectral lines fluctuating during its rotation period.

Alioth’s rotational and magnetic poles are positioned at almost a 90-degree angle to one another. The abundance of oxygen is about 100,000 times greater around the magnetic equator than near the magnetic poles and chromium is more densely concentrated in a band positioned at a right angle to the equator.

Alpha² Canum Venaticorum variables were named after the prototype for this class of stars, known by the proper name Cor Caroli. (Cor Caroli is the brightest star in the constellation Canes Venatici, located in the same area of the sky as Ursa Major, just under the handle of the Big Dipper.) Alioth has a very weak magnetic field compared to other variable stars in this class. Its magnetic field is about 15 times weaker than Cor Caroli‘s, but still about 100 times stronger than the Earth’s. Alioth has a rotation period of 5.1 days and both its magnetic field and brightness change during this period.

Facts

Alioth is a member of the Ursa Major Moving Group (Collinder 285), a stellar association that includes a number of bright stars, including most of those that form the Big Dipper (Alkaid and Dubhe are the only exceptions). These stars are believed to have a common origin and share common velocities in space. All the stars are thought to have formed about 300 million years ago. The core of the group lies approximately 80 light years away. The brighter members include Menkalinan (Beta Aurigae), Skat (Delta Aquarii), Gamma Leporis, Alphecca (Alpha Coronae Borealis) and Beta Serpentis.

Alioth is listed as one of the 58 navigational stars, along with its Big Dipper neighbours Dubhe (Alpha Ursae Majoris) and Alkaid (Eta Ursae Majoris).  The stars selected for navigation are typically both bright and easy to identify because they either belong to a prominent asterism or are located near a recognizable pattern. Alioth is both bright enough to be visible even from light-polluted areas and easy to identify because it is a part of a familiar asterism, the Big Dipper.

Big Dipper stars, image: Wikisky

Alioth has the Greek letter designation Epsilon, not Alpha, even though it is Ursa Major’s brightest star. When German uranographer Johann Bayer assigned Greek letter designations to stars in his star atlas Uranometria (1603), he mostly followed the magnitude class rule, assigning the letters Alpha and Beta to any first magnitude stars in a constellation. However, he did not necessarily assign letters in the order of brightness and, in Ursa Major, he named the stars from west to east. For this reason, Alioth is designated Epsilon Ursae Majoris even though it is the constellation’s brightest star, and the Alpha designation went to Dubhe, the second brightest star in Ursa Major and the westernmost star of the Big Dipper.

Name

The name Alioth (pronunciation: /ˈæliɒθ/) is derived from the Arabic alyat al-hamal, meaning “the sheep’s fat tail.” The name was officially approved for Epsilon Ursae Majoris A by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on June 30, 2016.

The Chinese know the star as the Fifth Star of Northern Dipper (北斗五), referring to the Chinese asterism Northern Dipper (北斗), which corresponds to the Big Dipper. The Chinese also call Alioth Yù Héng (玉衡), or the Star of Jade Sighting-Tube.

The star’s Hindu name is Angiras, after a Vedic sage mentioned by some sources as one of the Saptarishi (seven rishis or sages). Angiras was said to teach divine knowledge and mediate between gods and humans. He was considered to be the first of the fire gods, or Agni-devas.

In the Arabic-speaking world, Alioth, Mizar and Alkaid (the stars of the Great Bear‘s tail) represented mourners gathered around a bier marked by Megrez, Phecda, Dubhe and Merak (the stars of the Big Dipper’s bowl). The constellation represented a funeral procession, with Alioth, Mizar and Alkaid representing the children of Al Na’ash, who met his end at the hands of Al Jadi, represented by Polaris. The slow motion of the Big Dipper stars around the north celestial pole represented the children of Al Na’ash circling Al Jadi, wanting vengeance.

Location

Alioth is very easy to find because it is part of one of the best known asterisms in the night sky. It is the third star of the Big Dipper‘s handle, closest to the bowl, and noticeably brighter than most of its neighbours.

Alioth lies in the same area of the sky as the bright spiral galaxy Messier 106, located in the neighbouring constellation Canes Venatici, and the famous Pinwheel Galaxy (Messier 101), a large, face-on spiral galaxy with an apparent magnitude of 7.86, which can be observed in larger telescopes. The galaxies can be found using Alioth’s neighbours Mizar and Alkaid.

Big Dipper and the Pinwheel Galaxy, image: Wikisky

The stars of the Big Dipper’s handle can be used to find two other bright stars, Arcturus and Spica. Arcturus, Alpha Boötis, is the brightest northern star and the fourth brightest star in the sky, while Spica, Alpha Virginis, is the 16th brightest star and the luminary of the constellation Virgo. Arcturus and Spica can be found by following the arc of the Dipper’s handle away from the bowl. Arcturus is the first bright star that appears along this imaginary line and Spica, the second.

The Big Dipper, Arcturus and Spica, image: Wikisky

Constellation

Alioth is located in the constellation Ursa Major. It is one of the stars in the bear’s tail, closest to its rear end.

Ursa Major is the largest northern constellation and the third largest of all 88 constellations, after Hydra and Virgo. It contains a number of well-known deep sky objects. Famous galaxies within its borders include the Pinwheel Galaxy (M101), Bode’s Galaxy (M81), the Cigar Galaxy (M82), and the barred spiral galaxies Messier 108 and Messier 109. The constellation is also home to the bright (mag. 9.9) planetary nebula Messier 97, also known as the Owl Nebula. Many of these objects can be located using the bright stars of the Big Dipper.

Ursa Major constellation map by IAU and Sky&Telescope magazine

The Big Dipper stars are circumpolar to most northern locations and can be seen throughout the year. The best time of year to observe them is during the month of April, when Ursa Major is particularly prominent in the evening sky.

The 10 brightest stars in Ursa Major are Alioth (Epsilon UMa, mag. 1.77), Dubhe (Alpha UMa, mag. 1.79), Alkaid (Eta UMa, mag. 1.86), Mizar (Zeta UMa, mag. 2.04), Merak (Beta UMa, mag. 2.37), Phecda (Gamma UMa, 2.438), Psi Ursae Majoris (mag. 3.01), Tania Australis (Mu UMa, mag. 3.06), Talitha (Iota UMa, mag. 3.14), and Theta Ursae Majoris (mag. 3.166).

Alioth – Epsilon Ursae Majoris