Star Facts: Alioth

Star Facts: Alioth
Image Credit: Jerry Lodriguss

Alioth (Epsilon Ursae Majoris), despite its “epsilon” tag, is the most luminous star in Ursa Major but only just since at magnitude +1.76 it is a mere 2% brighter than the star Dubhe, the second brightest star in the constellation. Situated 82.6 light years from Earth, this blue star is also the 33rd brightest star in the entire night sky, and is listed as one of the 57 navigational stars, which are useful in obtaining a celestial fix position, as well as determining true north and not just magnetic north, with the help of a ship’s gyroscope. Alioth has the distinction of being a member of the Ursa Moving Group of Stars, a large, diffuse group of stars that share a largely common motion across the sky.

Quick Facts

• Constellation: Ursa Major
• Coordinates: RA 12h 54m 01.74959s |Dec. +55° 57′ 35.3627″
• Distance: 82.6 light years
• Star Type: A1III-IVp kB9
• Mass: 2.91 sol
• Radius: 4.2 sol
• Apparent Magnitude: +1.77
• Luminosity: 108 sol
• Surface Temperature: 10,800K
• Rotational Velocity: 33 km/sec
• Age: Undetermined
• Other Designations: Alioth, Allioth, Aliath, e UMa, 77 Ursae Majoris, BD+56°1627, FK5 483, GC 17518, HD 112185, HIP 62956, HR 4905, PPM 33769, SAO 28553

Visibility

Since the Big Dipper asterism, and by extension the constellation Ursa Major, is north circumpolar the star Alioth is visible for almost the whole night the year round. Look for Alioth in the Big Dipper’s handle, as the star closest to the Dipper’s bowl.

Physical Properties

Alioth is classified as an “Alpha Canum Venaticorum” variable star, with a spectral classification of A1p. In the modern system of stellar classification, the “p” refers to its spectrum being “peculiar”, which is characteristic of an Alpha2 Canum Venaticorum variable-type star. The peculiarity of its spectrum hinges on the fact that in stars of this type, two distinctly separate, but interacting processes may be at work:

1) The star’s magnetic field appears to be separating some of the constituents in its hydrogen fuel, with some elements sinking towards the core as the result of gravity, while others are being pushed upward by radiation pressure.

2) The stars’ magnetic field, which is oriented at almost a right angle relative to the stars’ plane of rotation, appears to be “collecting” some elements, such as chromium, along the magnetic field thus creating spots on the star that react differently to different frequencies of light. The net result of this is that the star’s spectrum contains peculiar lines that are brought to the fore as the star rotates once every 5.1 days. The last part of Alioth’s classification, the suffix “kB9”, refers to the fact that the star’s spectrum exhibits the presence of the calcium K line, which is representative of B9 spectral-type stars, even though the rest of the stars’ spectrum dictates that Alioth is an A1-type star.

More importantly though, Alioth turns out to be the brightest of the Ap-type stars, which are magnetic stars in which some elements are either enhanced, or depleted in different parts of the star. For instance, Alioth’s magnetic equator shown an abundance of oxygen that is about 100,000 times higher than at the magnetic polar regions, which regions, it must be noted, is displaced relative to the rotational poles and equator. Other heavy elements, such as chromium and europium (a type of rare earth) display similar displacements, although not quite to the same degree. Also note that while Alioth is the brightest of the Ap-type stars its magnetic field is among the weakest known for this type of star. In fact, it is fully 15 times weaker than the magnetic field of the star Cor Caroli (Alpha Canum Venaticorum), the brightest star in the constellation Canes Venatici, and the prototype for this type of variable star.

Despite the observed processes that cause the brightness variations in Alioth, a recent study is suggesting that the star’s observed variability may be caused by a 14.7 Jupiter-mass object that is orbiting the star once every 5.1 days in an eccentric orbit with an average separation of 0.055 astronomical units.

History

Alioth’s traditional name derives from the Arabic word “alya”, which means “fat tail of a sheep”. However, some sources and authorities hold that the name is a mistranslation, and that it refers to a black horse, instead. In Chinese astronomy, Alioth, along with the stars Alpha Ursae Majoris, Beta Ursae Majoris, Gamma Ursae Majoris, Delta Ursae Majoris, Zeta Ursae Majoris and Eta Ursae Majoris, forms part of an asterism known as “Bei Dou”, which translates into English as “[the] Northern Dipper”. Due to its location within this asterism, Epsilon Ursae Majoris is known either as “Bei Dou wu”, or as “Yù Héng”, which means “[the] Fifth Star of Northern Dipper”, or “Star of Jade Sighting-Tube”, respectively.

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