Alioth
Brightest star in Ursa Major, a peculiar magnetic variable.
Alioth, also designated Epsilon Ursae Majoris, is the brightest star in the northern constellation Ursa Major and the thirty-third brightest star in the sky at magnitude 1.77. Despite its Bayer designation of epsilon, it outshines all other stars in the constellation. It is the star in the tail of the bear closest to its body, and thus the star in the handle of the Big Dipper (or Plough) closest to the bowl. It is a member of the large and diffuse Ursa Major moving group and was historically used in celestial navigation in the maritime trade, listed as one of the 57 navigational stars.
- Bayer designation
- ε Ursae Majoris
- Apparent magnitude
- 1.77
- Distance from sun
- 81 ly (Hipparcos)
- Spectral type
- A1p (with kB9 suffix)
- Rotation period
- 5.1 days
- Magnetic field strength
- 100 times stronger than Earth's (15 times weaker than α Canum Venaticorum)
- Traditional name
- Alioth (from Arabic 'alyat al-hamal', 'the sheep's fat tail')
Lore & Background
Alioth's spectral type is A1p, where the 'p' stands for peculiar, as its spectrum is characteristic of an α2 Canum Venaticorum variable. As a representative of this type, it may harbor two interacting processes: a strong magnetic field separating different elements in its hydrogen 'fuel', and a rotation axis at an angle to the magnetic axis that spins different bands of magnetically sorted elements into the line of sight. The intervening elements react differently at different frequencies of light, causing very strange spectral lines that fluctuate over a period of 5.1 days. The kB9 suffix indicates that the calcium K line is present and representative of a B9 spectral type even though the rest of the spectrum indicates A1. Its rotational and magnetic poles are at almost 90 degrees to one another, and darker (denser) regions of chromium form a band at right angles to the equator.
Reader's Guide
Alioth has long been suspected of being a spectroscopic binary, possibly with more than one companion. A 2008 study suggested its 5.1-day variation might be due to a substellar object of about 14.7 Jupiter masses in an eccentric orbit (e=0.5) with an average separation of 0.055 astronomical units. However, it is now thought that the 5.1-day period is the rotation period of the star, and no companions have been detected using the most modern equipment. Observations with the Navy Precision Optical Interferometer also did not detect a companion. The star has a relatively weak magnetic field for a variable of this type, 15 times weaker than α Canum Venaticorum, but still 100 times stronger than that of the Earth. Historically, it was used in celestial navigation in the maritime trade. The traditional name Alioth comes from the Arabic 'alyat al-hamal' ('the sheep's fat tail'). In 2016, the International Astronomical Union's Working Group on Star Names approved Alioth as the star's proper name. The star was known to the Hindus as Añgiras, one of the Seven Rishis. In Chinese, the star is part of the Northern Dipper asterism. The United States Navy's Crater class cargo ship USNS Alioth was named after the star.
Did You Know?
- Despite being designated epsilon, Alioth is the brightest star in Ursa Major.
- Its magnetic field is 100 times stronger than Earth's but 15 times weaker than α Canum Venaticorum.
- A 2008 study proposed a 14.7 Jupiter-mass substellar object as the cause of its 5.1-day variation, but this is now thought to be the star's rotation period.
More in Variable Stars, Part 5 1-24
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