Star Facts: Alkaid

Alkaid (Eta Ursae Majoris)
Image Credit: Tom Wildoner

Alkaid (Eta Ursae Majoris) is a prominent star in the handle of the Big Dipper, an asterism which forms just a small part of the overall constellation of Ursa Major. With an apparent magnitude of +1.84, it is the third most luminous star in Ursa Major, and the 38th most luminous star in the entire night sky. However, note that even though Alkaid forms a part of the Big Dipper asterism, the star is not a member of the Ursa Major Moving Group, which means that its differential motion is hastening the dispersion of the Big Dipper asterism. In fact, Alkaid is traveling in the opposite direction from the star Dubhe at the far top of the bowl, meaning 50,000 years or so from now the asterism will resemble more a tadpole than a saucepan.

Quick Facts

• Constellation: Ursa Major
• Coordinates: RA 13h 47m 32.43776s |Dec. +49° 18′ 47.7602″
• Distance: 103.9 light years
• Star Type: B3 V
• Mass: 6.1 sol
• Radius: 3.4 sol
• Apparent Magnitude: +1.86
• Luminosity: 580 to 1,350 sol
• Surface Temperature: 15,700K to 17,900K
• Rotational Velocity: 166 km/sec
• Age: 10 to 15 million years
• Other Designations: Benetnash, Benetnasch, Elkeid, 85 UMa, BD+50°2027, FK5 509, GC 18643, HD 120315, HIP 67301, HR 5191, PPM 53742, SAO 44752.

Visibility

Ursa Major is a northern circumpolar constellation that can be seen from latitudes of between +90° and -30°. It is difficult to miss Alkaid, which can be found simply by looking at the end of the Big Dipper’s handle, at the point furthest away from the Dipper’s bowl.

Physical Properties

Located about 104 light years away, Alkaid is a young blue main sequence dwarf star with a B3 V stellar classification, and since 1943, its spectrum has served as a standard against which other similar stars are classified. However, despite this, there are some ambiguities around its mass, radius, color, and temperature that remain unresolved.

For instance, while models of stellar evolution predict that Alkaid should weigh in at five solar masses, various methods of determining stellar mass suggest that the star actually weighs 6.1 times as much as the Sun. Similarly, while Alkaid shines with the blue-white hue that is typical of its spectral class, its temperature falls into the low-end of the typical range for B-type stars, which translates into the fact that Alkaid is too “red” for its class, and not as blue as one would expect.

In addition, while the stars’ radius as obtained through interferometry turns out to be 3.2 times that of the Sun, which yields a luminosity of 1,350 sol, calculations based on distance and mean temperature show that Alkaid is only 2.9 sol, which radius yields a luminosity of only 580 times that of the Sun. The exact reason for these discrepancies is not known with any degree of certainty, although most investigators agree that gravity darkening likely plays a role in the star’s overall temperature.

Further note that there is some uncertainty about whether or not Alkaid is a source of x-rays. While most sources list the star as having an x-ray luminosity of around 9.3 × 1028 ergs, several other sources list the star as being below the limit at which it can emit any radiation in x-ray frequencies. Alkaid is also “oddly” placed; unlike the other stars in the Big Dipper asterism that are located in the galactic disc, Alkaid is located significantly above the galactic disc, and as such, it does not share the common proper motion of the central stars in the asterism.

History

The name Alkaid derives from the Arabic phrase meaning “The leader of the daughters of the bier”, as a reference to the mourning maidens that accompanied the funeral bier in ancient times, which was the movable frame on which a coffin was placed and carried on its way to the grave. For context, the stars Megrez, Phecda, Merak, and Dubhe in the Big Dipper asterism form the funeral bier itself.

In China, Alkaid is known as the Seventh Star of the Northern Dipper, or the Star of Twinkling Brilliance, while in Korea and Japan, Alkaid is known as “the military breaking star” or “most corner star” respectively; both meanings have their roots in ancient China’s influence in both cultures.

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