Epsilon Ursae Majoris
From Wikipedia, the free encyclopedia
 Alioth in Ursa Major. |
|
| Observation data Epoch J2000 |
|
|---|---|
| Constellation (pronunciation) |
Ursa Major |
| Right ascension | 12h 54m 01.6s |
| Declination | +55° 57′ 35.4″ |
| Apparent magnitude (V) | 1.76 |
| Characteristics | |
| Spectral type | A0pCr |
| U-B color index | 0.02 |
| B-V color index | -0.02 |
| Variable type | alpha2-CVn |
| Astrometry | |
| Radial velocity (Rv) | -9.3 km/s |
| Proper motion (μ) | RA: 111.74 mas/yr Dec.: -8.99 mas/yr |
| Parallax (π) | 40.3 ± 0.62 mas |
| Distance | 81 ± 1 ly (24.8 ± 0.4 pc) |
| Absolute magnitude (MV) | -0.22 |
| Details | |
| Mass | ~3 M☉ |
| Radius | 3.7 R☉ |
| Luminosity | 108 L☉ |
| Temperature | 9,400 K |
| Metallicity | ? |
| Rotation | 38 km/s. |
| Age | ? years |
| Other designations | |
Epsilon Ursae Majoris (ε UMa / ε Ursae Majoris) is the brightest star in the constellation Ursa Major (despite its Bayer designation being merely "epsilon"), and at magnitude 1.76 is the thirty-first brightest star in the sky. It also has the traditional name Alioth (from the Arabic word alyat—fat tail of a sheep).
It is known as 北斗五 (the Fifth Star of the Northern Dipper) or 玉衡 (the Star of Jade Sighting-tube) in Chinese.
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 closest to the bowl. It is also a member of the large and diffuse Ursa Major moving group. Historically, the star was frequently used in celestial navigation in the maritime trade, because it is listed as one of the 57 navigational stars.[1]
According to Hipparcos, Alioth is 81 light years (25 parsecs) from Earth. Its spectral type is A0p; the "p" stands for peculiar, as the spectrum of its light is quite odd, of a kind characteristic of an Alpha2 Canum Venaticorum variable. Alioth, as a representative of this type, is believed to look the way it does because of two interacting processes: first, the star's strong magnetic field separating different elements salting the star's hydrogen fuel, then a rotation axis at an angle to the magnetic axis spinning different bands of magnetically sorted elements into the line of sight between Alioth and the Earth. The intervening elements react differently at different frequencies of light as they whip in and out of view, causing Alioth to have very strange spectral lines that fluctuate over a period of 5.1 days. In the case of Alioth, the rotational and magnetic axes are at almost 90 degrees to one another; in the map of Alioth linked below, note how the darker (more dense) regions of chromium form a band at right angles to the equator. A recent study suggests the 5.1 days variation may be due to a substellar object with around 14.7 Jupiter masses in eccentric orbit (e=0.5) and average separation of 0.055 Astronomical units.
For its type, Alioth has a relatively weak magnetic field (15 times weaker than α CVn's), but it is still 100 times stronger than that of the Earth.
[edit] See also
[edit] References
- ^ This article incorporates content from the 1728 Cyclopaedia, a publication in the public domain.
