Gamma Cassiopeiae
 Location of γ Cassiopeiae (circled) |
|
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Cassiopeia |
| Right ascension | 00h 56m 42.50108s |
| Declination | +60° 43′ 00.2984″ |
| Apparent magnitude (V) | 2.47(1.6 - 3.0) |
| Characteristics | |
| Spectral type | B0.5IVe |
| U−B color index | –1.08 |
| B−V color index | –0.15 |
| Variable type | γ Cas |
| Astrometry | |
| Radial velocity (Rv) | –6.8 km/s |
| Proper motion (μ) |
RA: +25.17 mas/yr Dec.: –3.92 mas/yr |
| Parallax (π) | 5.94 ± 0.12mas |
| Distance | 550 ± 10 ly (168 ± 3 pc) |
| Details | |
| Mass | 17 M☉ |
| Radius | 10 R☉ |
| Luminosity | 34,000 L☉ |
| Surface gravity (log g) | 3.50 cgs |
| Temperature | 25,000 K |
| Rotational velocity (v sin i) | 432 km/s |
| Age | 8.0 ± 0.4 Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Gamma Cassiopeiae (γ Cas, γ Cassiopeiae) is the star at the center of the distinctive "W" asterism in the northern circumpolar constellation of Cassiopeia. Although it is a fairly bright star with an apparent visual magnitude that varies from 1.6 to 3.0, it has no traditional Arabic or Latin name. The Chinese name Tsih has been increasingly used since the end of the 20th century.
Gamma Cassiopeiae is a Be star, a variable star, and a binary. Based upon parallax measurements made by the Hipparcos satellite, it is located at a distance of roughly 550 light-years from Earth.
Gamma Cassiopeiae is an eruptive variable star, whose apparent magnitude changes irregularly between +1.6 and +3.0. It is the prototype of the class of Gamma Cassiopeiae variable stars. In the late 1930s it underwent what is described as a shell episode and the brightness increased to above magnitude +2.0, then dropped rapidly to +3.4. It has since been gradually brightening back to around +2.2. At maximum intensity, γ Cassiopeiae outshines both α Cassiopeiae (magnitude +2.25) and β Cassiopeiae (magnitude +2.3).
Gamma Cassiopeiae is a rapidly spinning star with a projected rotational velocity of 472 km s−1, giving it a pronounced equatorial bulge. When combined with the star's high luminosity, the result is the ejection of matter that forms a hot circumstellar disk of gas. The emissions and brightness variations are apparently caused by this "decretion" disk.
The spectrum of this massive star matches a stellar classification of B0.5 IVe. A luminosity class of IV identifies it as a subgiant star that has reached a stage of its evolution where it is exhausting the supply of hydrogen in its core region and transforming into a giant star. The 'e' suffix is used for stars that show emission lines of hydrogen in the spectrum, caused in this case by the circumstellar disk. This places it among a category known as Be stars; in fact, the first such star ever to be so designated. It has 17 times the Sun's mass and is radiating as much energy as 34,000 Suns. At this rate of emission, the star has reached the end of its life as a B-type main sequence star after a relatively brief 8 million years. The outer atmosphere has an intense effective temperature of 25,000 K, which is causing it to glow with a blue-white hue.
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