by neufer » Thu Dec 24, 2009 5:35 am
http://en.wikipedia.org/wiki/Gamma_Cassiopeiae wrote:
<<Gamma Cassiopeiae (γ Cas / γ Cassiopeiae) is an eruptive variable star, whose brightness changes irregularly between +2.20 mag and +3.40 mag. It is the prototype of the Gamma Cassiopeiae variable stars. Although it is a fairly bright star, it has no traditional Arabic or Latin name. In Chinese, however, it has the name
Tsih, meaning "the whip". It is located at the center of the distinctive "W" shape that forms the Cassiopeia constellation. The star was used as an easily identifiable navigational reference point during space missions.
The apparent magnitude of this star was +2.2 in 1937, +3.4 in 1940, +2.9 in 1949, +2.7 in 1965 and now it is +2.15. At maximum intensity, γ Cassiopeiae outshines both α Cassiopeiae (magnitude +2.25) and β Cassiopeiae (magnitude +2.3). This is a rapidly spinning star that bulges outward along the equator. When combined with the high luminosity, the result is mass loss that forms a disk around the star. The emissions and brightness variations are apparently caused by this "decretion" disk.>>
http://www.aavso.org/vstar/vsots/1001.shtml wrote:
<<What distinguishes Be stars from normal stars on or near the main sequence? One thing is their rapid rotation --
up to 450 km/sec at their equator. [Rotational velocity of the sun ~ 2 km/sec at the equator; solar orbital velocity ~ 437 km/sec at the equator.] This reduces the effective gravity at the equator of the star. The strong radiation of B stars (they are thousands of times more luminous than the sun) produces a "stellar wind" which, in Be stars, is focussed into an equatorial disc. The shell stars are mostly Be stars in which we see the disc edge-on: gases in the disc produce the deep, narrow absorption lines. Be stars are also a major contributor to "galactic ecology" -- the process by which stars lose mass, which becomes part of the raw material from which new stars and planets form. With modern optical interferometers -- two or more telescopes which image an object simultaneously -- it is possible to "see" these discs. Another possible factor in "the Be phenomenon" may be a magnetic field, but no field has yet been observed in most Be stars.
Be stars vary in brightness, and spectrum, on several different timescales. There are variations on time scales of weeks to decades, which are connected with the formation and dispersal of the disc. These variations may be cyclic in nature; according to one theory, this is due to a spiral wave which slowly circulates around the disc. There are variations on time scales of days to weeks which are often connected with the binary motion of some of these stars; one example -- CX Dra -- is described below. Finally, there are variations on time scales of 0.3 to 2 days, which are due to non-radial pulsation, or perhaps rotation. These variations, which occur on or near the surface of the star, may be connected with the formation of the disc around the star. AAVSO photoelectric observations are used primarily to study the slow variations, but a few photoelectric photometrists have participated in intensive "campaigns" to study the rapid variations.>>
[quote=" http://en.wikipedia.org/wiki/Gamma_Cassiopeiae"]
<<Gamma Cassiopeiae (γ Cas / γ Cassiopeiae) is an eruptive variable star, whose brightness changes irregularly between +2.20 mag and +3.40 mag. It is the prototype of the Gamma Cassiopeiae variable stars. Although it is a fairly bright star, it has no traditional Arabic or Latin name. In Chinese, however, it has the name [b]Tsih, meaning "the whip"[/b]. It is located at the center of the distinctive "W" shape that forms the Cassiopeia constellation. The star was used as an easily identifiable navigational reference point during space missions.
The apparent magnitude of this star was +2.2 in 1937, +3.4 in 1940, +2.9 in 1949, +2.7 in 1965 and now it is +2.15. At maximum intensity, γ Cassiopeiae outshines both α Cassiopeiae (magnitude +2.25) and β Cassiopeiae (magnitude +2.3). This is a rapidly spinning star that bulges outward along the equator. When combined with the high luminosity, the result is mass loss that forms a disk around the star. The emissions and brightness variations are apparently caused by this "decretion" disk.>>[/quote]
[quote=" http://www.aavso.org/vstar/vsots/1001.shtml"]
<<What distinguishes Be stars from normal stars on or near the main sequence? One thing is their rapid rotation -- [b]up to 450 km/sec[/b] at their equator. [Rotational velocity of the sun ~ 2 km/sec at the equator; solar orbital velocity ~ 437 km/sec at the equator.] This reduces the effective gravity at the equator of the star. The strong radiation of B stars (they are thousands of times more luminous than the sun) produces a "stellar wind" which, in Be stars, is focussed into an equatorial disc. The shell stars are mostly Be stars in which we see the disc edge-on: gases in the disc produce the deep, narrow absorption lines. Be stars are also a major contributor to "galactic ecology" -- the process by which stars lose mass, which becomes part of the raw material from which new stars and planets form. With modern optical interferometers -- two or more telescopes which image an object simultaneously -- it is possible to "see" these discs. Another possible factor in "the Be phenomenon" may be a magnetic field, but no field has yet been observed in most Be stars.
Be stars vary in brightness, and spectrum, on several different timescales. There are variations on time scales of weeks to decades, which are connected with the formation and dispersal of the disc. These variations may be cyclic in nature; according to one theory, this is due to a spiral wave which slowly circulates around the disc. There are variations on time scales of days to weeks which are often connected with the binary motion of some of these stars; one example -- CX Dra -- is described below. Finally, there are variations on time scales of 0.3 to 2 days, which are due to non-radial pulsation, or perhaps rotation. These variations, which occur on or near the surface of the star, may be connected with the formation of the disc around the star. AAVSO photoelectric observations are used primarily to study the slow variations, but a few photoelectric photometrists have participated in intensive "campaigns" to study the rapid variations.>>[/quote]