jeudi 4 septembre 2008

Cassiopeia 5

α Cassiopeiae is the second-brightest star in the constellation Cassiopeia (magnitude 2.25). It has the traditional name Schedar (which may also be spelt as Shedar, Shadar, Schedir, or Shedir).
It is an orange giant (spectral type K0 IIIa), a type of star cooler but much brighter than our Sun. In visible light only, it is well over 500 times brighter than the Sun. According to the Hipparcos astrometrical satellite, distance to the star is about 230 light years (or 70 parsecs).
Schedar has been sometimes classified as a variable star, but no variability has been detected since the 19th century. Also, three companions to the star have been listed in the Washington Double Star Catalog, but it seems that all of them are just line-of-sight optical components.
The name Schedar comes from the Arabic word صدر şadr, "breast".

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. American astronaut Virgil Ivan "Gus" Grissom nicknamed the star Navi after his own middle name spelled backwards.[1] 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.
Gamma Cassiopeiae is a spectroscopic binary with an orbital period of about 204 days and an eccentricity alternately reported as 0.26 and "near zero." The mass of the companion is believed to be about that of our Sun (Harmanec et al. 2000, Miroschnichenko et al. 2002)
Gamma Cas is also the prototype of a small group of stellar sources of X-ray radiation that is about 10 times higher that emitted from other B or Be stars, which shows very short term and long-term cycles. The character of the X-ray spectrum is be "thermal" and is possibly emitted from plasmas of temperatures up to least ten million kelvins. Historically it has been held the these X-rays might be excited by matter originating from the star, from a hot wind or a disk around the star, accreting onto the surface of a degenerate companion, such as a white dwarf or neutron star It is now realized that there are interpretational difficulties with either of these pictures. For example, it is not clear that enough matter can be accreted by the white dwarf at the distance of the secondary star (whose nature is not known), implied by the orbital period, is sufficient to power the X-rays (nearly 1033 erg/s or 100 YW). A neutron star could easily power this X-ray flux, but X-ray emission from neutron stars is known to be nonthermal, and thus in apparent variance with the spectral properties.
Recent evidence suggests that the X-rays may be associated with the Be star itself or in some complex interaction between the star and surrounding decretion disk. One line of evidence is that the X-ray production is known to vary on both short and long time scales with respect to various UV line and continuum diagnostics associated with a B star or with circumstellar matter close to the star (see Smith and Robinson 1999, Cranmer et al. 2000). Moreover, the X-ray emissions exhibit long-term cycles that correlate with the visible wavelength light curves (Smith et al. 2006). One intriguing property is that gamma cas exhibits characteristics consistent with a strong, disordered field (although no field can be measured directly by zeeman techniques because of its broad spectral lines). This inference comes from a coherent signature giving rise to robust period of 1.21 days suggesting a rooted magnetic field on its surface. The star's UV and optical spectral lines also show ripples moving from blue to red over several hours, which is indicative of clouds of matter frozen over the star's surface by strong magnetic fields. This evidence suggests that a magnetic field from the star interacting with the decretion disk are responsible for the X-rays. A disk dynamo has been advanced as a mechanism to explain the modulation of the X-rays (Robinson et al. 2002). However, difficulties remain with this mechanism, among which is that there are no disk dynamos are known to exist in other stars, rendering their behavior somewhat speculative.

β Cassiopeiae is a Delta Scuti variable star in the constellation Cassiopeia. It also has the traditional name Caph, from the Arabic word كف kaf, "palm" (i.e. reaching from the Pleiades), also known as al-Sanam al-Nakah.
β Cassiopeiae is a yellow-white F-type giant with a mean apparent magnitude of +2.28. It is classified as a Delta Scuti type variable star and its brightness varies from magnitude +2.25 to +2.31 with a period of 2.5 hours.

δ Cassiopeiae is a star in the constellation Cassiopeia. It also has the traditional names Ksora and Ruchbah, derived from an Arabic word ركبة rukbah meaning "knee".
Delta Cassiopeiae is an eclipsing binary with a period of 759 days. Its apparent magnitude varies between +2.68 mag and +2.74 with a period of 759 days. It is of spectral class A3, and is approximately 99 light years from Earth.

ε Cassiopeiae is a star system in the constellation Cassiopeia. It is approximately 441 light years from Earth. It has the traditional name Segin. One of the NASA designations for this star is Navi, in honor of astronaut Virgil Ivan "Gus" Grissom, one of the three astronauts who died in the Apollo 1 accident. [1]
It is a single, blue-white B-type giant with a luminosity 720 times that of the Sun. It has an apparent magnitude of +3.38 and is approximately 442 light years from Earth.

η Cassiopeiae is a star system 19.4 light years away from Earth, in the constellation Cassiopeia. Sometimes the traditional name Achird is used. It is known as 王良三 (the Third Star of Wang Liang) in Chinese. Achird; nearby ستاره دوتایی

The primary star in the η Cassiopeiae system is a yellow dwarf (main sequence) star of spectral type G0V, putting it in the same spectral class as our Sun, which is of spectral type G2V. It therefore resembles what our Sun might look like if we were to observe it from η Cassiopeiae. The star is of apparent magnitude 3.45. The star has a cooler and dimmer (magnitude 7.51) orange dwarf companion of spectral type K7V. The system is an RS Canum Venaticorum type variable star and its brightness varies by 0.05 magnitudes.
Based on an estimated semimajor axis of 12″ and a parallax of 0.168″, the two stars are separated by an average distance of 71 AU, where an AU is the average distance between the Sun and the Earth.[5] However, the large orbital eccentricity of 0.497 means that their periapsis, or closest approach, is as small as 36 AU. For comparison, the semi-major axis of Neptune is 30 AU. There are six dimmer optical components listed in the Washington Double Star Catalog. However, none of them are related to the η Cassiopeiae system and are in reality more distant stars.

ο Cassiopeiae is a triple star system in the constellation Cassiopeia. It is approximately 910 light years from Earth.چندستاره, γ Cas variable
The primary component, ο Cassiopeiae A, is a blue-white B-type giant with a mean apparent magnitude of +4.54. It is classified as a Gamma Cassiopeiae type variable star and its brightness varies from magnitude +4.50 to +4.62. The primary is a spectroscopic binary, and its close companion completes one orbit every 2.83 years. A more distant companion, ο Cassiopeiae B, lies 33.6 arcseconds away. It is an eleventh magnitude, yellow-white F-type main sequence dwarf.

ρ Cassiopeiae is a yellow hypergiant in the constellation Cassiopeia. It is about 11,650 light-years away, yet can still be seen by the naked eye (in the Northern Hemisphere only), as it is 550,000 times as luminous as the Sun. Its surface diameter is 450 times that of our sun's. On average, it has an absolute magnitude of −7.5, making it one of the most luminous stars known. Even though its surface temperature is similar to the Sun, its comfort zone for an Earthlike planet would be 450AU, which is more than 10 times the distance from the sun to Pluto. Being a yellow hypergiant, it is one of the rarest types of stars, one of only seven that are currently known.ابرغول زرد
Rho Cassiopeiae is somewhat unstable in its luminosity. Its apparent magnitude is currently about 4.5, but in 1946 it dimmed to 6th magnitude, before returning to its previous brightness. This happened again in 2000–2001, when it produced one of the largest outbursts known, ejecting 3% of a solar mass, the equivalent of 10,000 Earths. During the summer of 2000 it was observed (by the William Herschel Telescope) to have cooled from 7000 to 4000 degrees in the course of a few months. It seems to undergo these eruptions approximately once every 50 years (data suggests previous eruptions in 1893 and 1945).
Astronomers predict Rho Cassiopeiae may become a supernova in the near future because it will soon have consumed most of its nuclear fuel.

Aucun commentaire: