Gamma Cassiopeiae
Encyclopedia
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, derived from the word 策 (cè), 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. 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
.
Gamma Cassiopeiae is also the prototype of a small group of stellar sources of X-ray radiation that is about 10 times higher than 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", 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. Moreover, the X-ray emissions exhibit long-term cycles that correlate with the visible wavelength light curves. One intriguing property is that Gamma Cassiopeiae 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. 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.
Gamma Cassiopeiae is also an optical double, with a faint magnitude
11 companion B about 2 arc seconds distant, with the designation of ADS782AB, and a further, fainter, optical companion C.
The star was referenced in O. Henry
's short story The Skylight Room.
The star varies in its brightness by 40% about every 50 years.
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
, 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
Chinese language
The Chinese language is a language or language family consisting of varieties which are mutually intelligible to varying degrees. Originally the indigenous languages spoken by the Han Chinese in China, it forms one of the branches of Sino-Tibetan family of languages...
, however, it has the name Tsih, derived from the word 策 (cè), meaning "the whip". It is located at the center of the distinctive "W" shape that forms the Cassiopeia constellation
Cassiopeia (constellation)
Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivalled beauty. Cassiopea was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today...
. American astronaut
Astronaut
An astronaut or cosmonaut is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft....
Virgil Ivan "Gus" Grissom
Gus Grissom
Virgil Ivan Grissom , , better known as Gus Grissom, was one of the original NASA Project Mercury astronauts and a United States Air Force pilot...
nicknamed the star Navi after his own middle name spelled backwards. The star was used as an easily identifiable navigational reference point during space missions.
The apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
.
Gamma Cassiopeiae is also the prototype of a small group of stellar sources of X-ray radiation that is about 10 times higher than 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", 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
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
or neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
. 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. Moreover, the X-ray emissions exhibit long-term cycles that correlate with the visible wavelength light curves. One intriguing property is that Gamma Cassiopeiae 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. 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.
Gamma Cassiopeiae is also an optical double, with a faint magnitude
Magnitude (astronomy)
Magnitude is the logarithmic measure of the brightness of an object, in astronomy, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.-Background:...
11 companion B about 2 arc seconds distant, with the designation of ADS782AB, and a further, fainter, optical companion C.
The star was referenced in O. Henry
O. Henry
O. Henry was the pen name of the American writer William Sydney Porter . O. Henry's short stories are well known for their wit, wordplay, warm characterization and clever twist endings.-Early life:...
's short story The Skylight Room.
The star varies in its brightness by 40% about every 50 years.
External links
- Gamma Cassiopeiae by Jim Kaler.
- Gamma Cassiopeiae and the Be Stars.
- A New Class of X-ray Star?
- Gamma Cas and Friends, Astronomy Picture of the DayAstronomy Picture of the DayAstronomy Picture of the Day is a website provided by NASA and Michigan Technological University . According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation written by a professional astronomer."The photograph is not necessarily...
, 2009 December 24