Gamma Cassiopeiae, Latinized from γ Cassiopeiae, is a bright

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

at the center of the distinctive "W" asterism in the northern

circumpolar constellation A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets below the horizon due to its apparent proximity to one of the celestial poles. Circumpolar stars are therefore visible from said location toward the nearest po ...

of Cassiopeia. Although it is a fairly bright star with an

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's lig ...

that varies from 1.6 to 3.0, it has no traditional Arabic or Latin name. It sometimes goes by the informal name Navi. Gamma Cassiopeiae is a

Be star Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as ''classical Be stars'', is a non-supergiant B star whose spectrum has, or had at some time, one or more Balmer e ...

, a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...

, and a multiple star system. Based upon

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...

measurements made by the

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...

satellite, it is located at a distance of roughly 550

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s from Earth. Together with its common-proper-motion companion, HD 5408, the system could contain a total of eight stars.

Physical properties

Gamma Cassiopeiae and its associated nebulosity

Gamma Cassiopeiae is an eruptive variable star, whose

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...

changes irregularly between +1.6 and +3.0. It is the prototype of the class of

Gamma Cassiopeiae variable A Gamma Cassiopeiae variable (γ Cassiopeiae variable) is a type of variable star, named for its prototype γ Cassiopeiae. Variability γ Cassiopeiae variables show irregular changes in brightness on a timescale of decades. These typically hav ...

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 Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface. The rotation of a star produces an equatorial bulge ...

of 472 km s⁻¹, giving it a pronounced

equatorial bulge An equatorial bulge is a difference between the equatorial and polar diameters of a planet, due to the centrifugal force exerted by the rotation about the body's axis. A rotating body tends to form an oblate spheroid rather than a sphere. On Ea ...

. 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 A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...

of this massive star matches a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...

of B0.5 IVe. A

luminosity class In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

of IV identifies it as a

subgiant star A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of ...

that has reached a stage of its

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

where it is exhausting the supply of hydrogen in its core region and transforming into a

giant star A giant star is a star with substantially larger radius and luminosity than a main sequence, main-sequence (or ''dwarf'') star of the same effective temperature, surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moo ...

. The 'e' suffix is used for stars that show

emission line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

s of hydrogen in the spectrum, caused in this case by the circumstellar disk. This places it among a category known as

s; 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 late

O-type main sequence star An O-type main-sequence star (O V) is a main-sequence (core hydrogen-burning) star of spectral type O and luminosity class V. These stars have between 15 and 90 times the mass of the Sun and surface temperatures between 30,000 and 50,000 K. The ...

after a relatively brief 8 million years. The outer atmosphere has an intense

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of 25,000 K, which is causing it to glow with a blue-white hue.

X-ray emission

Gamma Cassiopeiae is the prototype of a small group of stellar sources of

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

radiation that is about 10 times stronger than emitted from other B or Be stars. The character of the X-ray spectrum is ''Be thermal'', possibly emitted from plasmas of temperatures up to least ten million kelvins, and shows very short term and long-term cycles. Historically, it has been held that 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 A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...

. However, there are difficulties with either of these hypotheses. For example, it is not clear that enough matter can be accreted by a white dwarf, at the distance of the purported secondary star implied by the orbital period, sufficient to power an X-ray emission of 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 non-thermal, and thus in apparent variance with the spectral properties. Evidence suggests that the X-rays may be associated with the Be star itself or caused by 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 changes 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 light curves in the visible wavelengths. Gamma Cassiopeiae exhibits characteristics consistent with a strong disordered

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

. No field can be measured directly from the

Zeeman effect The Zeeman effect (; ) is the effect of splitting of a spectral line into several components in the presence of a static magnetic field. It is named after the Dutch physicist Pieter Zeeman, who discovered it in 1896 and received a Nobel prize ...

because of the star's rotation-broadened spectral lines. Instead, the presence of this field is inferred from a robust periodic signal of 1.21 days that suggests a magnetic field rooted on the rotating star's surface. The star's UV and optical spectral lines show ripples moving from blue to red over several hours, which indicates clouds of matter being held frozen over the star's surface by strong magnetic fields. This evidence suggests that a magnetic field from the star is interacting with the decretion disk, resulting in the X-ray emission. A disk dynamo has been advanced as a mechanism to explain this modulation of the X-rays. However, difficulties remain with this mechanism, among which is that there are no disk dynamos known to exist in other stars, rendering this behavior more difficult to analyze.

Companions

Gamma Cassiopeiae has three faint companions, listed in double star catalogues as components B, C, and D. Star B is about 2

arc-second A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...

s distant and magnitude 11, and has a similar space velocity to the bright primary, making it likely to be physically associated. Component C is magnitude 13, nearly an

arc-minute A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...

distant, and is listed in

Gaia Early Data Release 3 The ''Gaia'' catalogues are star catalogues created using the results obtained by '' Gaia'' space telescope. The catalogues are released in stages that will contain increasing amounts of information; the early releases also miss some stars, espec ...

as having a very different proper motion and being much more distant than Gamma Cassiopeiae. Finally, component D, about 21 arc-minutes distant, is the naked-eye star HR 266 (HD 5408), itself a quadruple system. Gamma Cassiopeiae A, the bright primary, itself contains a spectroscopic binary with an orbital period of about 203.5 days and an eccentricity alternately reported as 0.26 and "near zero." The mass of the companion is believed to be about that of the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, but its nature is unclear. It has been proposed that it is a degenerate star or a hot helium star, but it seems unlikely that it is a normal star. Therefore, it is likely to be more evolved than the primary and to have transferred mass to it during an earlier stage of evolution. Additionally, Hipparcos data show a "

wobble Wobble or wobbles may refer to: * "Wobble" (song), a single by V.I.C. * Wobbles (equine disorder), a disorder of the nervous system in dogs and horses * Wobble base pair, a type of base pairing in genetics * Jah Wobble (born 1958), British musici ...

" with an amplitude of about 150 mas, that may correspond to the orbit of a third star. This star would have an orbital period of at least 60 years.

Names

The

Chinese name Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are often a ...

''Tsih'', "the whip" (), is commonly associated with this star. The name however originally referred to Kappa Cassiopeiae, and Gamma Cassiopeiae was just one of four horses pulling the chariot of legendary charioteer Wangliang. This representation was later changed to make Gamma the whip. The star was used as an easily identifiable navigational reference point during space missions and American

astronaut An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a human spaceflight program to serve as a commander or crew member aboard a spacecraft. Although generally r ...

Virgil Ivan "Gus" Grissom nicknamed the star Navi after his own middle name spelled backwards.

References

External links

* Philippe Stee's homepage
Hot and Active Stars ResearchGamma Cassiopeiae and the Be Stars

A New Class of X-ray Star?

Astronomy Picture of the Day Astronomy Picture of the Day (APOD) is a website provided by NASA and Michigan Technological University (MTU). According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation written ...