Shell star
Encyclopedia
Gamma Cassiopeiae
Gamma 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...
variable (GCAS), is a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
having a spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
that exhibits features indicating a circumstellar disc of gas surrounding the star at the equator. They exhibit irregular variations in their luminosity due to the outflow of matter. The shell stars are fast rotators, giving a partial explanation on the mechanism, but shell stars are still considered enigmatic. Shell stars belong to spectral types
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
O7.5 to F5, but their spectra are characterized by enormously widened absorption lines due to fast rotation and the disc that contributes also to other spectral peculiarities. Rotation velocities are about 200–250 km/s, not far from the point when the rotational acceleration would disrupt the star. The spectra and general qualities of shell stars are hard to interpret, since normal spectral features are overlaid with variable emission, so luminosity class and exact spectral class might easily be misinterpreted.
Subtypes
Shell stars are subdivided in four categories:- early Be stars of spectral types 07.5 to B2.5
- middle Be stars of types B3 to B6.5,
- late Be stars of types B7 to B9.5, and
- A-F shell stars ranging from A0 to F5.
The spectrum is variable in the long term, and the early type shell stars may switch from and to exhibiting a Be type spectrum or an ordinary B spectrum. All shell stars exhibit broadened emission instead of absorption lines in spectrum for the counterparting non-shell star type, so that B shell stars show emission of the Balmer
Balmer series
The Balmer series or Balmer lines in atomic physics, is the designation of one of a set of six different named series describing the spectral line emissions of the hydrogen atom....
Hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
lines where a normal B star show absorption. The earlier types usually exhibit He I
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
and usually Fe II
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
emission, the later shell stars exhibits Ca II and Ti II
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
emission. It is believed that the shell stars belong to somewhere in the range of main sequence up to giant star size, although the exact luminosity class is unclear by the general emission broadening caused by the rotation.
Examples
The prototype shell star is Gamma CassiopeiaeGamma Cassiopeiae
Gamma 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...
, while the brightest example is Achernar
Achernar
Achernar , sometimes spelled Achenar, is the brightest star in the constellation Eridanus and the ninth-brightest star in the night sky. Of the top ten apparent brightest stars —Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon, Achernar and Betelgeuse—Achernar is the hottest...
. A certain peculiar shell star is HR 2309 – a sharp-lined shell star, indicating a small and well defined ring structure. Other examples include:
- 27 Canis Majoris27 Canis Majoris27 Canis Majoris, also called 27 CMa, is a star system in the constellation Canis Major. It is approximately 1580 light years from Earth and has a mean apparent magnitude of +4.42....
- Beta Canis MinorisBeta Canis MinorisBeta Canis Minoris is a star in the constellation of Canis Minor. It has the traditional name Gomeisa.Beta Canis Minoris is a hot, B8-class main sequence star of apparent magnitude 2.9, easily visible to the naked eye. It is slightly variable, and belongs to the Gamma Cassiopeiae category of...
- Gamma CassiopeiaeGamma CassiopeiaeGamma 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...
- Phi PerseiPhi PerseiPhi Persei is a Class B2Vpe, fourth-magnitude star in the constellation Perseus. Due to its isolation and distance from Cassiopeia, Andromeda and the rest of Perseus, John Flamsteed never took thought to assign a number and constellation to Phi Persei.Phi Persei is binary consisting of a blue main...
- PleionePleione (star)Pleione is a binary star in the Pleiades star cluster , located roughly 390 light years away in the constellation of Taurus. Pleione was not given a Bayer designation, but did receive a Flamsteed number—hence its designation 28 Tauri...
- Psi PerseiPsi PerseiPsi Persei is a Be star in the constellation Perseus. This is a shell star that has a circumstellar disc of gas surrounding the equator. As a result of this disc, the spectrum of this star shows emission lines and the magnitude varies over a period of about a day...