An FS Canis Majoris variable (FS CMa star) is a type of
eruptive variable star. The class of variable stars are named after its prototype,
FS Canis Majoris. They are somewhat poorly understood, but are probably
binary star systems in which mass exchange is ongoing or has recently happened. These most likely consist of at least one
B-type main-sequence star
A B-type main-sequence star (B V) is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type star ...
in a dust envelope. They undergo irregular light variations over long periods of time, with
magnitude
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
variation of approximately 2 magnitudes.
The
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
emission lines
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 ...
from FS CMa stars are much stronger than in normal
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 ...
s, and certain
forbidden line
In spectroscopy, a forbidden mechanism (forbidden transition or forbidden line) is a spectral line associated with absorption or emission of photons by atomic nuclei, atoms, or molecules which undergo a transition that is not allowed by a particu ...
s are present in the spectrum as well so they are classified as a sub-type of
B star.
[
FS CMa stars are rare and it has been difficult to determine their properties well enough to determine what causes their unusual character. Several have been detected in massive star clusters and this has ruled out some theories about their origin. The preferred explanation is still for a binary origin, possibly as the result of ]stellar merger
A stellar collision is the coming together of two stars caused by stellar dynamics within a star cluster, or by the orbital decay of a binary star due to stellar mass loss or gravitational radiation, or by other mechanisms not yet well understood ...
s.[
]
References
{{variable star topics
FS Canis Majoris variables