FS Canis Majoris (FS CMa), also known as HD 45677 or MWC 142, is a B-type star in the constellation of Canis Major. It has 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 ...

of approximately 8.50, having varied between magnitudes 7.35 and 8.58. Feinstein and colleagues reported in 1976 that it had decreased by 0.9 magnitude between 1969 and 1976, whereas it had only varied by 0.3 magnitude in the 70 years beforehand. Sometimes it could vary by up to 0.5 magnitude in a year or 0.1 magnitude in a night, and there did not appear to be any regular period to its variability. Astronomer Anatoly Miroshnichenko has made it the prototype of a new type of variable star, the FS Canis Majoris variables. These are hot blue-white stars that exhibit

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 ...

emission and strong infra-red excess suggestive of very young ( pre-main-sequence) stars yet they do not lie in star-forming regions. Nor did they appear to be stars that had evolved off the main sequence into giant or supergiant stars. It is now thought that they are main-sequence stars that have absorbed or are absorbing matter, most likely from a companion star, and are surrounded by a compact dusty shell. These stars are thought to be significant contributors to interstellar dust. FS Canis Majoris has been well studied due to its dusty disk, which is inclined 51° relative to the plane of sky. The disk has a gap within 5 au of the star. Its spectral type has been previously classified as B2III to B2V and its

bolometric magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it ...

as -4.89. However, investigation of its spectrum in 2006 showed that FS Canis Majoris is a

binary system A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that th ...

. The system is between 1,250 and 8,000 times as luminous as 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 ...

. One calculation has the masses of the primary and secondary at 9.3 ± 0.5 and 4.8 ± 0.9 solar masses and radii 6.6 ± 0.5 and 2.9 ± 0.6 times that of the Sun, and surface temperatures of 21,600 ± 350 and 16,380 ± 1,670 K, respectively.

References

{{DEFAULTSORT:FS Canis Majoris Canis Major B-type main-sequence stars 045677 030800 Canis Majoris, FS FS Canis Majoris variables Shell stars Binary stars Durchmusterung objects