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 in a dust envelope. They undergo irregular light variations over long periods of time, with magnitude variation of approximately 2 magnitudes.
The hydrogen emission lines from FS CMa stars are much stronger than in normal Be stars, and certain forbidden lines are present in the spectrum as well so they are classified as a sub-type of B[e] star. [1]
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 mergers. [2]
Canis Major is a constellation in the southern celestial hemisphere. In the second century, it was included in Ptolemy's 48 constellations, and is counted among the 88 modern constellations. Its name is Latin for "greater dog" in contrast to Canis Minor, the "lesser dog"; both figures are commonly represented as following the constellation of Orion the hunter through the sky. The Milky Way passes through Canis Major and several open clusters lie within its borders, most notably M41.
A variable star is a star whose brightness as seen from Earth 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 either:
Epsilon Canis Majoris is a binary star system and the second-brightest object in the constellation of Canis Major. Its name is a Bayer designation that is Latinised from ε Canis Majoris, and abbreviated Epsilon CMa or ε CMa. This is the 22nd-brightest star in the night sky with an apparent magnitude of 1.50. About 4.7 million years ago, it was the brightest star in the night sky, with an apparent magnitude of −3.99. Based upon parallax measurements obtained during the Hipparcos mission, it is about 430 light-years distant from the Sun.
Gamma Canis Majoris, also named Muliphein, is a star in the constellation of Canis Major. It is unclear exactly why this relatively faint star was given the 'gamma' designation, but possibly because it is in the same part of the constellation as Sirius (alpha) and Mirzam (beta).
Nu2 Canis Majoris (ν2 Canis Majoris) is a single star in the southern constellation of Canis Major.
Omicron2 Canis Majoris is a star in the constellation Canis Major. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It has an apparent visual magnitude of 3.043, making it one of the brighter members of the constellation. The distance to this star is roughly 2,800 light years (800 parsecs), with a 34% margin of error.
Pi Canis Majoris is a binary star system in the southern constellation of Canis Major. It is visible to the naked eye, having an apparent visual magnitude of +4.69. Based upon an annual parallax shift of 33.80 mas as seen from Earth, this system is located 96.5 light years from the Sun. The star is moving in the general direction of the Sun with a radial velocity of −37.9 km/s. It will make its closest approach in around 733,000 years when it comes within 23 ly (7 pc).
Tau Canis Majoris is a multiple star system in the constellation Canis Major. It is approximately 5,000 light years distant from Earth and is the brightest member of the open cluster NGC 2362.
Omega Canis Majoris, Latinized from ω Canis Majoris, is a solitary, blue-white-hued star in the equatorial constellation of Canis Major. It is visible to the naked eye with an apparent visual magnitude of about 4. Based upon an annual parallax shift of just 3.58 mas as seen from Earth, this system is located roughly 910 light-years from the Sun.
27 Canis Majoris is a binary star system in the northern constellation of Canis Major, located approximately 1,700 light years away from the Sun. It has the variable star designation EW Canis Majoris; 27 Canis Majoris is the Flamsteed designation. This system is visible to the naked eye as a dim, blue-white hued star with an apparent visual magnitude of +4.65. It is moving away from the Earth with a heliocentric radial velocity of 16 km/s.
UW Canis Majoris is a star in the constellation Canis Major. It is classified as a Beta Lyrae eclipsing contact binary and given the variable star designation UW Canis Majoris. Its brightness varies from magnitude +4.84 to +5.33 with a period of 4.39 days. Bode had initially labelled it as Tau2 Canis Majoris, but this designation had been dropped by Gould and subsequent authors.
Z Canis Majoris (Z CMa) is a B-type star in the constellation of Canis Major. It has an average apparent visual magnitude of approximately 9.85, though has brightened by 1-2 magnitudes in irregular outbursts in 1987, 2000, 2004 and 2008.
EZ Canis Majoris is binary system in the constellation of Canis Major. The primary is a Wolf-Rayet star and it is one of the ten brightest Wolf-Rayet stars, brighter than apparent magnitude 7.
VY Canis Majoris is an extreme oxygen-rich (O-rich) red hypergiant (RHG) or red supergiant (RSG) and pulsating variable star 1.2 kiloparsecs from the Solar System in the slightly southern constellation of Canis Major. It is one of the largest known stars, one of the most luminous and massive red supergiants, and one of the most luminous stars in the Milky Way.
NR Canis Majoris is a binary star system in the southern constellation of Canis Major, located to the east of Sirius and Gamma Canis Majoris near the constellation border with Puppis. It has a yellow-white hue and is dimly visible to the naked eye with a combined apparent visual magnitude that fluctuates around 5.60. It is located at a distance of approximately 297 light years from the Sun based on parallax. The system is drifting closer to the Sun with a radial velocity of −29 km/s, and in about three million years it is predicted to approach within 14.1+4.7
−4.0 ly. At that time, the star will become the brightest in the night sky, potentially reaching magnitude −0.88.
Mu Canis Majoris is a binary star system in the southern constellation of Canis Major. The pair can be located a little to the southwest of the point midway between Gamma and Theta Canis Majoris, and the components can be split with a small telescope. The system is faintly visible to the naked eye with an apparent visual magnitude of 5.12. Based upon an annual parallax shift of just 2.62 mas as seen from Earth, this system is located roughly 1,200 light years from the Sun.
LS Canis Majoris is an eclipsing binary star located around 1,510 light years from Earth in the constellation Canis Major. Its apparent magnitude ranges from 5.63 to 5.79. The orbital period of binary pair and the period of light variation is 70.048 days.
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 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.
CX CMa is a blue variable star in the Canis Major constellation. Discovery of this variable is usually credited to German Astronomer Cuno Hoffmeister in 1931, although this remains uncertain.
FY Canis Majoris, also known as HD 58978, is a star about 1,800 light years from the Earth, in the constellation Canis Major. It is usually a 5th magnitude star, so it will be visible to the naked eye of an observer far from city lights. It is a Gamma Cassiopeiae variable star, whose brightness varies from magnitude 4.8 to 6.25.