R Canis Majoris is an eclipsing interacting binary star system in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Canis Major 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 C ...

. It varies from magnitude 5.7 to 6.34. The system is unusual in the low mass ratio (ie. big mass difference) between the main two components and shortness of the orbital period. It is faintly visible to the

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...

of an observer with very good observing conditions.

Variability

In 1887, Edwin F. Sawyer discovered that the star, then generally referred to as 155 Canis Majoris (after its listing in ''Uranometria Argentina''), is a

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

. It appears with its

variable star designation In astronomy, a variable-star designation is a unique identifier given to variable stars. It extends the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive of the name of the constellation in whic ...

, R Canis Majoris, in

Annie Jump Cannon Annie Jump Cannon (; December 11, 1863 – April 13, 1941) was an American astronomer whose cataloging work was instrumental in the development of contemporary stellar classification. With Edward C. Pickering, she is credited with the creation of ...

's ''Second catalogue of variable stars'', published in 1907. Eclipse timings for R Canis Majoris have been being measured since 1887, and whilst at present the time period appears constant at 1.1359 days, periodic quasi-sinusoidal variations of the eclipse arrival times have been taking place with a periodicity of around 93 years. This has led to the suggestion that there exists a third non-eclipsing body in the system whose gravitational pull is responsible for these variations.

Interacting binary star

R Canis Majoris is thought to be an interacting binary star. The secondary star, originally the more massive of the two, has evolved away from the

main sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...

, expanded, and exceeded its

Roche lobe In astronomy, the Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately teardrop-shaped region bounded by a critical gravitational equipotential, ...

. The primary has stripped away mass from the secondary, with much of it being lost completely from the system. The secondary now has a very reduced mass relative to its temperature and luminosity, while the primary has gained helium-rich material and become hotter and more luminous. Reanalysis of the system using high-resolution spectroscopy yields its two main components to have masses and times that of the Sun respectively and radii and times that of the Sun respectively. Their surface temperatures are and . A third star in the system is very faint, presumed to be an orange dwarf. A tentative orbit has been derived with a period of 102 years. The third star has a mass 80% that of the Sun and a radius around 83% that of the Sun.

References

{{DEFAULTSORT:R Canis Majoris Eclipsing binaries G-type subgiants F-type main-sequence stars M-type main-sequence stars Canis Majoris, R Durchmusterung objects 057167 035487 2788 Canis Major