Tau Canis Majoris (τ CMa, τ Canis Majoris, 30 CMa) is a multiple 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 is approximately 5,000 light years distant from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

and is the brightest member of the

open cluster An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...

NGC 2362 NGC 2362, also known as Caldwell 64, is an open cluster of stars in the southern constellation of Canis Major. It was discovered by the Italian court astronomer Giovanni Batista Hodierna, who published his finding in 1654. William Herschel called ...

System

τ Canis Majoris lies at the center of the very young open cluster NGC 2362 which contains several hundred stars. It is by far the brightest member of the cluster and the only one to have evolved away from the main sequence. A number of stars were catalogued by

John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor and experimental photographer who invented the blueprint and did botanical work. ...

as companions: component B is a 10th magnitude star 8.6 arc-seconds distant; component C is a 14th magnitude star 14.2 arc-seconds away; and component D is an 8th magnitude star at 85 arc-seconds. These stars are all thought to be main sequence members of NGC 2362. A period of 94,000 years has been suggested for the AB pair assuming they are gravitationally bound. In 1951, component A was resolved into a double separated by only 0.15", with an estimated orbital period of 250 years. The Washington Double Star catalogue lists the pair as magnitude 4.89 Aa and magnitude 5.33 Ab, but the CCDM designates the components as A and P. A 10th magnitude component E was discovered in 2010. It is less than 1" away from the 4th magnitude main component. TauCMaLightCurve

The primary component A is itself a spectroscopic binary with a period of 154.918 days. The variable radial velocity was discovered in 1906 and the first orbit published in 1928. More recently, the Hipparcos satellite data revealed the existence of a 1.282 day eclipsing binary within the system, with two equal minima where the brightness dips by half a magnitude. The short period eclipsing pair is the fainter component of the two longer period spectroscopic binary, forming an unusual triple star with the most massive component orbiting pair of smaller stars. These three stars are referred to as Aa, Ab1, and Ab2, not to be confused with the fainter visible star 0.15 arc-seconds distant.

UW Canis Majoris 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 ...

is another 4th magnitude star less than half a degree away, and is itself an eclipsing binary system associated with NGC 2362. It has been catalogued as τ² CMa, but that name is now rarely used.

Properties

τ Canis Majoris appears as a class O supergiant at a distance of 5,000 light years within the open cluster NGC 2362, but this is composed of multiple stars. The properties of the minor components of the τ CMa system are poorly known. Component D has been described as having spectral type B2V, but this is uncertain. The relative brightnesses of the three spectroscopic components have been calculated from the eclipses and orbital movement. The spectrum, mass, and luminosity is dominated by component Aa which is now considered to be an O9 bright giant. It is thought to have a mass around , a temperature of 32,000 K, and a luminosity of . The two eclipsing stars are almost identical class B main sequence stars with masses around . This star is sometimes known as the 'Mexican Jumping Star' by amateur astronomers, because it can appear to 'jump around' with respect to the other stars in the cluster because of its marked contrast in brightness.

References

External links

The Mexican Jumping Star
{{DEFAULTSORT:Tau Canis Majoris Canis Majoris, Tau Beta Lyrae variables Canis Major Spectroscopic binaries O-type bright giants B-type main-sequence stars Canis Majoris, 30 2782 057061 035415 Durchmusterung objects 3 NGC 2362