Beta Canis Majoris (β Canis Majoris, abbreviated Beta CMa, β CMa), also named Mirzam/ˈmɜːrzəm/,[13] is a star in the southern constellationofCanis Major, the "Great Dog", located at a distance of about 500 light-years (150 parsecs) from the Sun.[1] In the modern constellation it lies at the position of the dog's front leg.
Beta Canis Majoris is the star's Bayer designation. The traditional names Mirzam, Al-MurzimorMurzim,[11] derive from the Arabic (مرزم) for 'The Herald', and probably refers to its position, heralding (i.e., rising before) Sirius in the night sky. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[15] included a table of the first two batches of names approved by the WGSN; which included Mirzam for this star.
Mirzam is a Beta Cephei variable that varies in apparent magnitude between +1.97 and +2.01 over a six-hour period, a change in brightness that is too small to be discerned with the naked eye. It exhibits this variation in luminosity because of periodic pulsations in its outer envelope, which follow a complex pattern with three different cycles; all about six hours in length. The two dominant pulsation frequencies have a combined beat period of roughly 50 days. The strongest pulsation mode is a radial first overtone, while the second is non-radial.[8]
This star has a mass of about 13–14 times the mass of the Sun[8] with 8–11 times the Sun's radius.[9] The effective temperature of the star's outer envelope is about 23,150 K, which is much higher than the Sun's at 5,778 K. The energy emitted at the high temperature of the former is what gives this star a blue-white hue characteristic of a B-type star.[10][21] The estimated age of Mirzam is 12–13 million years, which is long enough for a star of this mass to have evolved into a giant star. The stellar classification of B1 II-III[4] indicates that the spectrum matches a star part way between a giant star and a bright giant.
Beta Canis Majoris is located near the far end of the Local Bubble,[22] a cavity in the local interstellar medium through which the Sun is traveling. It is located within the Mirzam Tunnel, a region of less dense concentration between the stars and HII regions surrounding the Ori OB1 and Vel OB2 associations. Beta Canis Majoris was the brightest star in the night sky around four million years ago, peaking with an apparent magnitude of -3.65, or more than seven times as bright as Sirius today.[23]
^ abcCousins, A. W. J. (1972), "UBV Photometry of Some Very Bright Stars", Monthly Notes of the Astronomical Society of Southern Africa, 31: 69, Bibcode:1972MNSSA..31...69C
^Balona, L. A.; Bregman, L.; Letsapa, B. A.; Magoro, B. T.; Walsh, S. E. (1996). "The Pulsation Frequencies of beta CMa". Information Bulletin on Variable Stars. 4313: 1. Bibcode:1996IBVS.4313....1B.
^Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1: 02025. Bibcode:2009yCat....102025S.
^Wilson, Ralph Elmer (1953), "General Catalogue of Stellar Radial Velocities", Washington, Bibcode:1953GCRV..C......0W
