Beta Canis Majoris is a star in the southern constellation of Canis Major, the "Great Dog". Its name is a Bayer designation; it has the proper name Mirzam, pronounced /ˈmɜːrzəm/.[12] Visible to the naked eye, this is a variable star that ranges in apparent visual magnitude from 1.97 to 2.01 over a period of six hours.[3] Based on parallax measurements, it is located at a distance of about 500light-years (150parsecs) from the Earth.[1] The star is drifting further away from the Sun with a line of sight velocity of +34km/s.[6] In the modern constellation it lies at the position of the dog's front leg.
Nomenclature
Beta Canis Majoris is the star's Bayer designation, which is Latinized from β Canis Majoris and abbreviated Beta CMa or β CMa. The traditional names Mirzam, Al-Murzim or Murzim,[10] derive from the Arabic (مرزم) for "The Announcer" or "The Herald", and probably refer to its position, because it rises before Sirius in the night sky. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[14] 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 50days. The strongest pulsation mode is a radial first overtone, while the second is non-radial.[8]
This star has a mass of about 12–13 times the mass of the Sun with 7.4 or 8.2 times the Sun's radius.[9] The effective temperature of the star's outer envelope is about 24,700K,[20] which is much higher than the Sun's at 5,778K. 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.[21] The estimated age of Mirzam is 12–14 million years,[9] which is long enough for a star of this mass to have evolved into a giant star. The stellar classification of B1II-III[4] indicates that the spectrum matches a star part way between a giant star and a bright giant. This star has a rotation period of 13.6days and is inclined 57.6° as seen from Earth.[9] Mirzam has sufficient mass to explode as a supernova.[22]
Beta Canis Majoris is located near the far end of the Local Bubble,[23] 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.[24]
123Cousins, 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.
12Balona, L. A.; etal. (1996). "The Pulsation Frequencies of beta CMa". Information Bulletin on Variable Stars. 4313: 1. Bibcode:1996IBVS.4313....1B.
↑Samus, N. N.; etal. (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.
12Wilson, Ralph Elmer (1953). "General Catalogue of Stellar Radial Velocities". Washington. Bibcode:1953GCRV..C......0W.
123456Shull, J. Michael; Curran, Rachel M.; Topping, Michael W. (2025-08-05). "Beta Canis Majoris: The Other Major Ionization Source of the Local Interstellar Clouds". arXiv:2508.03800 [astro-ph.GA].
↑Makemson, Maud Worcester (1941). The Morning Star Rises: An Account of Polynesian Astronomy. Yale University Press. p.239. Bibcode:1941msra.book.....M.
↑"Light Curve". Hipparcos ESA. ESA. Retrieved 17 February 2022.
