Beta Canis Majoris

Lore & Background

The traditional names Mirzam, Al-Murzim or Murzim derive from the Arabic (مرزم) for 'The Announcer' or 'The Herald', probably referring to its position rising before Sirius. In 2016, the International Astronomical Union's Working Group on Star Names approved Mirzam as the proper name. In Chinese, the star is part of an asterism called 'Market for Soldiers', and from this Chinese name arose the name Kuen She. The Dunhuang Star Chart noted it as Yeji 'Pheasant Cock', though it was located about 10 degrees too far north. The people of the Tuamotus called it Oupo.

Mirzam is a Beta Cephei variable that varies in apparent magnitude between +1.97 and +2.01 over a six-hour period, a change too small to be discerned with the naked eye. It exhibits this variation because of periodic pulsations in its outer envelope, following a complex pattern with three different cycles, all about six hours long. 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.

The 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. Its effective temperature is about 24,700 K, giving it a blue-white hue characteristic of a B-type star. The estimated age is long enough for a star of this mass to have evolved into a giant star. The stellar classification B1 II-III indicates a spectrum part way between a giant and a bright giant. The star has a rotation period of 13.6 days and is inclined 57.6° as seen from Earth. Mirzam has sufficient mass to explode as a supernova.

Beta Canis Majoris is located near the far end of the Local Bubble, 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.

Reader's Guide

Mirzam's significance lies in its role as a prominent Beta Cephei variable, a class of pulsating stars that help astronomers understand stellar structure and evolution. Its complex pulsation pattern, with three cycles and a beat period of roughly 50 days, provides a natural laboratory for studying non-radial oscillations in massive stars. The star's position near the far end of the Local Bubble and within the Mirzam Tunnel offers insight into the structure of the local interstellar medium. Historically, Mirzam served as a celestial marker for ancient cultures, its Arabic name 'The Herald' reflecting its role as a precursor to Sirius. Around four million years ago, it was the brightest star in the night sky, outshining Sirius by more than seven times. Its legacy extends to modern culture: it appears on the flag of Brazil, symbolizing the state of Amapá, and its alternative name Murzim was used for a United States Navy Crater class cargo ship. With sufficient mass to eventually explode as a supernova, Mirzam continues to be a subject of study for its pulsations, evolution, and eventual fate.

Did You Know?

• Mirzam varies in brightness between magnitude +1.97 and +2.01 over a six-hour period.
• The star has three different pulsation cycles, each about six hours long.
• Around four million years ago, Mirzam was the brightest star in the night sky, peaking at magnitude −3.65.
• Its traditional name Mirzam derives from Arabic for 'The Announcer' or 'The Herald'.