Mirzam, also spelled Murzim, is the fourth brightest star in Canis Major, the Larger Dog. Mirzam's Arabic name means that the star is "the Herald" of brilliant Sirius, as Mirzam rises first. At magnitude 1.98, Mirzam appears relatively dim compared to Sirius; but in fact Mirzam is by far the more luminous of the two. Its 500-light-year distance is 60 times greater than Sirius's; if Mirzam were at Sirius's distance, it would shine almost 15 times more brightly than Venus.
Properties
Mirzam is a very hot, blue class B1 II bright giant with a temperature around 25,800 K. Much of its radiation is therefore produced in the ultraviolet; with this taken into account, β CMa has a luminosity 34,000 times the Sun's - nearly 1300 times that of Sirius. Mirzam's temperature and luminosity indicate a mass around 15 times the Sun's. Mirzam is very close to the point of finishing its core hydrogen fusion. While Sirius will eventually evolve into a white dwarf, Mirzam seems to have sufficient mass for it to explode someday as a supernova.
Variability
Mirzam is the brightest of the β Cepheid variables; in fact, the class is also called the β CMa variables. Its instability is related to the cessation of hydrogen fusion in its core, and the corresponding changes in its internal structure. Though its 10% variations, between magnitudes 1.95 and +2.00, are too feeble to be seen with the naked eye, they are extremely complex and have multiple periods; the three strongest are about six hours.
Mirzam lies near the far end of the "Local Bubble" - a sort of cavity where the in the interstellar medium is especially hot and thin. Its light is used as a background to study the thin gas of interstellar space.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]