ADHARA (Epsilon Canis Majoris). The names of all the first magnitude stars ring clearly to us; even the names of those in the southern hemisphere are well known. Adhara has escaped the fame it deserves. Though better known as Epsilon Canis Majoris (using the fifth letter in the Greek alphabet), it's actually the second brightest star in its constellation (after Sirius), and helps form the western leg of Canis Major, the larger dog. Look just below Sirius to find an outstanding triangle of bright stars. Adhara is at the lower right. Bayer thus seems to have used position in the constellation rather than brightness. The ancient Arabs referred to this small three-star pattern as "The Virgins," to which the name "Adhara" actually refers (Adhara the westernmost of the three). No one knows why the name was given or who they were. Adhara has an apparent magnitude of 1.50, and therefore is sometimes referred to as the last of the first magnitude stars or as the brightest of the second magnitude depending on where one wants to draw the line. The latter view and the star's rather southern position in the sky has led to its being somewhat ignored. In fact, Adhara, a class B (B2) bright giant, is quite the magnificent star. Among the hotter of bright stars, Adhara shines with a surface temperature of some 21,900 degrees Kelvin, which gives it a sparkling bluish cast. From its distance of 405 light years (second Hipparcos reduction) we calculate a luminosity to the eye of 3500 times that of the Sun (including the effect of a six percent dimming by interstellar dust). If at the distance of Sirius, which dominates Canis Major, Adhara would shine at apparent magnitude -7, 7 times brighter than Venus at her most luminous. Because of the star's high temperature, it radiates a good portion of its energy in the invisible ultraviolet. If that is taken into account, Adhara is actually 22,300 times more luminous than the Sun. Indeed, if you had ultraviolet eyes, Adhara would be the brightest star in the sky. Adhara's angular diameter from luminosity and temperature leads to a physical radius 10.4 times solar, which is just three percent smaller than that found from the measured angular diameter, showing that the measured parameters are all very nearly correct. An equatorial rotation velocity of at least 38 kilometers per second then give a rotation period of under 14 days. Temperature and luminosity plus theory then lead to a mass 11 to 12 times that of the Sun, which is probably enough to make the star someday explode as a supernova (or at least to produce a rare neon-oxygen white dwarf, most of them being made of carbon and oxygen). Having ceased hydrogen fusion in its core, Adhara is actually in the beginning of its death stages (cessation of core hydrogen fusion), and is now more in its subgiant rather than giant state. Adhara is very popular among those who study local interstellar matter, as its simple and bright spectrum is used to examine the stuff that lies between it and the Sun. Seven seconds of arc away lies an 8th or 9th magnitude (depending on whom you ask) companion that from its brightness is probably on the line between classes A and F. The two are at least 900 Astronomical Units apart and take at least 7500 years to make a full orbit.
Written by Jim Kaler. Last updated 6/26/09. Return to STARS.