Epsilon Canis Majoris - Adhara

Adhara, or Epsilon Canis Majoris, is a bluish-white star of magnitude 1.50 in Canis Major. It is the second brightest in the constellation, about 12 degrees south of Sirius. Its traditional name, Adhara (sometimes spelled Adara), comes from the Arabic word for "The Virgins", which originally referred to the small three-star pattern including δ and ε CMa.

Properties

Adhara is a blue class B2 giant with a surface temperature of some 20,900 K. It is far more luminous than Sirius, but it is over 430 light-years away, so it appears less prominent. If it were at the distance of Sirius, Adhara would shine at magnitude -7.

About 4.7 million years ago, Adhara was much closer to us, only 34 light years from the Sun. At that time it was the brightest star in the sky, with a magnitude of -3.99. No other star has attained this brightness since, nor will any other for at least five million years.

Because of the star's high temperature, it radiates a good portion of its energy in the ultraviolet. With that taken into account, Adhara is actually 21,700 times more luminous than the Sun. In ultraviolet, Adhara is the brightest star in the sky. Its bright, simple spectrum is used to examine the interstellar matter that lies between it and the Sun. Adhara's measured angular diameter yields a physical radius 11.4 times the sun's, consistent with that implied by its temperature and luminosity. Having ceased fusing hydrogen in its core, Adhara is actually in the beginning of its death stages. Its mass of 10 to 12 suns is probably enough for the star to explode someday as a supernova.

Companion

Adhara is also a double star, with a faint 8th magnitude companion. The companion lies about 7" away, but can only be resolved in large telescopes, since it is approximately 250 times fainter than the primary. From its brightness, the companion is probably a mid-class A star. The two are at least 920 AU apart, and take at least 7500 years to make a full orbit.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]