Elnath, also spelled El Nath, Alnath, or Al Nath, lies at the north boundary of Taurus with Auriga. It marks the tip of the northern horn of the Taurus, the Bull. "El Nath", from Arabic, means "the butting one"; its name transferred over from stars in Aries. Elnath is one of the sky's two "linking stars", containing Bayer designations in two different constellations. (The other one is Alpheratz, α And or δ Peg.) Formally now in Taurus, Elnath is also labelled Gamma Aurigae, and is part of the classic pentagon of Auriga. The γ Aur designation is no longer used, however.
At magnitude 1.65, Elnath is one of the sky's brighter stars, ranking second in both Taurus and Auriga (behind Aldebaran and Capella), and is the 25th brightest in the sky. Elnath lies only three degrees to the west of the anticenter of the Galaxy - i.e. the point directly opposite the Galactic Center in Sagittarius. In this direction lie vast dark clouds of cold interstellar gas and dust in which stars are being born.
Physical Properties
Physically, Elnath is a hot (13,600 K) class B giant star. From its distance of 130 light years, we find a luminosity almost 700 times the Sun's. Its radius as derived from temperature and luminosity, 4.6 times the Sun's, agrees well with that found from the original interferometric measurement of its angular diameter, though not with a more recent measurement which makes the star 35% larger. Elnath has a mass of around 4.5 times the Sun's. Fusion of hydrogen into helium in its core is beginning to shut down, if it has not done so already. Within a couple of million years, it will expand to become a cool orange giant star.
Like so many stars of its class, Elnath is chemically peculiar, and appears to be a "mercury-manganese" star. Its manganese abundance is 25 times normal, with calcium and magnesium abundances reduced to only 1/8 the Sun's. These peculiarities are caused by the combined action of gravity and radiation, which cause some elements to drift downward out of sight, and causes others to rise. The best bright example is, by a rather amazing coincidence, the other "linking star," Alpheratz!
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]