At magnitude 2.95, α Ara is the second-brightest star in Ara, the Altar, the southernmost of the ancient constellations. It is just 0.10 magnitudes fainter than β Ara.
Properties
Shining from a distance of 240 light years, α Ara is a hot, blue, luminous, class B2 main-sequence star. It is in the most stable part of its life, fusing hydrogen into helium at its core. With a surface temperature of 22,000 K, its luminosity is 2900 times the Sun's, and is over three times the radius of the Sun. It has a mass of about 8 solar masses. Hot class B stars are common near the Milky Way, as they are bright enough to be seen over great distances.
Alpha Arae, however, falls into a special subclass known as "Be" (B-emission) stars, as its spectrum shows hydrogen emission lines from a thick disk that surrounds it. Moreover, it is in a subclass of Be stars called "shell stars," as the disk is seen nearly edge on, obscures part of the star, and superimposes absorptions on the star's spectrum.
Alpha Arae's disk is related to its huge rotation velocity of 300 km/sec, though the actual process that makes the disk is still not known. Though the rotation speed is only a lower limit (the tilt of the star's axis not known), it cannot be much higher, since the disk's thickness makes it appear to be nearly edge-on. The emission lines are also coupled to the star's stellar wind, which causes it to lose mass at a rate of over a tenth of a billionth of a solar mass per year.
α Ara shows eruptive variations in its brightness, which is typical of such shell stars - perhaps at times it is indeed the brightest in its constellation. At just under the mass limit required for a supernova explosion, it will die as a massive white dwarf.
Companion
Alpha Arae may have a nearby companion, but its existence has never been confirmed. It does, however, have a distant class K companion at least 4100 AU away. If it is physically related, it takes at least 94,000 years to complete an orbit.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]