Beta Arietis - Sheratan

At magnitude 2.64, Sheratan is the second brightest star in Aries, behind Hamal. Its name originally referred to both Sheratan and Mesarthim, and evoked "two things" whose meanings have been lost to time. The name is now applied to β Ari alone.

Properties

At first, the star appears to be a very ordinary, white, class A5 main sequence star, fusing hydrogen to helium in its core. At a distance of 60 light years, and with a temperature of 8200 K, it emits 22 solar luminosities into space.

However, β Ari has been known for a century to have a hidden companion that is only detectable through Doppler shifts in its spectrum. While such discoveries are not unusual, Sheratan stands out as because of the record-holding eccentricity of the orbit (0.88). As the stars wheel around each other in their hugely eccentric orbit, the smaller one (a class G star like our Sun) approaches as close as 0.08 AU, and then recedes to 1.2 AU - sixteen times farther away. No close planets could survive the gravitational onslaught.

Moreover, the star is an observational treasure. The two stars too close together to be separated visually through the telescope; detection via spectrograph requires the stars to be close together and moving quickly. However, sophisticated observation of Sheratan with an interferometer allows the pair to be resolved directly. The masses of the stars can then be computed accurately. Averaging 0.64 AU apart, a star with the 1.02 times the mass of the Sun orbits a 2.0 solar-mass star every 107 days.

Since stellar luminosity is very dependent on mass, 95% of the light of the system is produced by the heavier star. Such double stars, in which the components are detectable by two techniques, provides powerful evidence that the theoretical relation between stellar mass and luminosity is correct. The higher-mass star will become a shrunken, dim white dwarf first, and the lower mass G star will be the brighter of the pair.

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