SHERATAN (Beta Arietis). The most prominent part of Aries, the Ram, historically the "first" constellation of the Zodiac (as it held the Vernal Equinox in ancient times), is a thin flat triangle of stars that Bayer lettered (from east to west) Alpha (Hamal), Beta (our Sheratan), and Gamma (Mesarthim). The first two are also in order of brightness, just- barely-third magnitude (2.64) Sheratan ranking second behind Hamal. The name originally referred to both Sheratan and Mesarthim, and evoked "two things" whose natures have been lost to time, the name now applied to Beta Arietis alone. At first, the star looks like a very ordinary, white, mid-class A (A5) main sequence dwarf, one in the normal process of fusing hydrogen to helium in its core. At a distance of 60 light years, and with a temperature of 8200 Kelvin, it is pumping 22 solar luminosities into space. However, it keeps a secret from the eye, a companion that has been known for a century and that is visible only by means Doppler motions in the spectrum (that detect line of sight movement). While such discoveries are not at all unusual, Sheratan stands out as a result of the extremely high eccentricity of the orbit (0.88), the companion trapped in a record-holding elongated path. Moreover, the star is an observational treasure. The two stars are so close together that they cannot be separated directly through the telescope; all we ever actually see is one star (again common, as to allow detection via the spectrum requires the stars to be close and moving quickly). However, sophisticated observation of Sheratan with an interferometer, a device that makes use of the interfering properties of light to resolve ultra-fine detail, allow (as for the brighter component of Mizar) the pair to be resolved. The masses of the stars (through gravitational theory) can then be measured with high accuracy. Averaging 0.64 Astronomical Units apart (89 percent Venus's distance from the Sun), a star with the mass of the Sun (1.02 solar) orbits a double-solar-mass (2.00) star every 107 days. Since luminosity is very sensitive to mass, 95 percent of the light of the system is produced by the heavier star. The huge eccentricity adds the spice. As they wheel around each other, the smaller one (undoubtedly a class G star like the Sun) approaches as close as 0.08 AU (only 20 percent Mercury's distance from the Sun), and then half an orbit later loops around at 1.2 AU, 16 times farther away and 20 percent farther than Earth from the Sun. No close planets could survive the gravitational onslaught. Such stars, in which the doubling is "visible" by two techniques (only about 40 are known, Sheratan one of the brighter), allows accurate assessment of the theoretical relation between stellar mass and luminosity, and provides powerful evidence that the theory is correct. The higher mass star will die first. In a couple billion years, the lower mass G star will be the king of the pair, while the current luminary will be a shrunken dim white dwarf.
Written by Jim Kaler. Return to STARS.