BETA CRT (Beta Crateris). Among the dimmest of constellations, ancient Crater (the Cup) balances on the back of Hydra (the Water Serpent) to the west of Corvus (the Crow), which does the same, the three once considered a sort of "triplet." Though Crater's outlining stars are only fourth and fifth magnitude, under a dark sky it quite stands out. To the ancient Greeks it was Zeus's Goblet. Oddly the brightest star is Delta Crateris, while the Alpha designation went to fainter number two, which is in a virtual tie with Gamma but has the constellation's only proper name (Alkes). At nearly fifth magnitude (4.48), Beta Crt is actually number four, which shows once again that Bayer had something else also in mind (usually position) when he Greek-lettered the stars, Beta falling at the figure's base. A spectral class A (A2) "giant," Beta Crt is 340 light years away, give or take 24. The temperature of 8954 Kelvin, consistent with class, is of minor consequence since most of the light falls in the visual spectrum with only a small correction for ultraviolet. That along with distance yields a luminosity of 147 Suns and a radius of 5.1 times solar. The projected equatorial rotation velocity of 54 kilometers per second is low enough to expect separation of elements, resulting in a "metallic" star like Sirius. But the chemical composition is solar, which suggests atmospheric stirring from a faster spin, so that the rotation pole must be rather tipped toward us. Theory gives a mass of 2.9 to 3.0 times that of the Sun and shows the star to be not so much a giant as a subgiant that has just quit core hydrogen fusion after a lifetime of 350 million years. What makes Beta Crt really special is its 13th magnitude (13.4) hot (34,000 Kelvin or so) white dwarf companion, which puts the star back in league with Sirius. There is an apparent dearth of such binaries, so astronomers are avidly looking for them. Hints of a companion (from spectral Doppler shifts) go back to 1928, but the actual discovery from the white dwarf's strong ultraviolet radiation is only recent. White dwarfs, with masses that range from about half to 1.4 Suns, are the carbon/oxygen cores of stars initially born with masses from 0.8 to about 10 times solar, most of a star's mass being lost to space during late evolution. There are two kinds, "DA" wd's like Sirius B and Beta Crt B that have thin, pure hydrogen skins, and DB's with helium envelopes. Since higher mass stars have shorter lives, Beta Crt B must have evolved first and be the product of a star born with more than 3 Suns, yielding a white dwarf mass of 0.70 solar. But the mass, estimated at just at just 0.43 Suns, is far too low, suggesting strong ancient mass transfer to Beta Crt A, which then would have a birth mass under 3 Suns, confusing the issue. On the other hand, "B" seems too far away from "A" for any such action to have taken place. Factoring in an estimate of ultraviolet radiation, the white dwarf's luminosity is still some three-quarters that of the Sun. Given the temperature, its radius is just under three Earths, which is actually rather large (Sirius B is 0.92 the size of Earth), but consistent, as the smaller a white dwarf's mass, the larger it is thanks to a lowered pull of gravity.

Written byJim Kaler 5/22/15. Return to STARS.