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.