ACUBENS (Alpha Cancri). Though Bayer's Alpha star, Acubens, at
faint fourth magnitude (4.25), ranks only fourth in the
constellation Cancer, after Beta (Al Tarf), Delta (Asellus Australis), even
Iota, probably because of its position as a southern claw of the
celestial crab. The star's name, which it actually shares with a
northern claw (Iota), is derived from an Arabic word that means
just that, "the claw," and is the same root from which is derived
the name Zubenelgenubi (Alpha Librae),
the star that represents the southern claw of Scorpius. Acubens, 175 light years away, is something
of a mystery. Seemingly a simple white class A (A5) star, its
spectrum displays ultra-strong absorptions of particular metals
that make it a "metallic line" or "Am" star. Metallic line stars
are typically strongly enhanced in elements like zinc, strontium,
zirconium, barium and others. The phenomenon is a surface effect
in which some elements sink to lower layers under the action of
gravity, while others rise, pushed upward by radiation. To be such
chemically peculiar stars, they must rotate slowly, such that the
surface gases are not stirred up. Class A stars tend to rotate
quickly however, so there should be slowing mechanism, which for
the Am stars is duplicity (each acting tidally to slow the other).
Acubens, though, rotates more quickly than usual (at least 68
kilometers per second at the equator). It is, however, a double
star. It is close enough to the ecliptic plane that the Moon
occasionally passes over and occults it. Stars normally wink out
very quickly when the Moon covers them; Acubens winked out twice,
showing that it consists of two identical stars only 0.1 seconds of
arc (5.3 astronomical units) apart. Each is presumably a dwarf Am
star with double the mass of the Sun, each
shining with 23 solar luminosities. Given the masses and
separation, they should orbit each other every 6.1 years. Neither
can be discriminated from the other, however. Moreover, Acubens is
not just double, but quadruple. Eleven seconds of arc away is a
12th magnitude companion that is ITSELF double -- about all that is
known of it. If the two are identical, both would be dim class M
dwarfs. Separated by at least 600 astronomical units, the faint
pair must take at least 6300 years to orbit the bright pair (which
from the faint pair would appear about as far apart as the angular
diameter of the full Moon).