Alpha Cancri, also called Acubens, is the fourth brightest star in the constellation Cancer, at magnitude 4.25. The star's name is derived from an Arabic word that means "the claw."
Properties
Acubens is 175 light years away. It is a white class A5 V main sequence star whose spectrum displays very strong absorptions of particular metals. This makes it a "metallic line" or "Am" star, strongly enhanced in elements like zinc, strontium, zirconium, and barium. The phenomenon is a surface effect: some elements sink lower due to gravity, while others are pushed upward by radiation. Such chemically peculiar stars must rotate slowly, so that their surface gases are not stirred up. Class A stars tend to rotate quickly, however, so there must be a slowing mechanism. For the Am stars, this is usually a companion star, acting tidally to slow the other.
Components
While Acubens rotates more quickly than most Am stars (at least 68 km/sec at the equator), it is actually a double star. The pair cannot be resolved visually, however. Instead, its duplicity is revealed by lunar occultations. Acubens appears close enough to the ecliptic that the Moon occasionally passes in front of it. Stars normally disappear very quickly when the Moon occults them; Acubens "winks out" twice, showing that it consists of two identical stars only 0.1" apart (5.3 AU). Each is a dwarf Am star with twice the mass of the Sun, shining with 23 solar luminosities. Given their masses and separation, these components should orbit each other every 6.1 years.
In fact, Acubens is not just double, but quadruple. Eleven arcseconds away is a 12th-magnitude companion that is itself double. Little else is known about it. If its components are identical, both would be dim class M dwarfs. Separated by at least 600 AU from the bright pair, the fainter pair must take at least 6300 years to go around it. From the faint pair, the brighter two would appear about as far apart as the angular diameter of the full Moon.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]