Beta Capricorni is among the more complex of the sky's naked-eye stars. Second brightest in the constellation Capricornus, it shines at magnitude 3.08. It also has the traditional name Dabih, which comes from the Arabic, meaning "butchers". Because it is near the ecliptic, β Cap can be occulted by the Moon, and also (rarely) by planets.
Components
Dabih is a wide naked-eye (or at least binocular) double. The pair, separated by 205", is dominated by β1 Cap or Dabih Major, with an apparent magnitude of +3.05. Its magnitude 6.10 companion is β2 Cap, or Dabih Minor. At a distance of 330 light years, the two are separated by at least 21,000 AU, and take at least 700,000 years to orbit each other. Due to the complexity of this system, several different schemes have arisen to denote the subcomponents.
The fainter Dabih Minor (β2 Cap) is also the simpler of the two. Lunar occultations as well as space-based observations show that it is a binary star, dominated by a class B9.5 subgiant (β Cap B) about 40 times more luminous than our Sun. It is also an especially well known "mercury-manganese" star, with huge proportions of these elements in its atmosphere. Recent measures show platinum, gold, mercury, and bismuth elevated by 100,000 times the levels found in the Sun, caused by a combination of radiation and gravity acting in a quiet stellar atmosphere. The other component of Dabih Minor (β Cap C) is a much fainter and cooler class F main sequence star separated from its host by about 3" (30 AU at the system's distance). No orbit has yet been determined.
Dabih Major (β1 Cap), the brighter of the wide pair, is more complex. It has a composite spectrum, probably a cool K0 bright giant (β Cap Aa) coupled with a hot class B8 secondary (β Cap Ab). These two components are separated by 0.05" (4 AU); with spectroscopy, they are observed to have an orbital period of take 1374 days. The cooler Aa component has a mass just short of 4 times the Sun's, a temperature of 4900 K, a luminosity of 600 suns, and a radius around 35 suns. The hotter Ab component, with around the same mass, has an unseen companion (β Cap Ac), which orbits Ab with a period of 8.7 days at a distance of only a tenth of an AU. This component seems to have been detected by lunar occultation as well, but little else is known about it.
Two other nearby stars, sometimes referred to as β Cap D and E, were discovered by John Herschel. These lie 112" away from β1 Cap, and it is unclear whether they are simply optical doubles or physically related to β1 Cap. At least quintuple, the Dabih system could well make an astronomer's lifetime study.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]