NOTABLE STARS A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site Castor (Alpha Geminorum) The second brightest star in the constellation Gemini and, in mythology, the twin of Pollux. Though appearing as a single white star to the naked eye, Castor is really a remarkable multiple system, consisting of three stellar pairs. A modest telescope shows two similar A stars, of mid-second and mid-third magnitude, a couple of arcseconds apart, which orbit each other every 380 years. About 1' to the south lies a ninth magnitude third companion that orbits the bright pair at a distance of about 1,000 AU. A spectrograph shows that each of the two bright components, A and B, is itself a double. Castor A consists of almost identical stars, each of about 2 solar masses, orbiting each other every 9.22 days and about one-tenth the distance of Mercury from the Sun. Castor B’s twin stars orbit even faster, making their circuit in a mere 2.9 days. The faint, distant star, Castor C, is also double, consisting of nearly identical, low-mass M stars – red dwarfs – with temperatures of about 4,000 K, some two solar diameters apart with an orbital period of 20 hours. One or both are flare stars. Castor is part of a group of widely dispersed stars known as the Castor Moving Group. Visual magnitude 1.58 Absolute magnitude 0.58 Spectral type A1V + A2V Distance 52 light-years Position R.A. 07h 34m 36s, Dec. +31° 53' 18" Related entry    • brightest stars Related category    • NOTABLE STARS Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP