At magnitude 1.59, the reddish star Gamma Crucis, or Gacrux, is the third-brightest star in constellation Crux. It is one of the four stars in the Southern Cross, and it marks the north end of the asterism. A line through the two "Pointers", α and β Centauri, leads to within a few degrees of this star.
Since Gamma Crucis lies at roughly declination -57°, it is only visible south of the Tropic of Cancer, and never received an ancient traditional name. Thus, "Gacrux" is simply a combination of the "Ga" in "Gamma" plus the name of the constellation, Crux. It is known as the First Star of the Cross in Chinese.
Properties
Gamma Crucis has a prominent reddish-orange color, well in keeping with its spectral classification of M3.5 III, and surface temperature of 3400 K. From a distance of 88 light years, it shines with about 140 times the Sun's visual luminosity, and 1,500 times its total luminosity if infrared radiation is included. The star may be three times the Sun's mass, and as much as 113 times its diameter. It has evolved off of the main sequence to become a red giant, and may even be evolving past the helium-fusing stage.
Gacrux is also semi-regular variable, fluctuating by a few tenths of a magnitude, with a period of about 90 days. If Gacrux has gone through its core helium-fusion stage, it is in the process of becoming a "second-ascent" giant, a speculation reinforced by its variability. Perhaps it will become a full-blown pulsating Mira variable.
Companions
The atmosphere of Gacrux is enriched with barium. Such barium stars are usually binaries with a white dwarf companion, which has already thrown off its outer gas envelopes as a planetary nebula, and contaminated the other with the by-products of its more advanced nuclear reactions. Hence Gacrux may have an unseen white dwarf companion.
Gamma Crucis is a visual double star, with a main sequence companion of spectral class A3 V. The magnitude 6.4 companion star lies 25' away, and can be observed with binoculars. However, the star now appears to be a optical companion, as astronomers now believe that it lies four times farther away from Sol than Gacrux. Note that our Sun, as seen from this companion star, would also appear to be a companion of Gacrux! In addition to the A3 star, Gacrux has two other optical companions that are not gravitationally bound.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]