Gamma Crucis

Gamma Crucis

Location of γ Crucis (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Crux
Right ascension	12h 31m 09.95961s
Declination	−57° 06′ 47.5684″
Apparent magnitude (V)	+1.64
Characteristics
Spectral type	M3.5 III
Apparent magnitude (J)	−1.99
U−B color index	+1.78
B−V color index	+1.59
Variable type	SRV
Astrometry
Radial velocity (Rv)	+20.6 km/s
Proper motion (μ)	RA: +28.23 mas/yr Dec.: −265.08 mas/yr
Parallax (π)	36.83 ± 0.18 mas
Distance	88.6 ± 0.4 ly (27.2 ± 0.1 pc)
Absolute magnitude (MV)	−0.52
Details
Mass	1.3 ± 0.1 M☉
Radius	84 R☉
Luminosity	1,500 L☉
Temperature	3,626 K
Other designations
Gacrux, CD−56 4504, Gamma Crucis, GJ 470, HD 108903, HIP 61084, HR 4763, LTT 4752, SAO 240019.
Database references
SIMBAD	data

Gamma Crucis (γ Cru, γ Crucis), often called Gacrux, is the nearest red giant star to the Sun. The distance to Gacrux has been determined using parallax measurements made during the Hipparcos mission, which yielded a value of 88.6 light-years (27.2 parsecs) away from Earth. With an apparent visual magnitude of +1.63, this is the third-brightest star in the southern circumpolar constellation of Crux, the Southern Cross, and one of the brightest stars in the night sky. Among Portuguese-speaking peoples it is also named "Rubídea" (or Ruby-like), in reference to its color. A line from the two "Pointers", Alpha Centauri through Beta Centauri, leads to within a few degrees of this star.

Physical properties

Pulsation Periods

Period (days)	Amplitude (magnitude)
12.1	0.016
15.1	0.027
16.5	0.016
54.8	0.026
82.7	0.015
104.9	0.016

Gacrux has a stellar classification M3.5 III. It has evolved off of the main sequence to become a red giant star, but is most likely on the red giant branch rather than the asymptotic giant branch. Although only 30% more massive than the Sun, at this stage the star has expanded to 84 times the Sun's radius. It is radiating 1,500 times the luminosity of the Sun from its expanded outer envelope. With an effective temperature of 3,626 K, the colour of Gamma Crucis is a prominent reddish-orange, well in keeping with its spectral classification. It is a semi-regular variable with multiple periods. (See table at left.)

The atmosphere of this star is enriched with barium, which is usually explained by the transfer of material from a more evolved companion. Typically this companion will subsequently become a white dwarf. However, no such companion has yet been detected. A +6.4 magnitude companion star lies about 2 arcminutes away at a position angle of 128° from the main star, and can be observed with binoculars. But it is only an optical companion, which is about 400 light years distant from Earth.

In culture

Since Gamma Crucis, "Gacrux", is at roughly −60° declination, it lacks a traditional name. Nonetheless, it was known to the ancient Greeks and Romans, in whose era it was visible north of 40° latitude due to the precession of equinoxes. The astronomer Ptolemy counted it as part of the constellation Centaurus.

In Chinese, 十字架 (Shí Zì Jià), meaning Cross, refers to an asterism consisting of γ Crucis, α Crucis, β Crucis and δ Crucis. Consequently, γ Crucis itself is known as 十字架一 (Shí Zì Jià yī, English: the First Star of Cross.).

The people of Aranda and Luritja tribe around Hermannsburg, Central Australia named Iritjinga, "The Eagle-hawk", a quadrangular arrangement comprising this star, δ Cru (Palida), γ Cen (Muhilfain) and δ Cen (Ma Wei).

γ Cru is represented in the flags of Australia, New Zealand and Papua New Guinea as one of five stars which comprise the Southern Cross. It is also featured in the flag of Brazil, along with 26 other stars, each of which represents a state. γ Cru represents the State of Bahia.

See also

• Aldebaran
• Alpha Crucis
• Beta Crucis
• Betelgeuse