Epoch J2000.0 Equinox J2000.0
|Right ascension||12h 31m 09.95961s|
|Declination||−57° 06′ 47.5684″|
|Apparent magnitude (V)||+1.63|
|Spectral type||M3.5 III|
|Apparent magnitude (J)||−1.99|
|U−B color index||+1.77|
|B−V color index||+1.59|
|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|
|Mass||1.3 ± 0.1 M☉|
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.
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.
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.
- van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357
- Johnson, H. L. et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory 4 (99): 99. Bibcode:1966CoLPL...4...99J.
- Tabur, V. et al. (December 2009), "Long-term photometry and periods for 261 nearby pulsating M giants", Monthly Notices of the Royal Astronomical Society 400 (4): 1945–1961, arXiv:0908.3228, Bibcode:2009MNRAS.400.1945T, doi:10.1111/j.1365-2966.2009.15588.x
- Wielen, R. et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb (Astronomisches Rechen-Institut Heidelberg) 35 (35): 1, Bibcode:1999VeARI..35....1W
- Elgarøy, Øystein; Engvold, Oddbjørn; Lund, Niels (March 1999), "The Wilson-Bappu effect of the MgII K line - dependence on stellar temperature, activity and metallicity", Astronomy and Astrophysics 343: 222–228, Bibcode:1999A&A...343..222E
- Murdoch, Kaylene; Clark, M.; Hearnshaw, J. B. (January 1992), "The radial-velocity variability of Gamma Crucis", Monthly Notices of the Royal Astronomical Society 254: 27–29, Bibcode:1992MNRAS.254...27M, doi:10.1093/mnras/254.1.27
- Ireland, M. J. et al. (May 2004), "Multiwavelength diameters of nearby Miras and semiregular variables", Monthly Notices of the Royal Astronomical Society 350 (1): 365–374, arXiv:astro-ph/0402326, Bibcode:2004MNRAS.350..365I, doi:10.1111/j.1365-2966.2004.07651.x
- Kaler, James B., "GACRUX (Gamma Crucis)", Stars (University of Illinois), retrieved 2012-03-03
- Cohen, Martin et al. (November 1996), "Spectral Irradiance Calibration in the Infrared. VII.New Composite Spectra, Comparison with Model Atmospheres, and Far-Infrared Extrapolations", Astronomical Journal 112: 2274, Bibcode:1996AJ....112.2274C, doi:10.1086/118180
- Gomez, A. E.; Luri, X.; Grenier, S. et al. (March 1997), "Absolute magnitudes and kinematics of barium stars", Astronomy and Astrophysics 319: 881–885, Bibcode:1997A&A...319..881G
- Richard Hinckley Allen, "Star Names: Their Lore and Meaning", Dover Press, 1963.
- (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
- (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
- p. 8, Explorers of the southern sky: a history of Australian astronomy, Raymond Haynes, Cambridge, Cambridge University Press, 1996.
- Astronomy of the Brazilian Flag, FOTW Flags Of The World website
- "Gacrux/Gamma Crucis 2". SolStation. Retrieved 2008-09-10.