Gacrux

Gacrux /ˈɡækrʌks/,[8][9] Gamma Crucis (Latinised from γ Crucis, abbreviated Gamma Cru, γ Cru), is the nearest class M giant star to the Sun.[10] With an apparent visual magnitude of +1.63,[11] it is the third-brightest star (hence its name) in the southern constellation of Crux (the Southern Cross) and is listed 26th of the brightest stars in the night sky. A line from the two "Pointers", Alpha Centauri through Beta Centauri, leads to within 1° north of this star. Distance has been determined using parallax measurements made during the Hipparcos mission, yielding 88.6 light-years (27.2 parsecs) from the Sun.[1]

γ Crucis
Location of Gacrux, γ Crucis (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Crux
Right ascension 12h 31m 09.95961s[1]
Declination −57° 06′ 47.5684″[1]
Apparent magnitude (V) +1.64[2]
Characteristics
Spectral type M3.5 III[3]
Apparent magnitude (J) −1.99
U−B color index +1.78[2]
B−V color index +1.59[2]
Variable type SRV[3]
Astrometry
Radial velocity (Rv)+20.6[4] km/s
Proper motion (μ) RA: +28.23[1] mas/yr
Dec.: −265.08[1] mas/yr
Parallax (π)36.83 ± 0.18[1] mas
Distance88.6 ± 0.4 ly
(27.2 ± 0.1 pc)
Absolute magnitude (MV)−0.52[5]
Details
Mass1.5 ± 0.3[6] M☉
Radius120[7] R☉
Luminosity758[7] L☉
Temperature3,689[7] K
Other designations
Gacrux, CD−56 4504, γ Crucis, Gl 470, HD 108903, HIP 61084, HR 4763, LTT 4752, SAO 240019.
Database references
SIMBADdata

Nomenclature
The constellation Crux, with the reddish star Gacrux at the top of the constellation (north) in this image

γ Crucis (Latinised to Gamma Crucis) is the star's Bayer designation. Since Gacrux is at roughly −60° declination. It was known to the ancient Greeks and Romans, but oddly in the era lacked a traditional name, and was visible north of 40° latitude due to the precession of equinoxes. The astronomer Ptolemy counted it as part of the constellation of Centaurus.[12] The historical name Gacrux was coined by astronomer Elijah Hinsdale Burritt (1794-1838).[13][14] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[15] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[16] included a table of the first two batches of names approved by the WGSN; which included Gacrux for this star.

In Chinese astronomy, Gamma Crucis was known as 十字架一 (Shí Zì Jià yī, English: the First Star of Cross.).[17]

The people of Aranda and Luritja tribe around Hermannsburg, Central Australia named Iritjinga, "The Eagle-hawk", a quadrangular arrangement comprising Gacrux, Delta Crucis (Imai), Gamma Centauri (Muhilfain) and Delta Centauri (Ma Wei).[18]

Gacrux, Gamma Crucis.

Among Portuguese-speaking peoples, especially in Brazil, it is also named Rubídea (or Ruby-like), in reference to its colour.[19]

Physical properties
Pulsation Periods[3]
Period
(days)		 Amplitude
(magnitude)
12.10.016
15.10.027
16.50.016
54.80.026
82.70.015
104.90.016

Gacrux has the MK system stellar classification of M3.5 III.[3] 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.[10] Although only 30% more massive than the Sun,[6] at this stage the star has expanded to 120[7] times the Sun's radius. It is radiating roughly 760[7] times the luminosity of the Sun from its expanded outer envelope. With an effective temperature of 3,689 K,[7] the colour of Gacrux is a prominent reddish-orange, well in keeping with its spectral classification. It is a semi-regular variable with multiple periods.[3] (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.[20] 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,[21] which is about 400 light years distant from Earth.

In culture

Gacrux 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 on the Republican flag of Brazil, along with 26 other stars, each of which represents a state. Gacrux represents the State of Bahia.[22] The position of the line passing through Gacrux and Acrux marks the local meridian of the sky observed from Rio de Janeiro, at 8:30 am. on 15 November 1889: the time when the republic was formally ratified.[23]

See also

References
  1. 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, S2CID 18759600
  2. Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
  3. 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, S2CID 15358380
  4. 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
  5. 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
  6. 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
  7. Rau, Gioia; Nielsen, Krister E.; Carpenter, Kenneth G.; Airapetian, Vladimir (December 2018). "HST/GHRS Observations of Cool, Low-gravity Stars. VI. Mass-loss Rates and Wind Parameters for M Giants". The Astrophysical Journal. 869 (1): 1. arXiv:1811.10679. Bibcode:2018ApJ...869....1R. doi:10.3847/1538-4357/aaf0a0. ISSN 0004-637X. S2CID 119364960.
  8. "Audio Pronunciation Guide". Starry Night Education. Simulation Curriculum Corp. 2009. Retrieved 26 Apr 2019.
  9. "IAU Catalog of Star Names". Retrieved 28 July 2016.
  10. 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, S2CID 15830460
  11. 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.
  12. Richard Hinckley Allen, "Star Names: Their Lore and Meaning", Dover Press, 1963.
  13. "Gacrux/Gamma Crucis 2?". SolStation.com. Retrieved 2011-11-03.
  14. Lesikar, Arnold V. "Gacrux". Dome Of The Sky. Archived from the original on 2011-09-28. Retrieved 2011-11-03.
  15. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  16. "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  17. (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived January 30, 2011, at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  18. Raymond Haynes; Roslynn D. Haynes; David Malin; Richard McGee (1996), Explorers of the Southern Sky: A History of Australian Astronomy, Cambridge: Cambridge University Press, p. 8, ISBN 978-0-521-36575-8
  19. da Silva Oliveira, R., "Crux Australis: o Cruzeiro do Sul", Artigos: Planetario Movel Inflavel AsterDomus.
  20. 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
  21. Kaler, James B., "GACRUX (Gamma Crucis)", Stars, University of Illinois, retrieved 2012-03-03
  22. Astronomy of the Brazilian Flag, FOTW Flags Of The World website
  23. Flag of Brazil, World Afropedia

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