Gacrux
File:Gacrux.jpg Gacrux, γ Crucis | |
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Crux |
| Right ascension | Template:RA[1] |
| Declination | Template:DEC[1] |
| Apparent magnitude (V) | +1.64[2] |
| Characteristics | |
| Evolutionary stage | Red giant branch |
| Spectral type | M3.5 III[3] |
| Apparent magnitude (J) | −1.99 |
| U−B Template:Engvar | +1.78[2] |
| B−V Template:Engvar | +1.59[2] |
| Variable type | SRV[3] |
| Astrometry | |
| Radial velocity (Rv) | +20.6[4] km/s |
| Proper motion (μ) | RA: +28.23 mas/yr[1] Dec.: −265.08 mas/yr[1] |
| Parallax (π) | 36.83±0.18 mas[1] |
| Distance | Template:Rnd ± Template:Rnd ly (Template:Rnd ± Template:Rnd pc) |
| Absolute magnitude (MV) | Script error: No such module "val".[5] |
| Details | |
| Mass | 1.5±0.2[6] Template:Solar mass |
| Radius | 73[7] Template:Solar radius |
| Luminosity | 830[6] Template:Solar luminosity |
| Surface gravity (log g) | Script error: No such module "val".[6] cgs |
| Temperature | Script error: No such module "val".[6] K |
| Metallicity [Fe/H] | {{{metal_fe2}}} dex |
| Other designations | |
| Template:Odlist | |
| Database references | |
| SIMBAD | data |
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Gacrux is the third-brightest star in the southern constellation of Crux, the Southern Cross. It has the Bayer designation Gamma Crucis, which is Latinised from γ Crucis and abbreviated Gamma Cru or γ Cru. With an apparent visual magnitude of +1.63,[8] it is the 26th brightest star in the night sky. A line from the two "Pointers", Alpha Centauri through Beta Centauri, leads to within 1° north of this star. Using parallax measurements made during the Hipparcos mission, it is Script error: No such module "convert". distant.[1] It is the nearest M-type red giant star.[9]
Nomenclature
γ Crucis (Latinised to Gamma Crucis) is the star's Bayer designation. Gacrux is currently at roughly 60° south declination. It was known and visible to the ancient Greeks and Romans as it was visible north of 40° latitude because of the precession of equinoxes. Oddly, it lacked a traditional name. The astronomer Ptolemy counted it as part of the constellation of Centaurus.[10] The name Gacrux was coined by astronomer Elijah Hinsdale Burritt (1794-1838) as a contraction of the Bayer designation.[11][12] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[14] 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 Script error: No such module "Lang". (Script error: No such module "Lang"., Template:Langx).[15]
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).[16] To the Boorong clan of the Wergaia, this star is known as Bunya.[17][18]
Among Portuguese-speaking peoples, especially in Brazil, it is also named Rubídea (or Ruby-like), in reference to its colour.[19]
Physical properties
| 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 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.[9] Although only 50% more massive than the Sun,[6] at this stage the star has expanded to 73[7] times the Sun's radius. It is radiating roughly 830 times the luminosity of the Sun from its expanded outer envelope.[6] With an effective temperature of 3,689 K,[6] 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, Samoa, and Papua New Guinea as one of five stars (four in the case of New Zealand) that compose the Southern Cross.
It is also featured on the 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
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- ↑ a b Script error: No such module "Citation/CS1".
- ↑ Richard Hinckley Allen, "Star Names: Their Lore and Meaning", Dover Press, 1963.
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- ↑ Template:In lang 香港太空館 - 研究資源 - 亮星中英對照表 Template:Webarchive, Hong Kong Space Museum. Accessed on line November 23, 2010.
- ↑ Script error: No such module "citation/CS1".
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- ↑ da Silva Oliveira, R., "Crux Australis: o Cruzeiro do Sul", Artigos: Planetario Movel Inflavel AsterDomus.
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Further reading
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Template:Stars of Crux Template:Authority control Template:Sky