 Gacrux, γ Crucis | |
| Observation data EpochJ2000.0 EquinoxJ2000.0 | |
|---|---|
| Constellation | Crux |
| Right ascension | 12h 31m 09.960s[1] |
| Declination | −57° 06′ 47.57″[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−Bcolor index | +1.78[2] |
| B−Vcolor index | +1.59[2] |
| Variable type | SRV[3] |
| Astrometry | |
| Radial velocity (Rv) | +20.6[4] km/s |
| Proper motion (μ) | RA: +28.23mas/yr[1] Dec.: −265.08mas/yr[1] |
| Parallax (π) | 36.83±0.18 mas[1] |
| Distance | 88.6 ± 0.4 ly (27.2 ± 0.1 pc) |
| Absolute magnitude (MV) | −0.68±0.01[5] |
| Details | |
| Mass | 1.5±0.2[6] M☉ |
| Radius | 73[7] R☉ |
| Luminosity | 830[6] L☉ |
| Surface gravity (log g) | 0.75±0.02[6] cgs |
| Temperature | 3689±125[6] K |
| Other designations | |
| Gacrux,γ Crucis,CD−56 4504,GJ 470,HD 108903,HIP 61084,HR 4763,SAO 240019,LTT 4752 | |
| Database references | |
| SIMBAD | data |
Gacrux is the third-brightest star in the southern constellation ofCrux, the Southern Cross. It has theBayer designationGamma Crucis, which isLatinised fromγ Crucis and abbreviatedGamma Cru orγ Cru. With anapparent visual magnitude of +1.63,[8] it is the26th brightest star in thenight sky. A line from the two "Pointers",Alpha Centauri throughBeta Centauri, leads to within 1° north of this star. Usingparallax measurements made during theHipparcos mission, it is 88.6light-years (27.2parsecs) distant.[1] It is the nearest M-typered giant star.[9]
γ Crucis (Latinised toGamma Crucis) is the star'sBayer designation. Gacrux is currently at roughly 60° southdeclination. It was known and visible to theancient Greeks andRomans as it was visible north of 40°latitude because of theprecession of equinoxes. Oddly, it lacked a traditional name. The astronomerPtolemy counted it as part of the constellation ofCentaurus.[10] The nameGacrux was coined by astronomer Elijah Hinsdale Burritt (1794-1838) as a contraction of the Bayer designation.[11][12] In 2016, theInternational Astronomical Union organized aWorking 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 includedGacrux for this star.
InChinese astronomy, Gamma Crucis was known as十字架一 (Shí Zì Jià yī, English:the First Star of Cross).[15]
The people ofAranda andLuritja tribe aroundHermannsburg,Central Australia namedIritjinga, "The Eagle-hawk", a quadrangular arrangement comprising Gacrux,Delta Crucis (Imai),Gamma Centauri (Muhilfain) andDelta Centauri (Ma Wei).[16] To the Boorong clan of theWergaia, this star is known as Bunya.[17][18]
Among Portuguese-speaking peoples, especially in Brazil, it is also namedRubídea (or Ruby-like), in reference to its colour.[19]
| 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 systemstellar classification of M3.5 III.[3] It hasevolved off of themain sequence to become ared giant star, but is most likely on thered giant branch rather than theasymptotic giant branch.[9] Although only 50% more massive than the Sun,[6] at this stage the star has expanded to 73[7] times theSun's radius. It is radiating roughly 830 times theluminosity of the Sun from its expandedouter envelope.[6] With aneffective temperature of 3,689 K,[6] the colour of Gacrux is a prominent reddish-orange, well in keeping with its spectral classification. It is asemi-regular variable with multiple periods.[3] (See table at left.)
The atmosphere of this star is enriched withbarium, which is usually explained by the transfer of material from a more evolved companion. Typically this companion will subsequently become awhite dwarf.[20] However, no such companion has yet been detected. A +6.4 magnitude companion star named HD 108925 (A3IVn) lies about 2 arcminutes away at aposition angle of 128° from the main star, and can be observed withbinoculars. But it is only an optical companion,[21] which is about 400 light years distant from Earth.
Gacrux is represented in the flags ofAustralia,New Zealand,Samoa, andPapua New Guinea as one of five stars (four in the case of New Zealand) that compose theSouthern Cross.
It is also featured on theflag of Brazil, along with 26 other stars, each of which represents a state. Gacrux represents the State ofBahia.[22] The position of the line passing through Gacrux andAcrux marks thelocal meridian of the sky observed fromRio de Janeiro, at 8:30 am on 15 November 1889, the time when the republic was formally ratified.[23]
