Location of α Gruis (circled) |
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Observation data Epoch J2000 Equinox J2000 |
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Constellation | Grus |
Right ascension | 22h 08m 13.98473s[1] |
Declination | –46° 57′ 39.5078″[1] |
Apparent magnitude (V) | +1.74[2] |
Characteristics | |
Spectral type | B6 V[3] |
U−B color index | –0.47[2] |
B−V color index | –0.13[2] |
Astrometry | |
Radial velocity (Rv) | +11.8[4] km/s |
Proper motion (μ) | RA: +126.69[1] mas/yr Dec.: –147.47[1] mas/yr |
Parallax (π) | 32.29 ± 0.21[1] mas |
Distance | 101 ± 0.7 ly (31 ± 0.2 pc) |
Details | |
Radius | 3.4[5] R☉ |
Surface gravity (log g) | 3.76 ± 0.11[6] |
Temperature | 13,920[7] K |
Metallicity [Fe/H] | –0.13 ± 0.02[8] dex |
Rotational velocity (v sin i) | 215[9] km/s |
Age | 1.0 × 108[10] years |
Other designations | |
Alpha Gruis (α Gruis, α Gru) or Alnair is the brightest star in the southern constellation Grus. Its first depiction in a celestial atlas was in Johann Bayer's Uranometria of 1603.[12]
It has a stellar classification of B6 V,[3] although some sources give it a classification of B7 IV.[13] The first classification indicates that this is a B-type star on the main sequence of stars that are generating energy through the thermonuclear fusion of hydrogen at the core. However, a luminosity class of 'IV' would suggest that this is a subgiant star; meaning the supply of hydrogen at its core is becoming exhausted and the star has started the process of evolving away from the main sequence. It has no known companions.[13]
The measured angular diameter of this star, after correcting for limb darkening, is 1.02 ± 0.07 mas.[5] At a distance of 31 parsecs, this yields a physical size of 3.4 times the radius of the Sun.[14] It is rotating rapidly, with a projected rotational velocity of about 215 km/s providing a lower bound for the rate of azimuthal rotation along the equator.[9] The effective temperature of Alpha Gruis's outer envelope is 13,920 K,[7] giving it the blue-white hue characteristic of B-type stars. The abundance of elements other than hydrogen and helium, what astronomers term the metallicity, is about 74% of the abundance in the Sun.[8] The Spitzer Space Telescope detected an excess of infrared emission from this star, suggesting that a circumstellar disk of dust may be in orbit around it.[10]
Based on the estimated age and motion, it may be a member of the AB Doradus moving group that share a common motion through space. This group has an age of about 70 million years,[15] which is consistent with α Gruis's 100-million-year[10] estimated age (allowing for a margin of error). The space velocity components of this star in the Galactic coordinate system are [U, V, W] = [–7.0 ± 1.1, –25.6 ± 0.7, –15.5 ± 1.4] km/s.[15]
Alpha Gruis has the proper name Alnair or Al Nair (sometimes Al Na'ir), from the Arabic al-nayyir [an-nai:r], meaning "the bright one".[16] It is derived from its Arabic name, al-Nayyir min Dhanab al-ḥūt (al-Janūbiyy), "the Bright (star) belongs to the Tail of (the constellation of) the (Southern) Fish".[16] Confusingly, "Alnair" is also given as the proper name for Zeta Centauri in an astronomical ephemerides in the middle of the 20th century.[17] With β, δ, θ. ι, and λ Gru, Alnair belonged to Piscis Austrinus in traditional Arabic astronomy.[18]
In Chinese, 鶴 (Hè), meaning Crane, refers to an asterism consisting of α Gruis, β Gruis, ε Gruis, η Gruis, δ Tucanae, ζ Gruis, ι Gruis, θ Gruis, δ2 Gruis and μ1 Gruis.[19] Consequently, α Gruis itself is known as 鶴一 (Hè yī, English: First Star of the Crane).[20] The Chinese name gave rise to another English name, Ke.[21]
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