Alpha Gruis

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Alpha Gruis
Diagram showing star positions and boundaries of the Grus constellation and its surroundings
Cercle rouge 100%.svg

Location of α Gruis (circled)
Observation data
Epoch J2000      Equinox J2000
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 ± 0.7 ly
(31.0 ± 0.2 pc)
Details
Mass 4.0[5] M☉
Radius 3.4[6] R☉
Luminosity 263[5] L☉
Surface gravity (log g) 3.76 ± 0.11[7] cgs
Temperature 13,920[8] K
Metallicity [Fe/H] –0.13 ± 0.02[9] dex
Rotational velocity (v sin i) 215[10] km/s
Age 0.10[11] Gyr
Other designations
Al Na'ir, CD−47° 14063, FK5 829, GCTP 5339.00, Gl 848.2, HD 209952, HIP 109268, HR 8425, SAO 230992.[12]
Database references
SIMBAD data

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.[13]

Properties

Alpha Gruis has a stellar classification of B6 V,[3] although some sources give it a classification of B7 IV.[14] 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.[14]

The measured angular diameter of this star, after correcting for limb darkening, is 1.02 ± 0.07 mas.[6] At a parallax-measured distance of 101 light-years (31 parsecs) from Earth, this yields a physical size of 3.4 times the radius of the Sun.[15] 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.[10] This star has around four times the Sun's mass and is radiating 263 times the luminosity of the Sun.[5]

The effective temperature of Alpha Gruis's outer envelope is 13,920 K,[8] giving it the blue-white hue characteristic of B-type stars.[16] The abundance of elements other than hydrogen and helium, what astronomers term the metallicity, is about 74% of the abundance in the Sun.[9]

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,[17] which is consistent with α Gruis's 100-million-year[11] 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.[17]

Name

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".[18] 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".[18] Confusingly, "Alnair" is also given as the proper name for Zeta Centauri in an astronomical ephemerides in the middle of the 20th century.[19] With β, δ, θ. ι, and λ Gru, Alnair belonged to Piscis Austrinus in traditional Arabic astronomy.[20]

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.[21] Consequently, α Gruis itself is known as 鶴一 (Hè yī, English: First Star of the Crane).[22] The Chinese name gave rise to another English name, Ke.[23]

References

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External links

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  9. ↑ 9.0 9.1 Lua error in package.lua at line 80: module 'strict' not found.. The fractional abundance relative to the Sun is given by:
    10−0.13 = 0.74, or 74%.
  10. ↑ 10.0 10.1 Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ 11.0 11.1 Lua error in package.lua at line 80: module 'strict' not found.
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  15. ↑ Lua error in package.lua at line 80: module 'strict' not found.. The radius (R*) is given by:
    \begin{align} 2\cdot R_* & = \frac{(10^{-3}\cdot 31\cdot 1.02)\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\ & \approx 6.8\cdot R_{\bigodot} \end{align}
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  19. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  20. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  21. ↑ (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.
  22. ↑ (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.
  23. ↑ Richard Hinckley Allen: Star Names — Their Lore and Meaning: Grus
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