Altair

This article is about the star in the constellation Aquila. For other uses, see Altair (disambiguation).

Altair

Altair in the constellation of Aquila.
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Aquila
Right ascension	19h 50m 46.99855s[1]
Declination	+08° 52′ 05.9563″[1]
Apparent magnitude (V)	0.77[2]
Characteristics
Spectral type	A7V[2]
U−B color index	+0.08 [3]
B−V color index	+0.22 [3]
V−R color index	0.0 [2]
R−I color index	+0.14 [3]
Variable type	Delta Scuti[2]
Astrometry
Radial velocity (Rv)	−26.1 ± 0.9 [2] km/s
Proper motion (μ)	RA: 536.23[1] mas/yr Dec.: 385.29[1] mas/yr
Parallax (π)	194.95 ± 0.57[1] mas
Distance	16.73 ± 0.05 ly (5.13 ± 0.01 pc)
Absolute magnitude (MV)	2.21 [nb 1]
Details
Mass	1.79 [4] M☉
Radius	1.63 to 2.03 [4][nb 2] R☉
Luminosity	10.6 [5] L☉
Temperature	6,900 to 8,500 [4][nb 2] K
Metallicity	[Fe/H] = −0.2 [4]
Rotation	8.9 hours [5]
Rotational velocity (v sin i)	240 [4] km/s
Age	<109  [6] years
Other designations
Atair, α Aquilae, α Aql, Alpha Aquilae, Alpha Aql, 53 Aquilae, 53 Aql, BD+08°4236, FK5 745, GCTP 4665.00, GJ 768, HD 187642, HIP 97649, HR 7557, LFT 1499, LHS 3490, LTT 15795, NLTT 48314, SAO 125122, WDS 19508+0852A.[2][3][7]
Database references
SIMBAD	data

Altair (Alpha Aquilae, Alpha Aql, α Aquilae, α Aql, Atair) is the brightest star in the constellation Aquila and the twelfth brightest star in the night sky. It is an A-type main sequence star with an apparent visual magnitude of 0.77 and is one of the vertices of the Summer Triangle; the other two are Deneb and Vega.^[2][6][8]

Altair rotates rapidly, with a velocity at the equator of approximately 286 km/s.^{[nb 3][4]} A study with the Palomar Testbed Interferometer revealed that Altair is not spherical, but is flattened at the poles due to its high rate of rotation.^[9] Other interferometric studies with multiple telescopes, operating in the infrared, have imaged and confirmed this phenomenon.^[4]

Star

Altair

Altair is located 16.7 light-years (5.13 parsecs) from Earth and is one of the closest stars visible to the naked eye.^[10] Along with Beta Aquilae and Gamma Aquilae, it forms the well-known line of stars sometimes referred to as the Family of Aquila or Shaft of Aquila.^[11]

Altair is a type-A main sequence star with approximately 1.8 times the mass of the Sun and 11 times its luminosity.^[4][5] Altair possesses an extremely rapid rate of rotation; it has a rotational period of approximately 9 hours.^[5] For comparison, the equator of the Sun requires just over 25 days for a complete rotation. This rapid rotation forces Altair to be oblate; its equatorial diameter is over 20 percent greater than its polar diameter.^[4]

Satellite measurements made in 1999 with the Wide Field Infrared Explorer showed that the brightness of Altair fluctuates slightly, varying by less than a thousandth of a magnitude.^[12] As a result, it was identified in 2005 as a δ Scuti variable star. Its light curve can be approximated by adding together a number of sine waves, with periods that range between 0.8 and 1.5 hours.^[13]

Oblateness and surface temperature

False-color image of the rapidly rotating star Altair, made with the MIRC imager on the CHARA array on Mt. Wilson

The angular diameter of Altair was measured interferometrically by R. Hanbury Brown and his co-workers at Narrabri Observatory in the 1960s. They found a diameter of 3 milliarcseconds.^[14] Although Hanbury Brown et al. realized that Altair would be rotationally flattened, they had insufficient data to experimentally observe its oblateness. Altair was later observed to be flattened by infrared interferometric measurements made by the Palomar Testbed Interferometer in 1999 and 2000. This work was published by G. T. van Belle and his co-authors in 2001.^[9]

Theory predicts that, owing to Altair's rapid rotation, its surface gravity and effective temperature should be lower at the equator, making the equator less luminous than the poles. This phenomenon, known as gravity darkening or the von Zeipel effect, was confirmed for Altair by measurements made by the Navy Prototype Optical Interferometer in 2001, and analyzed by Ohishi et al. (2004) and Peterson et al. (2006).^[5][15] Also, A. Domiciano de Souza et al. (2005) verified gravity darkening using the measurements made by the Palomar and Navy interferometers, together with new measurements made by the VINCI instrument at the VLTI.^[16]

Altair is one of the few stars for which a direct image has been obtained.^[17] In 2006 and 2007, J. D. Monnier and his coworkers produced an image of Altair's surface from 2006 infrared observations made with the MIRC instrument on the CHARA array interferometer; this was the first time the surface of any main-sequence star, apart from the Sun, had been imaged.^[17] The false-color image was published in 2007 and can be seen above and to the left; the brighter regions are shown in white and darker regions in blue. In this image, North (the direction towards the North Celestial Pole) is up and East is left, and the white line is the rotational axis of Altair. The black grid shows lines of latitude and longitude in an Altair-centric coordinate system. The von Zeipel effect can be observed in the image, which shows a white spot near the pole and a darker equator. The equatorial radius of the star was estimated to be 2.03 solar radii, and the polar radius 1.63 solar radii—a 25% increase of the stellar radius from pole to equator.^[4]

Etymology, mythology, and culture

The name Altair has been used since medieval times. It is an abbreviation of the Arabic phrase النسر الطائر, an-nasr aṭ-ṭā’ir (English: The flying eagle). The term Al Nesr Al Tair appeared in Al Achsasi Al Mouakket catalogue, which was translated into Latin as Vultur Volans.^[18] This name was applied by the Arabs to the asterism of α, β, and γ Aquilae and probably goes back to the ancient Babylonians and Sumerians, who called α Aquilae the eagle star.^[19] The spelling Atair has also been used.^[20]

The Koori people of Victoria also knew Altair as Bunjil, the Wedge-tailed Eagle, and β and γ Aquilae are his two wives the Black Swans. The people of the Murray River knew the star as Totyerguil.^[21] The Murray River was formed when Totyerguil the hunter speared Otjout, a giant Murray Cod, who, when wounded, churned a channel across southern Australia before entering the sky as the constellation Delphinus.^[22]

In Chinese, the asterism consisting of α, β, and γ Aquilae is known as 河鼓 (Hé Gŭ, English: River Drum).^[20] Altair is thus known as 河鼓二 (Hé Gŭ èr, English: the Second Star of the Drum at the River, or more literally, Riverdrum II).^[23] However, it is better known by its other names: 牵牛星 (Qiān Niú Xīng) or 牛郎星 ( Niú Láng Xīng), or in English, Cowherd Star.^[24][25] These names are from the love story of 七夕, Qī Xī, in which Niú Láng and his two children, β and γ Aquilae, are separated from their mother, 织女 (Zhī Nŭ, English: Weaver Girl), the star Vega, by the Milky Way. They are only permitted to meet once a year, when the Milky Way is crossed by a bridge of magpies.^[25][26] The Japanese Tanabata (七夕^?) festival, in which Altair is known as Hikoboshi (彦星, , English: Cow Herder Star^?), is based on this legend.^[27]

The people of Micronesia called Altair as Mai-lapa, "big/old breadfruit", while the Māori people called this star as Poutu-te-rangi, "pillar of heaven".^[28]

Japan Airlines' Starjet 777-200 JA8983 was named Altair.

In astrology, the star Altair was ill-omened, portending danger from reptiles.^[20]

The NASA Constellation Program announced Altair as the name of the Lunar Surface Access Module (LSAM) on December 13, 2007.^[29] The Russian-made Beriev Be-200 Altair seaplane is also named after the star.^[30]

The 1956 science fiction film Forbidden Planet is set on Altair IV, a presumed planet of the star.

The Altair 8800 was one of the first microcomputers intended for home use.

Altair is the name of three United States navy ships: USS Altair (AD-11), edit] Visual companions

The A-type main sequence star has the multiple star designation WDS 19508+0852A and has three visual companion stars, WDS 19508+0852B, C, and D. Component B is not physically close to A but merely appears close to it in the sky.^[3]

Multiple/double star designation: WDS 19508+0852[7]
Component	Primary	Right ascension (α) Equinox J2000.0	Declination (δ) Equinox J2000.0	Epoch of observed separation	Angular distance from primary	Position angle (relative to primary)	Apparent magnitude (V)	Database reference
B	A	19h 50m 40.5s	+08° 52′ 13″[31]	2007	192.1″	287°	9.82	Simbad
C	A	19h 51m 00.8s	+08° 50′ 58″[32]	2007	189.6″	107°	10.3	Simbad
D	A			2007	31.7″	97°	11.9

See also

• Altair in fiction

Notes

1. ^ From apparent magnitude and parallax.
2. ^ ^{a b} Owing to its rapid rotation, Altair's radius is larger at its equator than at its poles; it is also cooler at the equator than at the poles.
3. ^ From values of v sin i and i in the second column of Table 1, Monnier et al. 2007.

References

1. ^ ^{a b c d e} van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, Bibcode 2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
2. ^ ^{a b c d e f g} NAME ALTAIR -- Variable Star of delta Sct type, database entry, SIMBAD. Accessed on line November 25, 2008.
3. ^ ^{a b c d e} HR 7557, database entry, The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), D. Hoffleit and W. H. Warren, Jr., CDS ID V/50. Accessed on line November 25, 2008.
4. ^ ^{a b c d e f g h i j} Imaging the Surface of Altair, J. Monnier et al., Science 317, #5836 (July 20, 2007), pp. 342–345, doi:10.1126/science.1143205, Bibcode: 2007Sci...317..342M, PubMed 17540860. Accessed on line November 25, 2008. See second column of Table 1 for stellar parameters.
5. ^ ^{a b c d e} Resolving the Effects of Rotation in Altair with Long-Baseline Interferometry, D. M. Peterson et al., The Astrophysical Journal 636, #2 (January 2006), pp. 1087–1097, doi:10.1086/497981, Bibcode: 2006ApJ...636.1087P; see Table 2 for stellar parameters.
6. ^ ^{a b} Altair, entry, The Internet Encyclopedia of Science, David Darling. Accessed on line November 25, 2008.
7. ^ ^{a b} Entry 19508+0852, The Washington Double Star Catalog, United States Naval Observatory. Accessed on line November 25, 2008.
8. ^ Summer Triangle, entry, The Internet Encyclopedia of Science, David Darling. Accessed on line November 26, 2008.
9. ^ ^{a b} Altair's Oblateness and Rotation Velocity from Long-Baseline Interferometry, Gerard T. van Belle, David R. Ciardi, Robert R. Thompson, Rachel L. Akeson, and Elizabeth A. Lada, Astrophysical Journal 559 (October 1, 2001), pp. 1155–1164, doi:10.1086/322340, Bibcode: 2001ApJ...559.1155V.
10. ^ p. 194, The Brightest Stars: Discovering The Universe Through The Sky's Most Brilliant Stars, Fred Schaaf, Hoboken, New Jersey: John Wiley & Sons, Inc., 2008, ISBN 978-0-471-70410-2.
11. ^ p. 190, Schaaf 2008.
12. ^ http://www.iop.org/EJ/article/0004-637X/619/2/1072/60499.text.html
13. ^ Altair: The Brightest δ Scuti Star, D. L. Buzasi et al., The Astrophysical Journal 619, #2 (February 2005), pp. 1072–1076, doi:10.1086/426704, Bibcode: 2005ApJ...619.1072B.
14. ^ The stellar interferometer at Narrabri Observatory-II. The angular diameters of 15 stars, R. Hanbury Brown, J. Davis, L. R. Allen, and J. M. Rome, Monthly Notices of the Royal Astronomical Society 137 (1967), pp. 393–417, Bibcode: 1967MNRAS.137..393H.
15. ^ Asymmetric Surface Brightness Distribution of Altair Observed with the Navy Prototype Optical Interferometer, Naoko Ohishi, Tyler E. Nordgren, and Donald J. Hutter, The Astrophysical Journal 612, #1 (September 1, 2004), pp. 463–471, doi:10.1086/422422, Bibcode: 2004ApJ...612..463O.
16. ^ Gravitational-darkening of Altair from interferometry, A. Domiciano de Souza, P. Kervella, S. Jankov, F. Vakili, N. Ohishi, T. E. Nordgren, and L. Abe, Astronomy and Astrophysics 442, #2 (November 2005), pp. 567–578, doi:10.1051/0004-6361:20042476, Bibcode: 2005A&A...442..567D.
17. ^ ^{a b} Gazing up at the Man in the Star?, Press Release 07-062, National Science Foundation, May 31, 2007. Accessed on line November 25, 2008.
18. ^ Knobel, E. B. (June 1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society 55: 429. Bibcode 1895MNRAS..55..429K. 
19. ^ pp. 17–18, A Dictionary of Modern Star Names, Paul Kunitzsch and Tim Smart, Cambridge, Massachusetts: Sky Publishing, 2006, ISBN 978-1-931559-44-7.
20. ^ ^{a b c} p. 59–60, Star-names and Their Meanings, Richard Hinckley Allen, New York: G. E. Stechert, 1899.
21. ^ p. 4, Aboriginal mythology : an A-Z spanning the history of aboriginal mythology from the earliest legends to the present day, Mudrooroo, London: HarperCollins, 1994, ISBN 1-85538-306-3.
22. ^ p. 115, Mudrooroo 1994.
23. ^ (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 26, 2008.
24. ^ pp. 97–98, 161, The Chinese Reader's Manual, William Frederick Mayers, Shanghai: American Presbyterian Mission Press, 1874.
25. ^ ^{a b} p. 72, China, Japan, Korea Culture and Customs: Culture and Customs, Ju Brown and John Brown, 2006, ISBN 978-1-4196-4893-9.
26. ^ pp. 105–107, Magic Lotus Lantern and Other Tales from the Han Chinese, Haiwang Yuan and Michael Ann Williams, Libraries Unlimited, 2006, ISBN 978-1-59158-294-6.
27. ^ pp. 238–240, The Romance of the Milky Way, Lafcadio Hearn, The Atlantic Monthly 96 (August 1905), pp. 238–250.
28. ^ p. 175, The Lexicon of Proto Oceanic: The Culture and Environment of Ancestral Oceanic Society: The Physical Environment, Volume 2, Malcolm Ross, Andrew Pawley and Meredith Osmond, Canberra, The Australian National University E Press, 2007.
29. ^ NASA names next-gen lunar lander Altair, December 13, 2007, collectspace.com. Accessed on line November 26, 2008.
30. ^ Press release #58, Beriev Aircraft Company, February 12, 2003. Accessed on line November 26, 2008.
31. ^ BD+08 4236B -- Star in double system, database entry, SIMBAD. Accessed on line November 25, 2008.
32. ^ BD+08 4238 -- Star in double system, database entry, SIMBAD. Accessed on line November 25, 2008.

External links

• Star with Midriff Bulge Eyed by Astronomers, JPL press release, July 25, 2001.
• Imaging the Surface of Altair, University of Michigan news release detailing the CHARA array direct imaging of the stellar surface in 2007.
• PIA04204: Altair, NASA. Image of Altair from the Palomar Testbed Interferometer.
• Altair at SolStation.
• Secrets of Sun-like star probed, BBC News, June 1, 2007.
• Astronomers Capture First Images of the Surface Features of Altair, astromart.com.
• Image of Altair from Aladin.

Coordinates: 19^h 50^m 46.9990^s, +08° 52′ 05.959″