Altair

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

Altair.
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Aquila
Right ascension 19h 50m 46.9990s[1]
Declination +08° 52′ 05.959″[1]
Apparent magnitude (V) 0.77[1]
Characteristics
Spectral type A7V[1]
U-B color index +0.08[2]
B-V color index +0.22[2]
V-R color index 0.0[1]
R-I color index +0.14[2]
Variable type Delta Scuti[1]
Astrometry
Radial velocity (Rv) −26.1 ± 0.9[1] km/s
Proper motion (μ) RA: 536.87[1] mas/yr
Dec.: 385.57[1] mas/yr
Parallax (π) 194.45 ± 0.94[1] mas
Distance 16.77 ± 0.08 ly
(5.14 ± 0.02 pc)
Absolute magnitude (MV) 2.21[note 1]
Details
Mass 1.79[3] M☉
Radius 1.63 to 2.03[3][note 2] R☉
Luminosity 10.6[4] L☉
Temperature 6,900 to 8,500[3][note 2] K
Metallicity [Fe/H] = −0.2[3]
Rotation 8.9 hours[4]
Rotational velocity (v sin i) 240[3] km/s
Age <109  [5] years
Other designations
Atair, α Aquilae, α Aql, Alpha Aquilae, Alpha Aql, 53 Aquilae, 53 Aql, STFB 10A, ADS 13009 A,BD+08°4236, CCDM J19508+0852A, FK5 745, GC 27470, GCTP 4665.00, GJ 768, HD 187642, HIP 97649, HR 7557, IDS 19459+0836 A, LFT 1499, LHS 3490, LTT 15795, NLTT 48314, PPM 168779, SAO 125122, TD1 25537, WDS 19508+0852A.[1][2][6]
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.[1][5][7]

Altair rotates rapidly, with a velocity at the equator of around 286 km/s.[note 3][3] A groundbreaking 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.[8] Synthetic aperture techniques with multiple telescopes, operating in the infrared, have imaged this phenomenon.[3]

Contents

Star

Altair in the constellation of Aquila.

Altair is located 16.8 light-years (5.14 pc) from Earth and is one of the closest stars visible to the naked eye.[1][9] 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.[10]

Altair is a type-A main sequence star with approximately 1.8 times the mass of the Sun and 11 times its luminosity.[3][4] It is notable for its extremely rapid rotation; it has a rotational period of approximately 9 hours.[4] For comparison, the equator of the Sun requires just over 25 days for a complete rotation. Rapid rotation forces Altair to be oblate: its equatorial diameter is over 20 percent greater than its polar diameter.[3]

Satellite measurements made in 1999 with the Wide-Field Infrared Explorer showed that the brightness of Altair fluctuates slightly, varying by a few thousandths of a magnitude, with a superposition of oscillatory periods ranging from 0.8 to 1.5 hours. As a result, it was identified in 2005 as a δ Scuti variable star.[11]

Oblateness and surface temperature

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 coworkers at Narrabri Observatory in the 1960s. They found a diameter of 3 milliarcseconds.[12] 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.[8]

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).[13][4] 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.[14]

Altair is one of the few stars for which a direct image has been obtained.[15] 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.[15] The image was published in 2007 and can be seen on the left. In this image, North 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 image is false-color, with brighter regions shown in white and darker regions in blue. 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 20% increase of the stellar radius from pole to equator.[3]

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 aţ-ţā’ir (English: The flying eagle). 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.[16] The spelling Atair has also been used.[17]

In Chinese, the asterism consisting of α, β, and γ Aquilae is known as 河鼓 (Hé Gŭ, English: River Drum).[17] 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).[18] 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, Star of the Cowherd.[19][20] 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.[20][21] The Japanese Tanabata (七夕?) festival, in which Altair is known as Hikoboshi (彦星 , English: Cow Herder Star?), is based on this legend.[22]

In astrology, the star Altair was ill-omened, portending danger from reptiles.[17]

The NASA Constellation Program announced Altair as the name of the Lunar Surface Access Module (LSAM) on December 13, 2007.[23] The Russian made Beriev Be-200 Altair seaplane is also named after the star.[24]

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.[2]

Multiple/double star designation: WDS 19508+0852[6]
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″[25] 2007 192.1″ 287° 9.82 Simbad
C A 19h 51m 00.8s +08° 50′ 58″[26] 2007 189.6″ 107° 10.3 Simbad
D A 2007 31.7″ 97° 11.9


See also

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 f g h i j k l m NAME ALTAIR -- Variable Star of delta Sct type, database entry, SIMBAD. Accessed on line November 25, 2008.
  2. ^ 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.
  3. ^ 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.
  4. ^ 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.
  5. ^ a b Altair, entry, The Internet Encyclopedia of Science, David Darling. Accessed on line November 25, 2008.
  6. ^ a b Entry 19508+0852, The Washington Double Star Catalog, United States Naval Observatory. Accessed on line November 25, 2008.
  7. ^ Summer Triangle, entry, The Internet Encyclopedia of Science, David Darling. Accessed on line November 26, 2008.
  8. ^ 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.
  9. ^ p. 194, The Brightest Stars: Discovering The Universe Through The Sky's Most Brilliant Stars, Fred Schaaf, Hoboken, NJ: John Wiley & Sons, Inc., 2008, ISBN 978-0-471-70410-2.
  10. ^ p. 190, Schaaf 2008.
  11. ^ 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.
  12. ^ 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.
  13. ^ 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.
  14. ^ 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.
  15. ^ 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.
  16. ^ 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.
  17. ^ a b c p. 59–60, Star-names and Their Meanings, Richard Hinckley Allen, New York: G. E. Stechert, 1899.
  18. ^ (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.
  19. ^ pp. 97–98, 161, The Chinese Reader's Manual, William Frederick Mayers, Shanghai: American Presbyterian Mission Press, 1874.
  20. ^ a b p. 72, China, Japan, Korea Culture and Customs: Culture and Customs, Ju Brown and John Brown, 2006, ISBN 9781419648939.
  21. ^ pp. 105–107, Magic Lotus Lantern and Other Tales from the Han Chinese, Haiwang Yuan and Michael Ann Williams, Libraries Unlimited, 2006, ISBN 9781591582946.
  22. ^ pp. 238–240, The Romance of the Milky Way, Lafcadio Hearn, The Atlantic Monthly 96 (August 1905), pp. 238–250.
  23. ^ NASA names next-gen lunar lander Altair, December 13, 2007, collectspace.com. Accessed on line November 26, 2008.
  24. ^ Press release #58, Beriev Aircraft Company, February 12, 2003. Accessed on line November 26, 2008.
  25. ^ BD+08 4236B -- Star in double system, database entry, SIMBAD. Accessed on line November 25, 2008.
  26. ^ BD+08 4238 -- Star in double system, database entry, SIMBAD. Accessed on line November 25, 2008.

External links