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Altair

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Altair
Aquila charta.png
Altair in the constellation of Aquila.
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Aquila
Pronunciation /ˈæltɛər/, /ˈæltaɪər/[1]
Right ascension 19h 50m 46.99855s[2]
Declination +08° 52′ 05.9563″[2]
Apparent magnitude (V) 0.76[3]
Characteristics
Evolutionary stage Main sequence
Spectral type A7 V[4]
U−B color index +0.09[3]
B−V color index +0.22[3]
V−R color index +0.14[3]
R−I color index +0.13[3]
Variable type Delta Scuti[5]
Astrometry
Radial velocity (Rv)−26.1 ± 0.9[4] km/s
Proper motion (μ) RA: +536.23[2] mas/yr
Dec.: +385.29[2] mas/yr
Parallax (π)194.95 ± 0.57[2] mas
Distance16.73 ± 0.05 ly
(5.13 ± 0.01 pc)
Absolute magnitude (MV)2.22[5]
Details
Mass1.79 ± 0.018[6] M
Radius1.63 to 2.03[6][nb 1] R
Diameter1.63 to 2.03[6][nb 1] D
Luminosity10.6[7] L
Surface gravity (log g)4.29[8] cgs
Temperature6,900 to 8,500[6][nb 1] K
Metallicity [Fe/H]−0.2[6] dex
Rotation8.9 hours[7]
Rotational velocity (v sin i)240[6] km/s
Age1.2[9] Gyr
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.[4][10][11]
Database references
SIMBADdata

Altair (/ˈæltɛər, -taɪər, ælˈtɛər, -ˈtaɪər/), also designated Alpha Aquilae (α Aquilae, abbreviated Alpha Aql, α Aql), is the brightest star in the constellation of Aquila and the twelfth brightest star in the night sky. It is currently in the G-cloud—a nearby accumulation of gas and dust known as an interstellar cloud.[12][13] Altair is an A-type main sequence star with an apparent visual magnitude of 0.77 and is one of the vertices of the asterism known as the Summer Triangle (the other two vertices are marked by Deneb and Vega).[4][14][15] It is 16.7 light-years (5.13 parsecs) from the Sun and is one of the closest stars visible to the naked eye.[16]

Altair rotates rapidly, with a velocity at the equator of approximately 286 km/s.[nb 2][6] This is a significant fraction of the star's estimated breakup speed of 400 km/s.[9] 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.[17] Other interferometric studies with multiple telescopes, operating in the infrared, have imaged and confirmed this phenomenon.[6]

Nomenclature[edit]

α Aquilae (Latinised to Alpha Aquilae) is the star's Bayer designation. The traditional name Altair has been used since medieval times. It is an abbreviation of the Arabic phrase النسر الطائر, al-nesr al-ṭā’ir ("the flying eagle"). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[18] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[19] included a table of the first two batches of names approved by the WGSN, which included Altair for this star. It is now so entered in the IAU Catalog of Star Names.[20]

Characteristics[edit]

Altair

Along with Beta Aquilae and Gamma Aquilae, Altair forms the well-known line of stars sometimes referred to as the Family of Aquila or Shaft of Aquila.[21]

Altair is a type-A main sequence star with approximately 1.8 times the mass of the Sun and 11 times its luminosity.[6][7] Altair possesses an extremely rapid rate of rotation; it has a rotational period of approximately 9 hours.[7] For comparison, the equator of the Sun requires a little more than 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.[6]

Satellite measurements made in 1999 with the Wide Field Infrared Explorer showed that the brightness of Altair fluctuates slightly, varying by just a few thousandths of a magnitude with several different periods less than 2 hours.[5] As a result, it was identified in 2005 as a Delta 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.[22] It is a weak source of coronal X-ray emission, with the most active sources of emission being located near the star's equator. This activity may be due to convection cells forming at the cooler equator.[9]

Oblateness and surface temperature[edit]

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.[23] 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, David R. Ciardi and their co-authors in 2001.[17]

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).[7][24] 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.[25]

Direct image of Altair

Altair is one of the few stars for which a direct image has been obtained.[26] 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.[26] The false-color image was published in 2007. 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.[6] The polar axis is inclined by about 60° to the line of sight from the Earth.[9]

Visual companions[edit]

The bright primary star has the multiple star designation WDS 19508+0852A and has three faint 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.[10]

Multiple/double star designation: WDS 19508+0852[11]
ComponentPrimaryRight
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″[27] 2007 192.1 287° 9.82 SIMBAD
C A 19h 51m 00.8s+08° 50′ 58″[28] 2007 189.6 107° 10.3 SIMBAD
D A 2007 31.7 97° 11.9

Notes[edit]

  1. 1.0 1.1 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.
  2. From values of v sin i and i in the second column of Table 1, Monnier et al. 2007.


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References[edit]

  1. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  2. 2.0 2.1 2.2 2.3 2.4 van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode 2007A&A...474..653V, doi:10.1051/0004-6361:20078357
  3. 3.0 3.1 3.2 3.3 3.4 Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues 2237: 0. Bibcode 2002yCat.2237....0D.
  4. 4.0 4.1 4.2 4.3 NAME ALTAIR -- Variable Star of delta Sct type, database entry, SIMBAD. Accessed on line November 25, 2008.
  5. 5.0 5.1 5.2 Buzasi, D. L.; Bruntt, H.; Bedding, T. R.; Retter, A.; Kjeldsen, H.; Preston, H. L.; Mandeville, W. J.; Suarez, J. C. et al. (2005). "Altair: The Brightest δ Scuti Star". The Astrophysical Journal 619 (2): 1072–1076. arXiv:astro-ph/0405127. Bibcode 2005ApJ...619.1072B. doi:10.1086/426704.
  6. 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 "Imaging the surface of Altair". Science 317 (5836): 342–345. 2007. arXiv:0706.0867. Bibcode 2007Sci...317..342M. doi:10.1126/science.1143205. PMID 17540860. See second column of Table 1 for stellar parameters.
  7. 7.0 7.1 7.2 7.3 7.4 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, Bibcode2006ApJ...636.1087P; see Table 2 for stellar parameters.
  8. Malagnini, M. L.; Morossi, C. (November 1990), "Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars", Astronomy and Astrophysics Supplement Series 85 (3): 1015–1019, Bibcode 1990A&AS...85.1015M
  9. 9.0 9.1 9.2 9.3 Robrade, J.; Schmitt, J. H. M. M. (April 2009), "Altair - the "hottest" magnetically active star in X-rays", Astronomy and Astrophysics 497 (2): 511–520, arXiv:0903.0966, Bibcode 2009A&A...497..511R, doi:10.1051/0004-6361/200811348.
  10. 10.0 10.1 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.
  11. 11.0 11.1 Entry 19508+0852, The Washington Double Star Catalog, United States Naval Observatory. Accessed on line November 25, 2008.
  12. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  13. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  14. Altair, entry, The Internet Encyclopedia of Science, David Darling. Accessed on line November 25, 2008.
  15. Summer Triangle, entry, The Internet Encyclopedia of Science, David Darling. Accessed on line November 26, 2008.
  16. Lua error in Module:Citation/CS1/Identifiers at line 738: attempt to concatenate a nil value.
  17. 17.0 17.1 Belle, Gerard T. van; Ciardi, David R.; Thompson, Robert R.; Akeson, Rachel L.; Lada, Elizabeth A. (2001). "Altair's Oblateness and Rotation Velocity from Long-Baseline Interferometry" (in en). The Astrophysical Journal 559 (2): 1155–1164. Bibcode 2001ApJ...559.1155V. doi:10.1086/322340. ISSN 0004-637X. http://stacks.iop.org/0004-637X/559/i=2/a=1155.
  18. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  19. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  20. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  21. p. 190, Schaaf 2008.
  22. 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, Bibcode2005ApJ...619.1072B.
  23. 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, Bibcode1967MNRAS.137..393H.
  24. 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, Bibcode2004ApJ...612..463O.
  25. 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, Bibcode2005A&A...442..567D.
  26. 26.0 26.1 Gazing up at the Man in the Star?, Press Release 07-062, National Science Foundation, May 31, 2007. Accessed on line November 25, 2008.
  27. BD+08 4236B -- Star in double system, database entry, SIMBAD. Accessed on line November 25, 2008.
  28. BD+08 4238 -- Star in double system, database entry, SIMBAD. Accessed on line November 25, 2008.
<comments />
As a reminder, article comments are only for discussions on how to improve the article. Please direct other comments to a user's talk page. Please be formal and do not use excessive uppercase. Please be advised you may receive an automatic block if you break the article comments policy. For information regarding what is acceptable/not acceptable in article comments, please message Icons-flag-ru.png Joey (talk), Natalia (talk), Icons-flag-fr.png ynoss (talk), or Icons-flag-ca.png Daniel (older account/talk).


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