7 Sagittarii is a massive star in the southern zodiac constellation of Sagittarius which is located in the Lagoon Nebula (NGC 6530),[9] although multiple sources have considered it a foreground star.[10] It is a dim star but visible to the naked eye with an apparent visual magnitude of 5.37.[2] The distance to this star can be determined from the annual parallax shift of 3.02±0.28mas,[1] yielding a value of roughly 1,100light years. It is moving closer to the Sun with a heliocentric radial velocity of −11km/s.[5]
7 Sagittarii is the brightest star in the region of the Lagoon Nebula, towards the right edge.
Gray and Garrison (1989) listed a stellar classification of F2II/III[3] for this star, suggesting it is a K-type star with a spectrum showing mixed traits of a giant/bright giant. Houk and Smith-Moore (1978) had a similar classification of F2/3II/III.[11] This may indicate it is not a member of NGC 6530, since it should not have evolved to this class from the O-type stars that still populate this cluster, and hasn't had time to evolve from a less massive cluster star.[10]
It is a suspected chemically peculiar star.[4][5] The spectral class from the calcium K line has been given as A8 while the class determined from other metallic lines was F4, making it an Am star.[12] This peculiarity is now considered doubtful.[4]
7 Sagittarii has an estimated 18[6] times the Sun's radius and is radiating 658[2] times the Sun's luminosity from its photosphere at an effective temperature of around 6,800K.[5]
Gray, R. O.; Garrison, R. F. (February 1989), "The early F-type stars: refined classification, confrontation with Stroemgren photometry, and the effects of rotation", Astrophysical Journal Supplement, 69: 301, Bibcode:1989ApJS...69..301G, doi:10.1086/191315.
Khalack, V.; LeBlanc, F. (July 2015), "Project VeSElkA: Analysis of Balmer Line Profiles of Slowly Rotating Chemically Peculiar Stars", The Astronomical Journal, 150 (1): 10, arXiv:1505.08158, Bibcode:2015AJ....150....2K, doi:10.1088/0004-6256/150/1/2, S2CID118283214, 2.
Pasinetti Fracassini, L. E.; etal. (February 2001), "Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS)", Astronomy and Astrophysics (Thirded.), 367: 521–524, arXiv:astro-ph/0012289, Bibcode:2001A&A...367..521P, doi:10.1051/0004-6361:20000451, S2CID425754.
Rutten, R. G. M.; Pylyser, E. (February 1988), "Magnetic structure in cool stars. XV. The evolution of rotation rates and chromospheric activity of giants", Astronomy and Astrophysics, 191: 227−236, Bibcode:1988A&A...191..227R.
"7 Sgr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2018-03-14.
Sowell, James R. (May 1987), "Yellow evolved stars in open clusters", Astrophysical Journal Supplement Series, 64: 241−267, Bibcode:1987ApJS...64..241S, doi:10.1086/191196.
See NGC 6530 on p. 257.
Houk, Nancy; Smith-Moore, M. (1978), Michigan catalogue of two-dimensional spectral types for the HD stars, vol.4, Ann Arbor: Dept. of Astronomy, University of Michigan, Bibcode:1988mcts.book.....H.
Hauck, B. (June 1973), "Catalogue of AM stars with known spectral types", Astronomy and Astrophysics Supplement, 10: 385, Bibcode:1973A&AS...10..385H.
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