Tau Herculis, a name Latinized from τ Herculis, is a variable star in the northern constellation of Hercules. It has a blue-white hue and is visible to the naked eye at night with an apparent visual magnitude that fluctuates around 3.91.[2] The star is located at a distance of approximately 307light years from the Sun based on parallax,[1] but it is drifting closer with a radial velocity of −16km/s.[2]
The stellar classification of Tau Hercules is B5IV,[5] and it serves as a standard spectrum in the modern Morgan–Keenan (MK) classification.[12] It is estimated to be just 26million years old with a relatively low projected rotational velocity of 32km/s.[6] Slowly rotating B-type stars are often chemically peculiar, so the mostly normal spectra of this star suggests we may be viewing it from near pole-on.[13] The abundance of most heavier elements in this star are about 85% of those in the Sun.[14] The star has four times the mass of the Sun[6] and around 3.8[8] times the Sun's radius. On average, it is radiating 574[2] times the luminosity of the Sun from its photosphere at an effective temperature of 15,615K.[7]
During the Hipparcos mission,[4] Tau Hercules was discovered to be a variable star of the slowly pulsating B-type. These are mid-B main sequence stars that vary with a period of about a day;[4] the brightness of Tau Hercules varies by 0.03 magnitude[3] over a period of 1.24970±0.00008days. The radial velocity of the star varies at a different rate than the photometric period, with the object showing both radial and non-radial pulsation modes.[4][15]
Historical significance and etymology
The path of the north celestial pole among the stars due to the precession.
Tau Herculis is located within 1° of the precessional path traced across the celestial sphere by the Earth's North pole. It could have served the northern pole star around the year 7400BCE, a phenomenon which is expected to reoccur in the year 18,400 due to precession.[13]
Its traditional name, Rukbalgethi Shemali, is of Arabic origin and shares certain etymological characteristics with the stars Ruchbah and Zubeneschamali, signifying Hercules' "northern knee".[16][bettersourceneeded]
David, Trevor J.; Hillenbrand, Lynne A. (2015), "The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets", The Astrophysical Journal, 804 (2): 146, arXiv:1501.03154, Bibcode:2015ApJ...804..146D, doi:10.1088/0004-637X/804/2/146, S2CID33401607.
Fitzpatrick, E. L.; Massa, D. (March 2005), "Determining the Physical Properties of the B Stars. II. Calibration of Synthetic Photometry", The Astronomical Journal, 129 (3): 1642–1662, arXiv:astro-ph/0412542, Bibcode:2005AJ....129.1642F, doi:10.1086/427855, S2CID119512018.
Gordon, Kathryn D.; etal. (2019), "Angular Sizes, Radii, and Effective Temperatures of B-type Stars from Optical Interferometry with the CHARA Array", The Astrophysical Journal, 873 (1): 91, Bibcode:2019ApJ...873...91G, doi:10.3847/1538-4357/ab04b2, S2CID125181833.
Smith, K. C.; Dworetsky, M. M. (1993), "Elemental Abundances in Normal Late B-Stars and Hgmn-Stars from Co-Added IUE Spectra - Part One - Iron Peak Elements", Astronomy and Astrophysics, 274 (2): 335, Bibcode:1993A&A...274..335S
"* tau Her". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2010-06-10.
Adelman, Saul J.; etal. (June 2001), "Elemental abundance analyses with DAO spectrograms. XXV. The superficially normal B and A stars alpha Draconis, tau Herculis, gamma Lyrae, and HR 7926", Astronomy and Astrophysics, 371 (3): 1078–1083, Bibcode:2001A&A...371.1078A, doi:10.1051/0004-6361:20010408
Briquet, M.; etal. (April 2003), "Spectroscopic mode identification for the slowly pulsating B star HD 147394", Astronomy and Astrophysics, 401: 281–288, Bibcode:2003A&A...401..281B, doi:10.1051/0004-6361:20030086.
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