The star'sapparent magnitude is 5.49, making it visible with the naked eye under suitable viewing conditions.
51 Pegasi was listed as a standard star for the spectral type G2IV in the 1989The Perkins catalog of revised MK types for the cooler stars. Historically, it was generally given astellar classification of G5V,[14] and even in more modern catalogues it is usually listed as amain-sequence star.[15] TheNStars project assign it a G2V spectral class.[3] It is generally considered to still be generating energy through thethermonuclear fusion of hydrogen at its core, but to be in a more evolved state than the Sun.[6] Theeffective temperature of thechromosphere is about 5,571 K (5,298 °C; 9,568 °F), giving 51 Pegasi the characteristic yellow hue of a G-type star.[16] It is estimated to be about 4.8 billion years old, about the same age as theSun, with a radius 11.5% larger and 9% more mass.[7] The star has a higher proportion of elements other than hydrogen/helium compared to the Sun; a quantity astronomers term a star'smetallicity. Stars with higher metallicity such as this are more likely to host giant planets.[17] In 1996, astronomers Baliunas, Sokoloff, and Soon measured a rotational period of 37 days for 51 Pegasi.[18]
Although the star was suspected of beingvariable during a 1981 study,[19] subsequent observation showed there was almost no chromospheric activity between 1977 and 1989. Further examination between 1994 and 2007 showed a similar low or flat level of activity. This, along with its relatively low X-ray emission, suggests that the star may be in aMaunder minimum period[14] during which a star produces a reduced number ofstar spots.
The star rotates at an inclination of 79+11 −30 degrees relative to Earth.[9]
51 Pegasi is theFlamsteed designation. On its discovery, the star's planet — and actually the first exoplanet discovered around a main-sequence star — was designated 51 Pegasi b by its discoverers and unofficially dubbed Bellerophon, in keeping with the convention of naming planets after Greek and Roman mythological figures (Bellerophon was a figure fromGreek mythology who rode the winged horsePegasus).[20]
In July 2014, theInternational Astronomical Union launchedNameExoWorlds, a process for giving proper names to certain exoplanets and their host stars.[21] The process involved public nomination and voting for the new names.[22] In December 2015, the IAU announced the names of Helvetios for this star and Dimidium for its planet.[23]
The names were those submitted by the Astronomische Gesellschaft Luzern,Switzerland. "Helvetios" isLatin for "theHelvetian" and refers to theCeltic tribe that lived in Switzerland during antiquity; 'Dimidium' isLatin for 'half', referring to the planet's mass of at least half the mass ofJupiter.[24]
In 2016, the IAU organized aWorking Group on Star Names (WGSN)[25] to catalog and standardize proper names for stars. In its first bulletin of July 2016,[26] the WGSN explicitly recognized the names of exoplanets and their host stars approved by the Executive Committee Working Group Public Naming of Planets and Planetary Satellites, including the names of stars adopted during the 2015 NameExoWorlds campaign. This star is now so entered in the IAU Catalog of Star Names.[12]
51 Pegasi b (51 Peg b) was the first discovered exoplanet around a main-sequence star. It orbits very close to the star, experiences estimated temperatures around 1,200 °C (1,500 K; 2,200 °F) and has a mass at least half that ofJupiter. At the time of its discovery, this close distance was not compatible with theories ofplanet formation and resulted in discussions ofplanetary migration.[28] However, severalhot Jupiters are now known to be oblique relative to the stellar axis.[29]
^abGray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Bubar, E. J.; McGahee, C. E.; O'Donoghue, A. A.; Knox, E. R. (2006-07-01). "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample".The Astronomical Journal.132 (1):161–170.arXiv:astro-ph/0603770.Bibcode:2006AJ....132..161G.doi:10.1086/504637.ISSN0004-6256.
^abJohnson, H. L.; et al. (1966). "UBVRIJKL photometry of the bright stars".Communications of the Lunar and Planetary Laboratory.4 (99): 99.Bibcode:1966CoLPL...4...99J.
^Skiff, B. A. (2014). "VizieR Online Data Catalog: Catalogue of Stellar Spectral Classifications (Skiff, 2009–2016)".VizieR On-line Data Catalog.Bibcode:2014yCat....1.2023S.
^"The Colour of Stars".Australia Telescope, Outreach and Education. Commonwealth Scientific and Industrial Research Organisation. December 21, 2004. Archived fromthe original on March 18, 2012. Retrieved2012-01-16.
^Kukarkin, B. V.; et al. (1981). "Nachrichtenblatt der Vereinigung der Sternfreunde e.V. (Catalogue of suspected variable stars)".Nachrichtenblatt der Vereinigung der Sternfreunde: 0.Bibcode:1981NVS...C......0K.
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