Alphard

This article is about the star. For the programming language, see Alphard (programming language). For the vehicle, see Toyota Alphard.

Alphard /ˈælfɑːrd/,^[10] designated Alpha Hydrae (α Hydrae, abbreviated Alpha Hya, α Hya), is the brightest star in the constellation of Hydra. It is a single giant star, cooler than the Sun but larger and more luminous. It is about 177 light-years away.

Alphard

Location of Alphard (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Hydra
Right ascension	09h 27m 35.2433s[1]
Declination	−08° 39′ 30.969″[1]
Apparent magnitude (V)	+2.00[2]
Characteristics
Evolutionary stage	Giant star
Spectral type	K3IIIa[3]
U−B color index	+1.73[4]
B−V color index	+1.45[3]
Astrometry
Radial velocity (Rv)	−4.3[5] km/s
Proper motion (μ)	RA: −14.49[1] mas/yr Dec.: 33.25[1] mas/yr
Parallax (π)	18.40 ± 0.78 mas[1]
Distance	177 ± 8 ly (54 ± 2 pc)
Absolute magnitude (MV)	−1.743±0.032[3]
Details
Mass	3.2±0.32[3] M☉
Radius	55.9±2.4[6] R☉
Luminosity	971+154 −133[3] L☉
Luminosity (visual, LV)	426[3][lower-alpha 1] L☉
Surface gravity (log g)	1.77[6] cgs
Temperature	4117±18[3] K
Metallicity [Fe/H]	−0.12[6] dex
Rotation	2,991 days[7]
Rotational velocity (v sin i)	1.1[7] km/s
Age	420±160[8] Myr
Other designations
Alphard, Alfard, Alphart, Kalbelaphard, Cor Hydrae, 30 Hydrae, HR 3748, BD−08°2680, HD 81797, SAO 136871, FK5 354, HIP 46390[9]
Database references
SIMBAD	data

Nomenclature

α Hydrae (Latinised to Alpha Hydrae) is the star's Bayer designation.

The traditional name Alphard is from the Arabic الفرد (al-fard), "the individual", there being no other bright stars near it. It was also known as the "backbone of the Serpent" to the Arabs. In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, it was designated Soheil al Fard, which was translated into Latin as Soheil Solitarius, meaning the bright solitary one.^[11] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[12] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016^[13] included a table of the first two batches of names approved by the WGSN; which included Alphard for this star. It is now so entered in the IAU Catalog of Star Names.^[14]

The European astronomer Tycho Brahe dubbed it Cor Hydræ, Latin for 'the heart of Hydra'.^[15]

In Chinese, 星宿 (Xīng Xiù), meaning Star, refers to an asterism consisting of Alphard, τ¹ Hydrae, τ² Hydrae, ι Hydrae, 26 Hydrae, 27 Hydrae, HD 82477 and HD 82428.^[16] Consequently, Alphard itself is known as 星宿一 (Xīng Xiù yī), "the First Star of Star".^[17] In ancient China it formed part of an asterism called the "red bird".

Properties

The western portion of Hydra, with Alphard the brightest star near the centre

Alphard has three times the mass of the Sun.^[3] Its estimated age is 420 million years^[8] and it has evolved away from the main sequence to become a giant star with a spectral classification of K3 and luminosity class III.^[3] The angular diameter has been measured using interferometry, after correction for limb darkening, it yields a value of 9.58±0.08 mas.^[6] Assuming the distance of 54.3 parsecs (177 ly),^{[lower-alpha 2]} it yields a physical size of 55.93±2.41 R_☉.^[6]

Alphard's spectrum shows a mild excess of barium, an element that is normally produced by the s-process of nucleosynthesis. Typically a barium star belongs to a binary system and the anomalies in abundances are explained by mass transfer from a companion white dwarf star.^[18]

Precise radial velocity measurements have shown variations in the stellar radial velocities and spectral line profiles. The oscillations are multi-periodic with periods from several hours up to several days. The short-term oscillations were assumed to be a result of stellar pulsations, similar to the solar ones. A correlation between the variations in the asymmetry of the spectral line profile and the radial velocity has also been found. The multi-periodic oscillations make HD 81797 (Alphard) an object of interest for asteroseismologic investigations.^[19]

Modern legacy

Alphard appears on the flag of Brazil, symbolising the state of Mato Grosso do Sul.^[20]

The Toyota Alphard is a minivan named after this star.

The character Roy Alphard from Japanese light novel series Re:Zero is named after this star.

Notes

1. Calculated using the absolute magnitude in the equation 10^{4(4.83-M_abs)}, where M_abs is the absolute magnitude
2. Calculated with parallax

References

1. Perryman, M. A. C.; et al. (1997). "The Hipparcos Catalogue". Astronomy & Astrophysics. 323: L49–L52. Bibcode:1997A&A...323L..49P.
2. Piau, L.; et al. (February 2011). "Surface convection and red-giant radius measurements". Astronomy and Astrophysics. 526: A100. arXiv:1010.3649. Bibcode:2011A&A...526A.100P. doi:10.1051/0004-6361/201014442. S2CID 118533297.
3. Rosas-Portilla, F.; Schröder, K.-P.; Jack, D. (2022-04-26). "On the physical nature of the Wilson-Bappu effect: revising the gravity and temperature dependence". Monthly Notices of the Royal Astronomical Society. 513 (1): 906–924. arXiv:2203.16593. doi:10.1093/mnras/stac929. ISSN 0035-8711.
4. Pfleiderer, J.; Mayer, U. (October 1971). "Near-ultraviolet surface photometry of the southern Milky Way". Astronomical Journal. 76: 691–700. Bibcode:1971AJ.....76..691P. doi:10.1086/111186.
5. Evans, D. S. (June 20–24, 1966). "The Revision of the General Catalogue of Radial Velocities". In Batten, Alan Henry; Heard, John Frederick (eds.). Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30. University of Toronto: International Astronomical Union. Bibcode:1967IAUS...30...57E.
6. Piau, L.; Kervella, P.; Dib, S.; Hauschildt, P. (2011-02-01). "Surface convection and red-giant radius measurements". Astronomy & Astrophysics. 526: A100. arXiv:1010.3649. Bibcode:2011A&A...526A.100P. doi:10.1051/0004-6361/201014442. ISSN 0004-6361.
7. Setiawan, J.; et al. (July 2004). "Precise radial velocity measurements of G and K giants. Multiple systems and variability trend along the Red Giant Branch". Astronomy and Astrophysics. 421: 241–254. Bibcode:2004A&A...421..241S. doi:10.1051/0004-6361:20041042-1.
8. da Silva, L.; et al. (November 2006). "Basic physical parameters of a selected sample of evolved stars". Astronomy and Astrophysics. 458 (2): 609–623. arXiv:astro-ph/0608160. Bibcode:2006A&A...458..609D. doi:10.1051/0004-6361:20065105. S2CID 9341088.
9. "Alphard". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2009-10-09.
10. Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
11. Knobel, E. B. (June 1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society. 55 (8): 429–438. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429.
12. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
13. "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
14. "IAU Catalog of Star Names". Retrieved 28 July 2016.
15. Olcott, William Tyler (2004). Star Lore: Myths, Legends, and Facts. Courier Dover Publications. p. 226. ISBN 0-486-43581-4.
16. (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
17. (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2008-10-25 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
18. Mennessier, M. O.; et al. (October 1997). "Barium Stars, Galactic Populations and Evolution". Astronomy and Astrophysics. 326: 722–730. Bibcode:1997A&A...326..722M.
19. Setiawan, J.; Roth, M.; Weise, P.; Dölinger, M. P. (2006). "Multi-periodic oscillations of HD 32887 and HD 81797". Memorie della Societa Astronomica Italiana. 77: 510–514. arXiv:astro-ph/0505184. Bibcode:2006MmSAI..77..510S.
20. "Astronomy of the Brazilian Flag". FOTW Flags Of The World website. 1 January 2019. Retrieved 3 May 2019.