Alphard Explained

Alphard,[1] 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.

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.[2] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[3] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[4] 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.[5]

The European astronomer Tycho Brahe dubbed it Cor Hydræ, Latin for 'the heart of Hydra'.[6]

In Chinese, Chinese: 星宿 (Chinese: 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.[7] Consequently, Alphard itself is known as Chinese: 星宿一 (Chinese: Xīng Xiù yī), "the First Star of Star".[8] In ancient China it formed part of an asterism called the "red bird".

Properties

Alphard has three times the mass of the Sun. Its estimated age is 420 million years and it has evolved away from the main sequence to become a giant star with a spectral classification of K3 and luminosity class III. The angular diameter has been measured using interferometry, after correction for limb darkening, it yields a value of . Assuming the distance of 54.3pc, it yields a physical size of .

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.[9]

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.[10]

Modern legacy

Alphard appears on the flag of Brazil, symbolising the state of Mato Grosso do Sul.[11]

The Toyota Alphard is a minivan named after this star.

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

Notes and References

  1. Book: Kunitzsch . Paul . Smart . Tim . 2006 . 2nd rev. . A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations . Sky Pub . Cambridge, Massachusetts . 978-1-931559-44-7.
  2. Knobel . E. B. . 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 . June 1895 . 1895MNRAS..55..429K . 10.1093/mnras/55.8.429. free.
  3. Web site: IAU Working Group on Star Names (WGSN). 22 May 2016.
  4. Web site: Bulletin of the IAU Working Group on Star Names, No. 1 . 28 July 2016.
  5. Web site: IAU Catalog of Star Names . 28 July 2016.
  6. Book: Olcott, William Tyler . 2004 . Star Lore: Myths, Legends, and Facts . 226 . . 0-486-43581-4 .
  7. 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, .
  8. 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
  9. Mennessier, M. O. . Barium Stars, Galactic Populations and Evolution . Astronomy and Astrophysics . 326 . 722–730 . October 1997 . 1997A&A...326..722M . etal.
  10. Setiawan, J. . Roth, M. . Weise, P. . Dölinger, M. P. . Multi-periodic oscillations of HD 32887 and HD 81797 . 77 . 510–514 . 2006 . Memorie della Societa Astronomica Italiana . 2006MmSAI..77..510S . astro-ph/0505184 .
  11. Web site: 1 January 2019. Astronomy of the Brazilian Flag. FOTW Flags Of The World website. 3 May 2019.