Alphard , designated Alpha Hydrae (α Hydrae, abbreviated Alpha Hya, α Hya), is the brightest

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Hydra Hydra generally refers to: * Lernaean Hydra, a many-headed serpent in Greek mythology * ''Hydra'' (genus), a genus of simple freshwater animals belonging to the phylum Cnidaria Hydra or The Hydra may also refer to: Astronomy * Hydra (constel ...

. It is a single giant star, cooler than the Sun but larger and more luminous. It is about 177

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s away.

Nomenclature

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

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. ...

. 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''. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 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. The European astronomer Tycho Brahe dubbed it ''Cor Hydræ'', Latin for 'the heart of Hydra'. In Chinese, (), meaning ''

Star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

'', refers to an asterism consisting of Alphard, τ¹ Hydrae, τ² Hydrae,

ι Hydrae Iota Hydrae (ι Hydrae, abbreviated Iota Hya, ι Hya), formally named Ukdah , is a star in the constellation of Hydra, about 8° to the north-northwest of Alphard (Alpha Hydrae) and just to the south of the celestial equator. Visible to ...

26 Hydrae 26 Hydrae is a binary star system located 334 light years away from the Sun in the equatorial constellation of Hydra. It is visible to the naked eye as a faint, yellow-hued point of light with a combined apparent visual magnitude ...

27 Hydrae 27 Hydrae is a member of a triple star system system in the equatorial constellation of Hydra, located 222 light years away from the Sun. It is visible to the naked eye as a faint, orange-hued star with a combined apparent visual ...

, HD 82477 and HD 82428. Consequently, Alphard itself is known as (), "the First Star of Star". In

ancient China The earliest known written records of the history of China date from as early as 1250 BC, from the Shang dynasty (c. 1600–1046 BC), during the reign of king Wu Ding. Ancient historical texts such as the '' Book of Documents'' (early chapte ...

it formed part of an asterism called the "red bird".

Properties

Alphard has three times the

mass of the Sun The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

. Its estimated age is 420 million years and it has evolved away from the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

to become a giant star with a spectral classification of K3 and luminosity class between II and III. The angular diameter has been measured using

long-baseline interferometry Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. The ...

, yielding a value of milliarcseconds, only beaten in it by

Betelgeuse Betelgeuse is a red supergiant of spectral type M1-2 and one of the largest stars visible to the naked eye. It is usually the tenth-brightest star in the night sky and, after Rigel, the second-brightest in the constellation of Orion ...

and R Doradus. It has expanded to 50 times the

radius of the Sun Solar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3: :1\,R_ = 6.9 ...

. Alphard's spectrum shows a mild excess of

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

, an element that is normally produced by the

s-process The slow neutron-capture process, or ''s''-process, is a series of reactions in nuclear astrophysics that occur in stars, particularly asymptotic giant branch stars. The ''s''-process is responsible for the creation (nucleosynthesis) of approximat ...

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. 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.

Modern legacy

Alphard appears on the flag of Brazil, symbolising the state of

Mato Grosso do Sul Mato Grosso do Sul () is one of the Midwestern states of Brazil. Neighboring Brazilian states are (from north clockwise) Mato Grosso, Goiás, Minas Gerais, São Paulo and Paraná. It also borders the countries of Paraguay, to the southwest, and ...

. 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.

References

{{Stars of Hydra Hydrae, Alpha Hydra (constellation) K-type bright giants K-type giants Alphard Hydrae, 30 046390 081797 BD-08 2680 3748