Rabu, 13 Juli 2011

Toyota Alphard 2011

The Toyota Alphard (α Hya, α Hydrae, Alpha Hydrae) is the brightest star in the constellation Hydra. The name Alphard is from the Arabic الفرد (al-fard), "the solitary one", there being no other bright stars near it. It was also known as the "backbone of the Serpent" to the Arabs. In ancient China it formed part of an asterism called the "red bird". The European astronomer Tycho Brahe dubbed it Cor Hydræ, the heart of the snake.[7]

In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Soheil al Fard, which was translated into Latin asSoheil Solitarius, meaning the bright solitary one.[8]

In Chinese, 柳宿 (Xīng Sù), meaning Star (asterism), refers to an asterism consisting of Alphard, τ1 Hydrae,τ2 Hydrae, ι Hydrae, 26 Hydrae, 27 Hydrae, HD 82477and HD 82428.[9] Consequently, Alphard itself is known as 星宿一 (Xīng Sù yī, English: the First Star of Star.)[10]

Properties

Alphard has three times the mass of the Sun. The estimated age of this star 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 between II and III.[4] The angular diameter of this star has been measured using long baseline interferometry, yielding a value of 9.09 ± 0.09milliarcseconds.[11] It has expanded to 50 times theradius of the Sun.[4]


Alphard's spectrum shows a mild excess of barium, an element that is normally produced by the s-process ofnucleosynthesis. Typically a barium star belongs to abinary system and the anom


alies in abundances are explained by mass transfer from a companion white dwarf star.[12]

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.

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