Alphard (Alpha Hydrae) has an apparent visual magnitude of +2.0, making it the brightest star in Hydra, the largest of the 88 recognized constellations. Alphard is at the midway point to becoming a bright giant star, and it is the third biggest star of this type known, after Betelgeuse and R Doradus.
• Constellation: Hydra
• Coordinates: RA 09h 27m 35.2433s | Dec. -08° 39′ 30.969″
• Distance: 177 ± 8 light years
• Star Type: K3 II-III
• Mass: 3.03 ± 0.36 sol
• Radius: 50.5 ± 4.0 sol
• Apparent Magnitude: +2.00
• Luminosity: 780 ± 78 sol
• Surface Temperature: 4,120K
• Rotational Velocity: 1.1 km/second
• Other Designations: Alfard, Alphart, Kalbelaphard, Cor Hydrae, 30 Hydrae, HR 3748, BD-08° 2680, HD 81797
Hydra is a southern hemisphere constellation located between latitudes +54° and -83° that is best seen at about 9 PM (Local Time) during the month of April. The serpent’s head starts just south of Cancer, with its heart represented by the star Alphard near to Sextans, and the rest of its long body winding its way all the way down between Centaurus and Libra. Note though that since Hydra runs roughly parallel to the ecliptic, it can take fully seven hours for the entire Water Serpent to rise above the horizon.
Given its mass of three times that of the Sun, it is not surprising that Alphard has evolved into the giant phase after only 420 million years. In fact, the star has now swollen to about fifty times the Sun’s radius, which places it among the top three biggest stars of its type.
Somewhat surprisingly, though, Alphard’s spectrum displays a distinct abundance of the metal barium, which is normally produced in massive stars through the s-process of nucleosynthesis. However, barium stars are typically members of binary systems in which one member is a white dwarf. This arrangement almost always explains the presence of abundances through the mechanism of mass transfer from the white dwarf to the normal star, but in Alphard’s case, no companion star of any type has been identified.
Alphard is also of scientific interest as an object for the study of asteroseismological events. Recent and very precise measurements of variations in the stars’ radial velocities and spectral lines have revealed multi-periodic oscillations with periods that range from a few hours to several days. Based on current knowledge, the short-term oscillations are thought to be similar to those that are caused by stellar pulsations such as those that occur on the Sun, but the exact reason(s) for the correlation between the variations in the stars’ spectral line and radial velocity remains unknown.
The star’s traditional name derives from the Arabic word al-fard, which translates into “the solitary one”, no doubt because there are no other bright stars in Alphard’s immediate vicinity. In the star catalogue Calendarium, compiled by Al Achsasi Al Mouakketa around 1650, the star was listed as Soheil al Fard, which was later latinized to Soheil Solitarius, meaning “the bright solitary one”. One other traditional name for Alphard is Cor Hydrae, meaning the heart of Hydra, a name given to the star by Tycho Brahe.
For Chinese observers, Alphard forms part of an asterism known simply as “Star”, and as a result of its position within this asterism it is known in Chinese as “the First Star of Star”. Alphard also appears on the national flag of Brazil, where it represents or symbolizes the state of Mato Grosso do Sul