Alpha Hydrae

Alpha Hydrae (Alphard, α Hydra, Alpha Hya, α Hya) is a bright giant star in the constellation of Hydra, the water snake.^[6] Its name of "Alphard," like many star names, derives from the Arabic al-fard, meaning "the solitary one."^[4] This comes from the fact it is the only bright star in a constellation of dim ones.^[6] Arabs also knew the star as the "backbone of the serpent". Tycho Brahe called the star "Cor Hydrae, or “the heart of the snake.” In China, it is part of the asterism known only as "Star", and the star is called the "First Star of Star." Its Bayer designation is Alpha Hydra. Alphard appears on the national flag of Brazil, where it represents or symbolizes the state of Mato Grosso do Sul.^[7]

Alpha Hydra and the rest of the constellation of Hydra can be seen ascending in the east in spring. The 47^th brightest star in the night sky, it can be seen without binoculars and appears slightly orange.^[8]

Located 177 light years from Earth, Alphard has an apparent magnitude of +2.0, making it the brightest star in the constellation of Hydra.^[4] It has a stellar classification of K3 II-III, placing it halfway between an orange giant star and a bright giant star. It is thought to have a mass of 3 solar masses. The star's angular diameter has been measured to be 9.09±0.09 milliarcseconds, corresponding to a radius 50 times larger than that of the Sun.^[9][10] The star's surface temperature is estimated at around 4,100 kelvin.^[8] Alphard is believed to have a luminosity roughly 946 times that of the Sun.^[11] The star has a rotational speed of 1.1 km/s at its equator.^[7]

The star is unusual as its spectrum suggests it contains a large abundance of barium.^[7][12] Barium is often produced through "s process nucleosynthesis." This is common in massive stars with white dwarf stars as companions, but no such companion has been observed for Alphard. Such stars are sometimes called "barium stars."^[13] The star is also of interest for studying astroseismological events. Multi-periodic oscillations with periods that range from a couple hours to nearly two days.^[9][14] 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 or reasons for the correlation between the variations in the star’s spectral line and radial velocity remains unknown.^[7]