Prima Hyadum

The star that forms the forward point or 'nose' of the triangular 'face' of the Bull Taurus, nearby to the west of the bright star Aldebaran in the sky, and to the southeast of the Pleiades. The group of stars that forms the Bull's head is the Hyades, an open cluster - the closest such cluster to the Solar System - and Prima Hyadum is a prominent member.

The third brightest of the Hyades (after Chamukuy and Ain), Prima Hyadum is the westernmost of the bright stars in the cluster, and thus leads the entire group through the skies of Earth. This is the source of its name, Prima Hyadum or Hyadum I (the following bright star in the cluster, Delta¹ Tauri, is named Secunda Hyadum (or Hyadum II) on the same system, but the sequence does not continue). Though apparently on the leading edge of the cluster to an observer on Earth, Prima Hyadum is actually close to its centre, at a distance of about 151 light years from the Sun.

Prima Hyadum has completed its development along the main sequence and has depleted its reserves of hydrogen. Its fusion processes are now consuming helium, and the star is therefore swelling and evolving from a yellow to an orange star, towards ultimately becoming a red giant. It is physically far larger than the Sun (about thirteen times larger, in terms of diameter) and much more inherently luminous. The pattern of variations in the brightness of Prima Hyadum raise the possibility that, as it has expanded, it may have consumed one or more inner members of its own planetary system.