Alpheratz

Alpheratz (α Andromedae / α And / 21 Andromedae) is the brightest star in the constellation Andromeda with apparent magnitude +2.06. Located northeast of the constellation Pegasus, it is a star common to both constellations, which is why it is sometimes called Delta Pegasi, although this is not a widely used name. Another more common name for Alpheratz is Sirrah or Sirah, from the old Arabic name Al Surrat al Faras, "the navel of the horse".

Located 97 light years from the solar system, Alpheratz is a spectroscopic binary star with an orbital period of 96.7 days. The primary component is a subgiant star of spectral type B8IV with a surface temperature of 13,000 kelvin, while the secondary star is about ten times less bright than the primary. The joint luminosity of both —including the emitted ultraviolet radiation— is about 200 times greater than that of our Sun.

Alpheratz is the brightest representative of a group of stars known as mercury-manganese stars. The atmospheres of these stars have very high abundances of mercury, gallium, manganese, and europium, and exceptionally low contents of other elements. It is thought that these anomalies are the result of the separation of the different elements as a consequence, on the one hand, of the gravitational attraction towards the interior, and on the other, of the radiation pressure exerted towards the exterior. Likewise, Alpheratz is classified as an Alpha2 Canum Venaticorum variable, with a variation in its brightness between magnitude 2.02 and 2.06 in a period of 23.19 hours. Some spectral lines vary with this same period, others with half the period, and others remain unchanged.