Alpha Andromedae - Alpheratz, Sirrah

At magnitude 2.06, Alpheratz, or α And, is tied along with β And for the title of brightest star in Andromeda. Alpheratz connects Andromeda with Pegasus, and is also the northeastern star of the Great Square of Pegasus.

Alpheratz's name has a confused origin that is now commonly taken to mean "the horse's shoulder". This shows that the star originally belonged to Pegasus, though now it is formally within the boundaries of Andromeda. As a star belonging to both constellations, Alpheratz carries two Greek letter names. It is also δ Peg, though that name is no longer commonly used.

Properties

Alpheratz is a B8 subgiant, with a temperature of about 13,000 K. At a distance of 97 light years, it has a total luminosity of about 200 times the Sun's. Alpheratz is a spectroscopic binary, with a period of 96.7 days; its companion seems to be about a tenth the brightness of the primary.

The bright member of the pair is also chemically peculiar, and is the brightest member of the odd class of "mercury-manganese" stars. Compared to our Sun, and to the vast majority of other stars, the atmospheres of these stars have vast enrichments of these elements. Mercury is overabundant by a factor of tens of thousands. Other elements such as gallium and europium are hugely enriched as well, while some others are depressed. We believe we are seeing a separation of elements as a result of the inward pull of gravity and the outward pressure of radiation that act differently on different elements.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]