Alpheratz

Alpheratz, also known as Alpha Andromedae, brightest star in the constellation Andromeda, the Princess. Although Alpheratz officially belongs to Andromeda, it is most easily identified as the bright star at the northeastern corner of the Great Square of Pegasus, the Horse, one of the most recognizable star patterns in the northern sky. The name Alpheratz comes from the Arabic phrase Al Surrat al Faras,”The Horse’s Navel,” a reflection of its historical association with Pegasus. Alpheratz is most visible in the evening skies from August through October. To observers in midnorthern latitudes, Alpheratz passes high overhead, while observers in the southern hemisphere see it low on the northern horizon.

Alpheratz has a companion star. By analyzing light from Alpheratz, astronomers estimate that the companion orbits with a radius of about 32 million km (about 20 million mi) and takes slightly more than 96 days to complete an orbit (see Binary Star: Spectroscopic Binary Stars).

Stars that are visible to the unaided eye, such as Alpheratz, belong to Earth’s home galaxy, the Milky Way, and tend to be very bright or relatively close. Alpheratz is fairly close to our solar system and shines in the night sky as one of the 100 brightest stars as seen from Earth.

Alpheratz’s estimated surface temperature is about 11,000°C (about 19,000°F), which is about double the surface temperature of the Sun and gives the star a white color. Its diameter is about 4 million km (about 2.5 million mi), or about three times the diameter of the Sun. From its composition, astronomers classify Alpheratz as a blue subgiant star—a massive star that has converted some of its core hydrogen fuel to helium, which it now also uses as fuel.