Alpheratz is one of four medium-bright stars mark the corners of an asterism – or noticeable star pattern – known as the Great Square of Pegasus. Unlike the other stars in the Great Square, Alpheratz does not belong to the constellation Pegasus. It belongs, officially, to the constellation Andromeda and in fact is designated as Alpha Andromedae. The star marks the point where Pegasus and Andromeda meet. Alpheratz can help you spot the famed Andromeda Galaxy, also known as M31, the nearest large spiral galaxy to Earth.
When can you see the star Alpheratz? This star reaches its midnight culmination – its highest point in the sky at midnight – on October 9. So it’s generally considered an autumn star for Northern Hemisphere observers. You can see it in summertime, too, of course; it rises late at night then. And by late winter, Alpheratz is setting with the sun.
At magnitude 2.07, Alpheratz isn’t as bright as the sky’s brightest stars. You can spot it easily, except in very light-polluted areas.
How can you use Alpheratz to help find the Andromeda Galaxy? The Andromeda Galaxy, also known as M31, is the nearest large spiral galaxy to Earth. It’s the most distant thing we can see with the unaided eye. You need a dark sky to see it. Autumn is a good time to look.
People often find the Andromeda galaxy by star-hopping from the star Alpheratz to two other stars in the constellation Andromeda, Mirach and Mu Andromedae. After you find those two stars, you can draw a line between them, and extend that line to find the galaxy. See the chart at the top of this post for an illustration.
Science of Alpheratz. By the best current estimates, Alpheratz is about 97 light-years away, making it a relatively nearby star. Its temperature is high – about 13,000 kelvins, more than twice that of our sun. As a result, its color is primarily blue. The bluer color also represents a relatively high energy output, probably about 200 times that of our sun.
Alpheratz appears single, but it is really two stars orbiting a common center of gravity every 97 days. We know of the binary nature of Alpheratz because its light spectrum shows two distinct sets of spectroscopic lines. The spectral class of this star is B9p. The B9 represents a star that is much hotter and more massive than our sun. The p indicating that this is a “peculiar” star. It is peculiar in the sense that its spectrum reveals an unexpected amount of manganese. It may also be peculiar due to effects of the nearby companion star.
Alpheratz in star history and mythology. The most interesting part of this star’s history, from our modern perspective, is its assignment to the constellation Andromeda in the 1930s by the International Astronomical Union. Prior to that, Alpheratz was often referred to as Delta Pegasi, indicating that it was the fourth-brightest star in Pegasus. But some classical star charts showed it as part of Andromeda. It was variously considered part of the constellation Pegasus, or part of the constellation Andromeda, or, by some people, part of both constellations. Today, Alpheratz is officially Alpha Andromedae, the brightest star in Andromeda.
The early Arabian stargazers certainly saw Alpheratz as part of Pegasus, not Andromeda. We know this because the name Alpheratz derives from an Arabic phrase meaning “the horse’s navel,” an obvious reference to Pegasus the Flying Horse. According to Richard Hinckley Allen in his classic book Star Names, Their Lore and Meaning, Alpheratz was called Al Surrat al Faras (the horse’s navel). That name emcompasses another name by which the star still is known today: Sirrah.
Alpheratz’s position is: RA: 00h 08m 23s, dec: +29° 05′ 27″
Larry Sessions has written many favorite posts in EarthSky's Tonight area. He's a former planetarium director in Little Rock, Fort Worth and Denver and an adjunct faculty member at Metropolitan State University of Denver. He's a longtime member of NASA's Solar System Ambassadors program. His articles have appeared in numerous publications including Space.com, Sky & Telescope, Astronomy and Rolling Stone. His small book on world star lore, Constellations, was published by Running Press.