Fall Constellation Spotlight: Andromeda

Notable Stars

Alpheratz (Alpha Andromedae) is a tight blue-white binary star system. This subgiant is 97 light-years away and has approximately 200 times the Sun's luminosity. Although it is officially one of the brighter stars of Andromeda at magnitude 2.06, it marks the head of Andromeda and the northeastern corner square star of Pegasus. Alpheratz is derived from an Arabic phrase meaning the horse's navel, but it is now commonly called "the horse's shoulder."

Mirach (Beta Andromedae) has an identical magnitude of 2.06 as Alpharetz. It is a cool, red giant that is 200 light-years away. Interesting fact: Mirach's was named somewhat by mistake. It was originally meant to be "Mizar," meaning "loin," but several misspellings gave the name in its present form. Today, Mizar is recognized as the middle star in the Big Dipper's handle.

Almach (Gamma Andromedae) is a stunning quadruple-star system approximately 350 light-years away. Although it appears as a single-star system to the naked eye, in a small telescope, it appears as a golden-yellow star next to a dimmer, bluish star, separated by approximately 10 arcseconds. Observers often compare the pair to a more famous and similarly colored double star, Albireo, as they are easy to observe. However, Almach's blue secondary is a triple star system seen in large aperture telescopes in steady seeing and can be split into components of magnitudes +5.1 and +6.3.

Deep Sky Objects

The Great Andromeda Galaxy, Messier 31, is Andromeda's most famous celestial object. It is the nearest spiral galaxy to our Milky Way, only 2.5 million light-years away, and the most distant object humans can see with their eyes from Earth. At one time, the Andromeda Galaxy was regarded by astronomers as a component of the Milky Way and called a spiral nebula. With an apparent magnitude of 3.4, it is visible as a hazy patch of light from dark sky locations on moonless autumn nights. As viewed from Earth, M31 is large, about 3 x 1 degrees, or six times the size of the Full Moon! Scientists believe the galaxy is between 5 and 10 billion years old and contains approximately 1 trillion stars—more than the amount estimated in the Milky Way. At least 13 dwarf galaxies are orbiting M31; however, its brightest companion galaxies, Messier 110 and Messier 32, are observable in amateur telescopes.

To locate the Andromeda Galaxy, use Mirach to form an imaginary line to the third-magnitude star Mu Andromedae. Extend the imaginary line approximately the same visual distance beyond Mu Andromedae to locate this famous galaxy.

NGC 752, also known as Caldwell 28, is a sixth magnitude open cluster located 5 degrees south and slightly west of the quadruple-star Almach. The cluster was discovered by German-born British astronomer Caroline Herschel in 1783, and her famous brother William Herschel cataloged it in 1786. NGC 752 is a large cluster that lies 1,400 light-years away and contains a loose collection of 60 to 70 stars that are no brighter than 9th magnitude.

NGC 891 is a famous edge-on spiral galaxy that appears like a flying saucer and was discovered by German-born British astronomer William Herschel in 1784. As seen in small telescopes, it appears as a faint 10th-magnitude light smear. In larger telescopes, NGC 891 is visible in a large star field, and its distinct dark dust lane separates its galactic plane.

NGC 7662 is a small but bright 8.3 magnitude planetary nebula discovered by German-born English astronomer William Herschel in 1784. Nicknamed "Blue Snowball," it is a popular celestial target easily observable in small telescopes. A 6-inch telescope at 100x reveals itself as a bluish disk, while larger aperture telescopes reveal more color and detail. A faint central star is variable within its nucleus with an estimated magnitude between 12 and 16.