Andromeda is one of the largest constellations in the northern sky and is associated with one of the night sky’s most famous stories: the princess rescued by Perseus in Greek mythology. Its brightest stars stretch away from the Great Square of Pegasus, forming an easy trail to follow across the autumn sky. And once you know its main stars, they can lead you to the Andromeda Galaxy, the most distant object visible to the naked eye.
How to Find Andromeda in the Night Sky
The Great Square of Pegasus holds a small surprise: its northeastern corner star, Alpheratz, actually belongs to Andromeda and is its brightest star. From Alpheratz, a long, sweeping trail of stars extends northeast through Delta Andromedae and Mirach to Almach, forming Andromeda’s most recognizable pattern.
And that’s only part of a much larger constellation. Andromeda covers 722 square degrees and ranks as the 19th-largest of the 88 constellations. It is best seen on autumn evenings in the Northern Hemisphere, although the constellation as a whole is visible from roughly 90°N to 40°S.
Andromeda in Greek Mythology
The Andromeda myth can also help you find your way around this part of the night sky. Look north and the distinctive W of Cassiopeia marks her mother, whose boast that she was more beautiful than the sea nymphs angered Poseidon. Nearby is Cepheus, Andromeda’s father, while farther south lies Cetus, the sea monster sent to punish their kingdom.
Andromeda was chained to a rock as a sacrifice to Cetus, but Perseus arrived and rescued her. His constellation can still be found beside Andromeda, toward Almach at the eastern end of her main star chain.
Brightest Stars in Andromeda
Three stars dominate Andromeda’s main pattern: Alpheratz, Mirach and Almach. All shine at around second magnitude, but they are very different stars at very different distances.
- Alpheratz (Alpha Andromedae / α And) — Andromeda’s brightest star at magnitude 2.1, about 97 light-years away. This hot blue-white binary is also the star at the northeastern corner of the Great Square of Pegasus. Its traditional name comes from an Arabic phrase meaning “the horse’s navel,” a reminder that it was once considered part of Pegasus.
- Mirach (Beta Andromedae / β And) — A reddish giant about 200 light-years away and nearly as bright in our sky as Alpheratz. Its name derives from an Arabic word associated with Andromeda’s girdle or waist.
- Almach (Gamma Andromedae / γ And) — A second-magnitude star roughly 350 light-years away. To the naked eye it looks like a single star, but Almach is actually a multiple-star system.
- Delta Andromedae (δ And) — An orange giant around 100 light-years away. At magnitude 3.3, it forms part of the main chain between Alpheratz and Mirach.
- Mu Andromedae (μ And) — A fainter white star of around magnitude 3.9, roughly 130 light-years away. It lies away from the main chain and provides a useful stepping stone toward the Andromeda Galaxy.
Almach Is Really a Four-Star System
Almach lies at the far end of Andromeda’s main trail, toward Perseus, where it appears as a single bright point to the naked eye. Through a small telescope, however, it splits into a striking pair, with a golden-orange primary beside a blue companion. Even that view is deceptive. The blue-looking companion is itself a system of three stars, meaning the single point of light we see as Almach is actually a system of four stars roughly 350 light-years away.
Mirach Has a Galaxy Hiding Beside It
Mirach lies near the middle of Andromeda’s main star trail. Close beside it is a faint galaxy, appearing almost like a ghostly reflection of the much brighter Mirach. Known as Mirach’s Ghost, the object is actually NGC 404, a galaxy about 10 million light-years away. Its closeness to Mirach is only an illusion: the star itself is just 200 light-years away inside our own Milky Way.
The Andromeda Galaxy Is Visible to the Naked Eye
Just beyond Andromeda’s main trail lies the Andromeda Galaxy (M31). Under a dark sky it appears to the naked eye as a faint, elongated glow.
Yet that tiny-looking smudge is actually a vast spiral galaxy about 2.5 million light-years away and the nearest large galaxy to the Milky Way. It contains hundreds of billions of stars, but without optical aid we mainly see its bright central region. Its full disk actually stretches across several times the apparent width of the full Moon. Most of the Andromeda Galaxy is simply too faint for our eyes to see, although its bright central core is much clearer through a telescope.
The Andromedid Meteor Shower
The Andromedids are a weak meteor shower seen each November. They occur when Earth passes through debris left behind by Biela’s Comet, but today’s modest display gives little hint of what the shower once produced.
It wasn’t always so quiet. After Biela’s Comet broke apart in the 19th century, Earth crossed much denser trails of its debris. The result was spectacular meteor storms in 1872 and 1885. During the 1872 storm, four observers in Italy counted more than 33,000 meteors in just 6½ hours. Those dense trails have since moved away from Earth’s path, leaving us with the much gentler Andromedids seen today.