Andromeda
What is Andromeda?
Constellations· Autumn evenings (N) · a chain from the Great Square, home of M31. The chained princess. Two lines of stars stream from a corner of the Great Square, and above Mirach lies the Andromeda Galaxy, M31, one of the most distant things the naked eye can see.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Alpheratz | 2.1 |
| Mirach | 2.1 |
| Almach | 2.3 |
The 3 brightest of the 4 stars used in the figure (3 lines). Lower magnitude means brighter.
Deep-sky objects near this constellation
Constellation basics
| Season | autumn evenings, October–December |
|---|---|
| Brightest star | Alpheratz (2.1) |
| How to find | two lines of stars streaming from the northeast corner of the Great Square |
| Area rank | 19th of 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Upsilon Andromedae — the first star found with several planets
Upsilon Andromedae, a fourth-magnitude star in Andromeda, was in 1999 the first Sun-like star confirmed to have several planets. Until then every known exoplanet had been the only one found at its star. Here three planets were confirmed at once, the first proof that stars with families of planets, like our Sun, really exist. At about 44 light-years it is visible to the naked eye.
All three planets are heavier than Jupiter, and the orbits of the outer two are tilted nearly 30 degrees to each other. The planets of our solar system orbit almost in one plane, so this arrangement came as a surprise. Scientists think the planets pushed and pulled on one another after they formed, knocking their orbits out of line. It was an early example showing that a planetary system may not stay the way it was born.
The constellation in depth
Andromeda was the daughter of King Cepheus and Queen Cassiopeia of Ethiopia. When the queen boasted of her beauty, the sea god Poseidon sent a sea monster. To save the kingdom, the king chained his daughter to a rock by the sea as a sacrifice, and the hero Perseus killed the monster and rescued her. Every character in this story now stands together in the autumn sky.
The Andromeda Galaxy, M31, is a spiral about 2.5 million light-years away. Together with the Milky Way it is one of the two largest galaxies in our nearby group, the Local Group. To the naked eye it looks like a long, pale smudge above Mirach. The two galaxies may collide in the distant future, but a 2025 study put the chance of a merger within the next 10 billion years at only about one in two.
On autumn evenings Alpheratz marks the north-east corner of the Great Square, and from it Mirach and Almach follow in turn toward the north-east. Hopping up two faint stars from Mirach reaches the dim glow of M31. Almach, at the end of the line, splits in a small telescope into a gold star and a blue one. With the Sky view set to an early November evening and deep-sky objects on, the galaxy's position is marked.
Frequently asked
- When can I see Andromeda?
- Andromeda is best seen in autumn evenings, October–December.
- How do I find Andromeda?
- To find Andromeda, look for two lines of stars streaming from the northeast corner of the Great Square.
- What is the brightest star in Andromeda?
- The brightest star in Andromeda is Alpheratz, at magnitude 2.1.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.