Messier 31: The Andromeda Galaxy

Messier 31, commonly known as the Andromeda Galaxy, is the nearest spiral galaxy to our own Milky Way and one of the most studied objects in the night sky. It lies approximately 2.5 million light-years away from Earth in the constellation of Andromeda and is the largest galaxy in the Local Group, which includes the Milky Way, the Triangulum Galaxy (M33), and dozens of smaller galaxies.

Andromeda's disk spans roughly 150,000 to 220,000 light-years, depending on how far out its faint edges are traced, and it contains roughly one trillion stars, more than the Milky Way. It is also notable because it is approaching the Milky Way at about 110 kilometers per second. Hubble measurements published in 2012 suggested the two galaxies would collide in about 4 to 5 billion years and merge into one large elliptical galaxy. A 2025 study that added newer Hubble and Gaia data and the pull of companion galaxies put the chance of a merger within the next 10 billion years at only about 50 percent, so whether and when the two galaxies merge is still being worked out. Despite its distance, the Andromeda Galaxy is visible to the naked eye from Earth under dark skies, appearing as a faint, elongated smudge.

Magnitude and Visibility

The apparent magnitude of the Andromeda Galaxy is about 3.1, making it the brightest galaxy in northern skies (only the Large and Small Magellanic Clouds, deep in the southern sky, are brighter) and one of the few deep-sky objects that can be seen without a telescope. However, observing it with the naked eye requires minimal light pollution and clear skies. Through binoculars or a small telescope, Messier 31 appears as an oval-shaped blur, with the central core being the brightest part.

Best Season for Viewing

Messier 31 is best observed during the autumn months, particularly from September to November. It rises in the northeast during the evening and culminates (reaches its highest point) around midnight in early October and around 9 p.m. in late November. From about 40°N it passes almost straight overhead. In the Northern Hemisphere, autumn offers the clearest and most accessible view of the galaxy, though it can also be observed in late summer and early winter. From the Southern Hemisphere it stays low in the north, and it is hard to see south of about 40°S.

Observing Tips for Telescopes

To observe Messier 31 through a telescope, a low magnification eyepiece is ideal. The galaxy is so large that higher magnifications can reduce the field of view, making it harder to appreciate the galaxy in its entirety. For smaller telescopes, start with a 25mm or 32mm eyepiece.

If you have access to a larger telescope, observing the central core of the galaxy reveals more detail, and under good conditions, you might also detect the spiral arms and dust lanes. The two nearby satellite galaxies, M32 and M110, can also be seen in the same field of view with moderate-sized telescopes.

Constellation and How to Find It

The Andromeda Galaxy is located in the constellation Andromeda, which is named after the princess in Greek mythology who was saved by Perseus. To find the galaxy, start by locating the Great Square of Pegasus, a prominent asterism that dominates the autumn night sky. The constellation Andromeda extends to the northeast of the Great Square.

1. Begin with the star Alpheratz, which forms the northeastern corner of the Great Square of Pegasus and is also part of Andromeda.
2. From Alpheratz, trace a line of stars toward Mirach, a bright red giant in Andromeda.
3. From Mirach, turn northwest and hop about 4 degrees to the fainter star Mu Andromedae.
4. Continue the same distance again in the same direction, past the 4th-magnitude star Nu Andromedae. Just over 1 degree west of Nu, you’ll see a faint, elongated blur of light. This is the Andromeda Galaxy, Messier 31.

Using binoculars or a small telescope will reveal more detail, including the galaxy's bright core; a small telescope also shows its two companion galaxies, M32 and M110, which appear as small, fuzzy patches near the main galaxy.

History and Discovery

The Andromeda Galaxy has been known to astronomers for centuries, with the earliest recorded mention dating back to Persian astronomer Abd al-Rahman al-Sufi in 964 AD. Al-Sufi described it as a "small cloud" in his Book of Fixed Stars, and it remained known as the "Little Cloud" for many centuries.

On August 3, 1764, French astronomer Charles Messier included it as M31 in his catalog of celestial objects. At the time, the nature of M31 was not fully understood, and it was referred to as the Great Andromeda Nebula. It wasn’t until 1923 that American astronomer Edwin Hubble, using the 100-inch telescope at Mount Wilson Observatory, found Cepheid variable stars in the Andromeda Galaxy. Their brightness let him measure its distance, which proved that M31 was not a nebula within the Milky Way but a separate galaxy, and his result, announced in 1925, fundamentally changed our understanding of the universe.

Messier 31, the Andromeda Galaxy, remains one of the most significant and easily observed deep-sky objects for amateur astronomers. Its visibility to the naked eye and accessibility through binoculars and telescopes makes it a favorite target for stargazers, particularly during the autumn months. As the closest spiral galaxy to our own Milky Way, it holds a unique place in both our past and future as a constant companion in the night sky and a glimpse into the eventual future collision of our galactic neighborhood.