Explore the Andromeda Galaxy

The Andromeda Galaxy is the closest spiral galaxy to the Milky Way. Adam Evans/Wikimedia Commons.

The Andromeda Galaxy is the closest spiral galaxy in the universe to the Milky Way Galaxy. For many years, it was called a "spiral nebula" and up until about a hundred years ago, that's all astronomers thought it was — a fuzzy object inside our own galaxy. However, observational evidence suggested that it was too far away to be inside the Milky Way.

When astronomer Edwin Hubble measured Cepheid variable stars (a special type of star that varies in brightness on a predictable schedule) inside Andromeda, that helped him calculate its distance. He found that it lay more than a million light-years from Earth, far outside the bounds of our home galaxy. Later refinements of his measurements pinned down a more accurate distance to Andromeda of a little over 2.5 million light-years. Even at that great distance, it's still the closest spiral galaxy to our own. 

Observing Andromeda for Yourself

Andromeda is one of only a few objects outside of our galaxy that are visible to with the naked eye (though dark skies are needed). In fact, it was first written about more than a thousand years ago by the Persian astronomer Abd al-Rahman al-Sufi. It's high in the sky beginning around September and through February for most observers in the Northern Hemisphere. (Here's a guide for September's evening skies to get you started looking for this galaxy.) Try to find a dark location from which to view the sky, and bring along a pair of binoculars to magnify your view. 

Properties of The Andromeda Galaxy

The Andromeda Galaxy is the largest galaxy in the Local Group, a collection of more than 50 galaxies that contains the Milky Way. It is a barred barred spiral that contains well more than a trillion stars, which is easily more than double the number in our Milky Way. However, while there are certainly more stars in our neighbor, the total mass of the galaxy is not all that dissimilar to our own. Estimates place the relative mass of the Milky Way to between 80% and 100% of Andromeda's mass. 

Andromeda also has 14 satellite galaxies. The two brightest show up as smaller blobs of light near the galaxy; they're called M32 and M110 (from the Messier list of observing objects). Chances are good that most of these companions formed about the same time in a tidal interaction in Andromeda's past.

Collision and Merger with the Milky Way

Current theory suggests that Andromeda itself was formed from the merger of two smaller galaxies more than five billion years ago. There are several galaxy mergers happening currently in our local group, with at least three smaller dwarf spheroidal shaped galaxies currently being absorbed by the Milky Way. Recent studies and observations of Andromeda have determined that Andromeda and the Milky Way are on a collision course and will merge in around four billion years.

It is not clear how this would effect any life that exists on planets circling stars in either galaxy.There won't be any life left on Earth, the continual increase in our Sun's luminosity will have damaged our atmosphere too much to support life by that point. So unless humans have developed the technology to travel to other solar systems, we won't be around to see the merger. Which is too bad, because it will be spectacular.)

Most researchers believe that it will have little effect on individual stars and solar systems. It will likely spark another round of star formation because of the collisions of gas and dust clouds and there could be some gravitational effects on groups of stars. But for the most part, the individual stars, on average, will find a new path around the center of the new, combined galaxy.

Because of the size and current shape of both galaxies - Andromeda and the Milky Way are both barred spiral galaxies - it is expected that when merged they will form a giant elliptical galaxy. In fact, it is hypothesized that virtually all large elliptical galaxies are the result of mergers between normal (non dwarf) galaxies.

Edited and updated by Carolyn Collins Petersen.