The Andromeda Galaxy is the closest major galaxy to our own home, the Milky Way Galaxy. Like our own, it is a spiral galaxy, meaning the core is orbited by massive "arms," or bands of star-rich regions. Together with a few dozen other galaxies, Andromeda and the Milky Way are part of a cluster of galaxies called the Local Group. In several billion years, the Milky Way and Andromeda galaxies will collide, probably merging to form a single new galaxy.
- Properties -
The Andromeda Galaxy is a massive spiral galaxy that is currently believed to stretch over 200,000 light-years from one edge to the other, running through a dense core region containing a significant portion of the overall mass (just as in our own). The core is so dense, in fact, that pictures of the galaxy often look like a single, massive glowing star surrounded by the cosmic dust of the rest of the galaxy; of course, this is largely a function of the distance from which we are viewing it.
The Andromeda Galaxy is currently orbited by several smaller galaxies, similar to the Magellanic Clouds through which our own Milky Way is passing. These include another galaxy included in the Messier star catalogue but never actually given its own name, designated M32. Andromeda's massive gravity is gradually pulling stars out of the orbiting galaxies, resulting in a stream of dislocated stars in its wake. We currently have relatively little knowledge of how orbiting satellites interact, but Andromeda offers an excellent opportunity for research.
- The Future Collision -
One of the more startling aspects of the study of Andromeda was the discovery that it is on a collision course with the Milky Way Galaxy: the two cores are currently hurtling toward one another at over 60 miles per second. This places Andromeda in a small group of galaxies which are "blue shifted" rather than red shifted - that is, their light shifts into the blue portion of the visual spectrum because they are heading toward us at very high speeds.
More importantly, though, it also indicates that the two galaxies will collide, in several billion years. What precisely will happen is unknown - especially on the level of individual solar systems like our own, which may be disrupted, relocated, shot out into the void of empty space, or relatively unaffected. The two galaxies themselves will effectively merge to become a single galaxy, probably an elliptical (oval) one rather than the more beautiful spiral type, because the "arms" of each will be ripped apart by the gravitational forces.
- History of Discovery -
Andromeda was first spotted by a Muslim astronomer named Azophi, who named it a "cloud." It subsequently became a standard inclusion on all star charts and, in the 1760s, was included by Charles Messier in his star catalogue, from which it derives its formal designation (M31). Messier did not have a theory of galaxies which he could apply to Andromeda; even after his time, William Herschel, who discovered the planet Uranus, continued to argue that Andromeda was only a nebula and to estimate its distance from the Earth on that basis.
It took the scientific progress of the 19th century to determine otherwise. By then, astronomers had begun to classify the different light spectra emitted by stars and nebulae, and discovered that Andromeda's light actually resembled that emitted by stars. Even so, it was not unil the early 20th century that early telescope-based photography showed a number of novas exploding in Andromeda, and astronomers such as Heber Curtis began to suggest that Andromeda might not just be a dense group of distant stars, but in fact a massive galaxy identical to our own.
Studies of stars within Andromeda currently continue, and have important relevance for determining how galaxies like ours evolved and what they look like from the outside.