The Andromeda Galaxy, considered something of a twin to our own Milky Way Galaxy, is less like our cosmic home than previously believed, with a much more violent past, astronomers have discovered.

Researchers at the University of California, Santa Cruz, have completed a survey of the movements of various populations of stars within Andromeda's galactic disk, saying they've found some significant differences when compared to our galaxy.

The movements of stars within Andromeda suggest it has experienced some violent collisions and mergers with smaller with a succession of smaller nearby galaxies in its past, UC Santa Cruz researchers Puragra Guhathakurta and Claire Dorman say.

The Andromeda galaxy, also known as M31, the nearest spiral galaxy to our Milky Way and the biggest in our local neighborhood of galaxies, was studied for insights into its structure and internal motions, keys to understanding the history of how it formed, they say.

"In the Andromeda galaxy we have the unique combination of a global yet detailed view of a galaxy similar to our own," says Guhathakurta, a professor of astrophysics and astronomy.

The view of a spiral galaxy from outside is something we are denied when studying our own galaxy, he says.

"We have lots of detail in our own Milky Way, but not the global, external perspective," he explains.

In their study, the researchers analyzed data gathered by the Hubble Space Telescope and the W.M. Keck Observatory in Hawaii.

That data allowed them to plot the velocities of more that 10,000 stars within Andromeda, revealing the different velocities of young stars, intermediate-age and old stars in the galaxy's disk.

It is the first such stellar measurement conducted for another galaxy.

The researchers' work revealed a clear link between the age of stars and their motions, with the galaxy's youngest stars displaying a relatively regular motion -- all moving coherently, with almost the same velocity -- around its center, while much more irregular motions were observed for older stars.

"If you could look at the disk edge on, the stars in the well-ordered, coherent population would lie in a very thin plane, whereas the stars in the disordered population would form a much puffier layer," Dorman explained in presenting the study at the winter meeting of the American Astronomical Society last week in Seattle.

One possibility for the galaxy's present structure is disturbances of an originally well-ordered galactic disk during mergers with smaller, satellite galaxies, the researchers suggested.

Substantial differences between it and our Milky Way strongly suggests Andromeda has exerienced a more violent history of such mergers in its recent past, they say.

"Even the most well ordered Andromeda stars are not as well ordered as the stars in the Milky Way's disk," Dorman says.

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