Andromeda had collided with another galaxy

Instead of having the flat plane and outflung arms that are the hallmarks of a mature spiral galaxy, Andromeda has a warped plane and several rather chaotic, overlapping outer rings, like ripples of stars.

The reason, according to an international team of astronomers: Andromeda suffered a head-on collision with a smaller galaxy some 210 million years ago.

The evidence comes from infrared images of Andromeda taken by the orbiting Spitzer Space Telescope, and these reveal a picture quite different to that seen by the naked eye.

It shows a previously-hidden, tilted second ring that protrudes from the heart of the galaxy. This ring is likely to be the shockwave of gas and dust from a colossal collision.

The theory has been put to the test in a computer simulation. It suggests that a "dwarf" galaxy, M32, was the likely impactor, driving straight into the heart of Andromeda, also called M31, to create a behemoth of a trillion stars.

"While head-on collisions may have been common in the early Universe, only a handful are known nearby," says the paper, lead-authored by David Block of the University of Witwatersrand in South Africa.

"The discovery of one in our near-neighbour M31 affords the unique opportunity of studying such a collision at unprecedented spatial resolution."

Andromeda, first spotted as "a little cloud" by the Persian astronomer Abd-al-Rahman Al Sufi in 964, is heading our way.

However, the two galaxies are more than two million light years apart and the collision will take some time -- between three and six billion years from now, according to various estimates.