The Andromeda galaxy is the most familiar of all the spiral galaxies in our sky. Wider and possibly brighter than our own Milky Way, Andromeda is about 2.4 to 2.9 million light years (one light year: the distance light travels in a year) away from the Solar System. It was previously thought that the Andromeda galaxy is 70,000 to 80,000 light years across, but now astronomers using the Keck II telescope in Hawaii have discovered that the galaxy is actually about 220,000 light years across. In comparison, our Milky Way is only about 100,000 light years across!

Andromeda Galaxy (Courtesy: NASA)Astronomers used the telescope to make new observations of the motions of stars in the most distant outskirts of the spiral galaxy. They found the movement of this sparse smattering of far-flung stars is actually synchronised with the rest of the galaxy's stars, rotating in an orderly way around its galactic centre. The stars surrounding Andromeda's spiral arms had been seen before, but astronomers had assumed they were captured fragments of other galaxies that would retain their own, essentially random, stellar motions.
The findings are hard to reconcile with current theories and computer models of galaxy formation, according to Rodrigo Ibata of the Observatoire Astronomique de Strasbourg, France, and another member of the research team. According to them, you just don't get giant rotating discs from the accretion of small galaxy fragments.
So back to rewriting the theories :):).