Our Galaxy, the Milky Way, engulfed several other small star systems during the course of its history, according to evidence presented in Nature1.

The Milky Way is visible on a clear night as a band of silvery light across the sky: countless millions of stars, too distant to see individually with the naked eye. Although it looks as though we are outside this vast field of stars we are, in fact, situated on one spiral arm of the Galaxy close to its outer edge. The Milky Way is what we see looking back into the flat Galactic disk.

Surrounding the Milky Way is a diffuse spherical cloud of stars called the Galactic halo. The disk-shaped region of the Galaxy inside the orbit of our own Sun – which circulates, along with the rest of the stars, around the Galactic centre – is thought to have been formed by the rapid collapse of a galaxy-sized gas cloud billions of years ago. The formation of the halo is happening more slowly, and in a piecemeal fashion.

For example, in 1994 astronomers at Cambridge discovered an entire galaxy, very close to our own, which is apparently being pulled apart and sucked up by the gravitational force of our own Galaxy. This 'dwarf' galaxy had not been seen before because it lies in the constellation of Sagittarius on the opposite side of the Galactic centre to the Sun, and so was obscured by stars in the central bulge. In the future this dwarf galaxy will merge with our own.

Merging events like this are thought to have been common in the early days of our Galaxy, but until now hard evidence for this was sparse.

The collapse of the gas cloud into the stars of the Galactic disk would have left bits scattered around the edges, which would have formed their own 'mini-galaxies'. Some of these still exist: the halo region is peppered with so-called 'globular clusters', groups of between a hundred thousand and a million stars. Many of the other medium-sized groups of stars from the Galaxy's early days will have long since been pulled apart and incorporated into the main spiral mass. But can we still see fossils of these building blocks of the Milky Way?

Amina Helmi from the Leiden Observatory in the Netherlands, and colleagues have searched for such remnants in the Galactic halo by locating the old stars that have been there since the time of galaxy formation and so might once have composed parts of the ancient star families. Old stars can be recognized by their colour and brightness – features which change as they age.

Helmi and colleagues found that one in ten old stars in the Galactic halo seem to be moving together, like a flock of birds. This suggests that they were once all part of some huge precursory cluster to the present-day Galaxy, most of which has now been swallowed. These stars are, the researchers say, a "fossil stream" from the Galaxy's formation.

Another facet of this birth process is revealed by Y W Lee of Yonsei University, Korea2, and colleagues. They have looked at the stars of the largest globular cluster in the halo, called 'ω-Centauri', and found that it contains at least two sub-populations of stars with differing ages, about two billion years apart. This implies that the cluster must have experienced at least two periods of star formation – which is a surprise, since globular clusters are generally considered to have done nothing since they first got together.

Thus Lee and colleagues infer that ω-Centauri was formed in some separate sub-galaxy, perhaps like the Sagittarius dwarf galaxy, that then got sucked into our Galaxy, stimulating a new era of star formation. Here, then is another fingerprint of events that have been altering the Milky Way from its birth to the present day.