Milky Way's star 'doughnut'

A ring of stars surrounds the Milky Way

A vast, but previously unknown structure has been discovered around the edges of our galaxy, the Milky Way.

The first large area surveys of the sky have revealed several hundred million stars surrounding the galaxy's main disc.

The ring, which has the appearance of a giant doughnut, could be the remains of a satellite galaxy.

Astronomers believe it could hold clues as to how the Milky Way and other galaxies evolved.

Giant doughnut

An international team of astronomers looked at images of the Milky Way from previous surveys.

They found that the brightness and colour of some objects did not match up to their expectations.

Dr Mike Irwin, from the Institute of Astronomy at the University of Cambridge, UK, was one of the researchers behind the work.

He told BBC News Online: "There's a component that doesn't fit in with the standard picture of our own galaxy."

Arms and disc

This giant doughnut-like ring of stars appears to encircle the Milky Way.

The ring is estimated to contain several hundred million stars - about 1% of the total number of stars in our galaxy.

The disc is not completely round, though, as it has been warped, most probably from encounters with orbiting satellite galaxies.

The Milky Way is a spiral galaxy - it has arms that sweep away from a nucleus of star formation. Its disc is roughly 100,000 light-years across, with the Sun (and the planets) embedded in it.

Galaxy evolution

The astronomers say they cannot be sure how the ring formed, but the idea that it represents the remains of a satellite galaxy consumed by the Milky Way seems plausible.

Another explanation could be that the stars originally came from the disc of the Milky Way itself, and their orbits have been warped or spread over time so that they now wander far from the disc plane.

Detailed studies of the ring could help explain how galaxies evolve.

According to Dr Irwin, something odd happened about 10 billion years ago. As yet, they cannot satisfactorily explain it, but researchers will now be looking for the phenomenon in other galaxies.

The findings were announced at the American Astronomical Society's meeting in Seattle, Washington, US.