Milky Way's Star 'Doughnut'

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.

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."

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's 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.

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.

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.

Astronomers find evidence of small galaxy torn apart by Milky Way

The Milky Way may have ripped apart a smaller galaxy billions of years ago and scattered its stars into a faint surrounding ring.

A new survey scanning the outskirts of the Milky Way has found a belt of stars that are different in chemistry and in motion from stars within the galaxy.

This suggests they are the remnants of a galactic collision that may have occurred 10 billion years ago.

The ring, some 120,000 light years across, is shaped like a doughnut with the Milky Way in its centre.

"This is something new that we had not expected to find," said Heidi Jo Newberg of Rensselaer Polytechnic Institute.

She and Brian Yanny of the Fermi National Accelerator Laboratory discovered the ring during the Sloan Digital Sky Survey, a project to plot the position and brightness of 100 million celestial objects.

A European team, using different instruments, has confirmed the existence of the ring.

Newberg and Yanny presented their discovery Monday at the national meeting of the American Astronomical Society.

Bruce Margon of the Space Telescope Science Institute, said the new finding by Yanny and Newberg "is a vivid smoking gun evidence of the disruption of galaxy" and said it was "embarrassing" that the structure had not been found earlier.

"It has been accepted folklore that big galaxies tear apart and eat smaller galaxies," said Margon and the new evidence shows that an entire small galaxy was once gobbled up by the Milky Way.

Yanny said that the ring has 100 million to a half billion stars circling the Milky Way in an orbital period of tens of thousands of years. Newberg said the stars in the ring show that they are moving "in a very similar fashion to (the) way that planets orbit the sun."

Story filed: 21:37 Monday 6th January 2003

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