Ring of stars circling the Milky Way

Evidence has been found for a vast ring of stars that may completely encircle the Milky Way, astronomers announced on Monday.

	An artist's impression shows the ring of stars outside the Milky Way's spiral arms (Image: Geraint F. Lewis)

Astronomers' view of the outer edge of our own galaxy has previously been restricted by intervening stars and limited telescope time. But now two teams of researchers have discovered parts of what could be a ring of about a billion stars beyond the Milky Way's spiral arms.

"Clearly we don't yet know for certain if it encircles the entire galaxy but it looks quite promising," says Mike Irwin, at Cambridge University, UK, and one of the researchers behind the discovery.

"The next steps are to probe other regions around the galaxy to measure their stellar colours, magnitudes and velocities, and to begin detailed modelling of the structure using the information we have," he told New Scientist.

Outer edge

Two teams of researchers were involved in the work. The first, led by Heidi Newberg, at the Rensselaer Polytechnic Institute, US, discovered an arc of stars all of which were equidistant from the centre of the galaxy. The team was monitoring the Milky Way's outer edge using the Sloan Digital Sky Survey, based in Sacramento, New Mexico.

The second team, which included Irwin, performed a survey of a section of sky 100 degrees away using the Isaac Newton Telescopes on La Palma, one of the Canary Islands. Their original aim was to study another galaxy altogether - Andromeda.

But they found a similar pattern of stars at the same distance as the first group, suggesting that a giant ring of stars could stretch around our galaxy.

However, some astronomers wish to see further evidence before accepting the existence of a giant ring. James Binney, at Oxford University, UK, told New Scientist: "Something very interesting is happening here, but the physical picture is as yet totally unclear."

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Torn to pieces

It is also unclear how such a structure might have formed. One possibility is that the stars originated within our galaxy but were flung out over time. Another is that the gravity of the Milky Way tore a smaller satellite galaxy to pieces leaving this outer debris.

If confirmed, the existence of a ring of stars encircling the Milky Way would not be a complete surprise, says Michael Merrifield, at Nottingham University, UK. "There are quite a number of other spiral galaxies similar to the Milky Way that have been observed to be surrounded by faint rings of stars," he says.

The research was presented on Monday at the American Astronomical Society meeting in Seattle, Washington State, US.

Will Knight