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'Alien' invaders seen in the Milky Way

The report also suggests that there may be as many as six dwarf galaxies yet to be discovered within the Milky Way, besides the two that were previously found.

'Some of the stars and star clusters you see when you look into space at night are aliens from another galaxy, just not the green-skinned type you find in a Hollywood movie,' says Terry Bridges, astronomer at Queen's University in Kingston, Canada.

'These 'alien' star clusters (are ones) that have made their way into our galaxy over the last few billion years,' adds Bridges.

Previously, astronomers had suspected that some star clusters, each of which contains between 100,000 and a million stars, were foreign to our galaxy, but it was difficult to identify which ones.

Using mostly Hubble Space Telescope data, Bridges and Duncan Forbes, co-author, from Swinburne University of Technology, Australia, examined old star clusters within the Milky Way galaxy.

From the research they compiled the largest ever high-quality database to record the age and chemical properties of each of these clusters.

'We looked at all the data we could find. The best data are from the Hubble Telescope because it has the best imaging,' Bridges says. 'We looked at the ages and the amounts of heavy elements in these clusters, which can be measured from their stars.'

The researchers' work also suggests that the Milky Way may have swallowed up more dwarf galaxies than was previously thought, said a Queen's varsity release.

They found that many of the foreign clusters originally existed within dwarf galaxies -- 'mini' galaxies of up to 100 million stars that sit within our larger Milky Way.

The study is slated for publication in the Monthly Notices of the Royal Astronomical Society.