As many as one quarter of the star clusters in our Milky Way -- many more than previously thought -- are invaders from other galaxies, according to a new study. The report also suggests there may be as many as six dwarf galaxies yet to be discovered within the Milky Way rather than the two that were previously confirmed.

"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. These 'alien' star clusters that have made their way into our galaxy over the last few billion years," says Terry Bridges, an astronomer at Queen's University in Kingston, Canada.

The study (co-authored by Duncan Forbes of Swinburne University of Technology in Australia) has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

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

Using mostly Hubble Space Telescope data, Mr. Bridges and Mr. Forbes 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. 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 suggests that there are more of these accreted dwarf galaxies in our Milky Way than was thought.