"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," said Dr Terry Bridges, an astronomer at Queen’s University in Ontario, Canada.

"These ‘alien’ star clusters have made their way into our galaxy over the last few billion years."

Astronomers previously suspected that some star clusters -- which contain from 100,000 to a million stars each -- were foreign to our galaxy.

But identifying those star clusters with origins in other galaxies was a difficult task.

Dr Bridges collaborated with Duncan Forbes of Swinburne University of Technology in Australia on solving this problem.

They examined old star clusters within the Milky Way galaxy using data from the Hubble Space Telescope.

This yielded 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," Dr Bridges said.

"The best data are from the Hubble Telescope because it has the best imaging."

"We looked at the ages and the amounts of heavy elements in these clusters, which can be measured from their stars."

One of the key findings of the study is that the Milky Way may have swallowed-up more dwarf galaxies than was previously thought.

These ‘mini’ galaxies of up to 100 million stars sit within our larger Milky Way.

The study suggests that many 'invading' star clusters came from dwarf galaxies -- which would mean that that the Milky Way may have far more dwarf galaxies than previously suspected.