Stars in the Milky Way move up and down like dancers in a mosh pit as they orbit the centre of the galaxy, an Australian study has found.
The findings from the Radial Velocity Experiment survey (RAVE), found stars are moving in an unexpected wave like pattern, making the galaxy wobble.
"The stellar movement looks like waves in a fluttering flag," says Dr Fred Watson of the Australian Astronomical Observatory, who is one of the study's authors.
Led by Dr Mary Williams of the Leibniz Institute for Astrophysics Potsdam in Germany, the team of astronomers combined the RAVE survey data with the movement of stars across the sky within 6500 light years of the Sun.
"RAVE has been surveying about six degrees of the sky, using the UK Schmidt telescope at Siding Spring, to conduct detailed spectroscopic observations of the chemical signatures and radial velocities of half a million stars," says Watson.
The team focused on specific stars known as red clump stars to obtain their exact distances. They then used this data to develop a detailed three-dimentional stellar map of moving stars.
"This allows us to identify many kinds of streaming motion amongst the stars," says Watson. "We can see groups of stars moving in the same direction and at the same speed."
"But what we didn't expect to see is significant vertical motion in these stars as well."
"At one place in the galaxy they're all moving upwards together, and at another place in the galaxy, they're all moving downwards."
According to Watson, the cause is still not understood.
"Everything's on the table here, because we have a measurement of something going on in the galaxy that wasn't predicted," says Watson.
Study co-author Professor Ken Freeman says there could be several causes including the effects of the galaxy's central 'bar' of stars and its spiral arms.
"Our galaxy has swallowed other small galaxies, and this could have disturbed the stars," says Freeman.
"A lump of dark matter could have gone through the disk of the galaxy, or a cloud of hydrogen gas could have fallen into it."
The team's results appear in the Monthly Notices of the Royal Astronomical Society.
