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Jumping stars hold clues to Milky Way's past

Old and new Young stars in the spiral arms of the Milky Way bounce less than their older counterparts, unless they're in their senior years, according to a new study.

The new finding suggests our galaxy may have collided with smaller galaxies early in its evolutionary history.

Our Sun and solar system reside in a thin spiral arm of stars orbiting around the centre of the Milky Way.

In a paper published late last year, researchers working on the Australian Astronomical Observatory's RAdial Velocity Experiment (RAVE), found that stars in the galactic arm move up and down as they orbit the Milky Way's core.

"RAVE lets us see how fast and in which direction the stars are moving, so we can determine how far they're moving above and below the galactic plane," says study co-author Professor Joss Bland-Hawthorn of the University of Sydney.

In this latest study, published in Astrophysical Journal Letters, Bland-Hawthorn and colleagues reveal that some stars are moving up and down faster than others.

Initially, they hypothesised that the amount of vertical movement was linked to the star's age.

To determine age of the stars, they calculated the ratio of magnesium to iron of each of the stars — the higher the ratio, the older the star.

"We find that younger stars go up and down by a smaller amount, and the older ones go up and down by a greater amount," says Bland-Hawthorn.

Not so simple

However, the researchers also found that the oldest stars moved significantly less than their slightly younger siblings.

"We don't know what it means yet, and we don't know what caused it," says Bland-Hawthorn.

To understand this somewhat complex observation, the team developed a computer model of the Milky Way. They found that a collision with a smaller galaxy early in our galaxy's evolution could drag stars from the centre of the galaxy, which generally don't move vertically, into the spiral arms.

"It may have been a collision with a galaxy, the size of the Large Magellanic Cloud, maybe 12 billion years ago when our galaxy had just formed in the very early universe."

According to lead author Dr Ivan Minchev of the Leibniz Institute for Astrophysics in Potsdam, Germany, the findings will enable astronomers to trace the history of the Milky Way more accurately than ever before.

"By looking at the chemical composition of stars around us, and how fast they move, we can deduce the properties of satellite galaxies interacting with the Milky Way throughout its lifetime," says Minchev.

"This can lead to an improved understanding of how the Milky Way may have evolved into the galaxy we see today."