Rethinking the Milky Way’s evolution

An investigation into the odd orbits of the galaxy’s oldest stars may prompt astronomers to rethink how the Milky Way evolved.

Australian telescopes teamed up with the European Space Agency’s Gaia satellite to discover that some of the galaxy’s most metal-poor stellar giants travel in surprising patterns.

“Metal-poor stars – containing less than one-thousandth the amount of iron found in the Sun – are some of the rarest objects in the galaxy,” says astronomer Gary Da Costa from the Australian National University. 

“We’ve studied 475 of them and found that about 11% orbit in the almost flat plane that is the Milky Way’s disc.

“They follow an almost circular path – very much like the Sun. That was unexpected, so astronomers are going to have to rethink some of our basic ideas.”

Da Costa was part of an international collaboration of researchers from Australia, Europe and the US. An advanced version of their study is published in the journal Monthly Notices of the Royal Astronomical Society.

The stars’ orbits fell into several different patterns, most of which matched what had already been studied or predicted.  Previous research, for example, had observed these rare stars almost exclusively in the Milky Way’s halo and bulge, and this study confirmed that most of the stars had large spherical orbits around the halo.

Some, however, were orbiting retrograde – the “wrong way” – around the galaxy. Around 5% were in the process of escaping the galaxy altogether; and 50 stars were orbiting the disc of the galaxy itself.

According to lead author Giacomo Cordoni, from Italy’s University of Padova, finding metal-poor stars orbiting the disc is particularly intriguing.

“Future scenarios for the formation of our galaxy will have to account for this finding – which will change our ideas quite dramatically,” he says.

“This discovery is not consistent with the previous galaxy formation scenario and adds a new piece to the puzzle that is the Milky Way.

“Their orbits are very much like that of the Sun, even though they contain just a tiny fraction of its iron. Understanding why they move in the way that they do will likely prompt a significant reassessment of how the Milky Way developed over many billions of years.”

201117 str graphic
Representation of the orbit of the star 232121.57-160505.4 in the Galactocentric cartesian frame, colour coded according to the time. The white dot represents the current position of the star. The black circled dot and the dashed circular line indicate the position and the approximated orbit of the Sun, respectively. Credit: Cordoni et al

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