The orbits of ancient stars push us to rethink the evolution of the Milky Way

Theories about how the Milky Way formed will be rewritten following discoveries about the behavior of some of its oldest stars.

An investigation into the orbits of the galaxy’s metal-poor stars – presumably among the oldest in existence – found that some of them travel in previously unforeseen patterns.

“Metal-poor stars – which contain less than a thousandth of the amount of iron found in the Sun – are some of the rarest objects in the galaxy,” said Professor Gary Da Costa of the ARC Center of Excellence in All Sky Astrophysics in 3 Dimensions of Australia (ASTRO 3D) and the Australian National University.

“We studied 475 and found that about 11% orbits in the nearly flat plane that is the disk of the Milky Way.

“They follow an almost circular path, very similar to the Sun. It was unexpected, so astronomers will have to rethink some of our basic ideas.”

Previous studies had shown that metal-poor stars were almost exclusively confined to the halo and bulge of the Galaxy, but this study revealed a significant number in orbit around the disk itself.

The Sun also orbits within the disk, which is why it manifests itself as a relatively thin ribbon structure easily visible from Earth in the night sky. In fact, we are seeing it from the front.

“Over the past year our view of the Milky Way has changed dramatically,” said lead author Giacomo Cordoni of the University of Padua in Italy, who performed most of the study during a recent internship at ANU, funded from the European Union Project GALFOR of the Research Council.

“This discovery is inconsistent with the Galaxy’s previous formation scenario and adds a new piece to the puzzle which is the Milky Way. Their orbits are very similar to that of the Sun, although they contain only a tiny fraction of its iron. Understanding why they move the way they do will likely require a significant reassessment of how the Milky Way has developed over many billions of years. “

Ancient stars have been identified using three highly technological pieces of equipment: ANU’s SkyMapper and 2.3-meter telescopes and the European Space Agency’s Gaia satellite.

The low metal content was identified by telescopes and the satellite was then used to determine their orbits.

The results, compiled by researchers from Australia, Italy, Sweden, the United States and Germany, found that the orbits of ancient stars fell into a number of different patterns, all but one that matched previous predictions and observations.

As expected, many of the stars had largely spherical orbits clustered around the galaxy’s “stellar halo,” a structure thought to be at least 10 billion years old.

Others had irregular and “wobbly” paths that are presumed to have been the result of two catastrophic collisions with smaller galaxies in the distant past – creating structures known as Gaia Sausage and Gaia Sequoia.

Some stars were in retrograde orbit – actually going in the wrong direction around the Galaxy – and some, about five percent, appeared to be about to leave the Milky Way entirely.

And then there were the remaining 50 or so, with orbits aligned with the disk of the Galaxy.

“I think this work is full of important and new results, but if I had to pick one, it would be the discovery of this extremely metal-poor population of disk stars,” Cordoni said.

“Future scenarios for the formation of our galaxy will have to take into account this discovery, which will change our ideas quite dramatically.”

Cordoni’s team included scientists from the Italian Center for Space Studies and Activities, the Max Planck Institutes for Astrophysics and Astronomy in Germany, the Massachusetts Institute of Technology in the United States, the Swedish universities of Uppsala and Stockholm, and the Monash University of Australia. University of New South Wales and ANU.

The team included Australian Brian Schmidt, winner of the 2011 Nobel Prize in Physics.

An early version of the study is now available in Royal Astronomical Society Monthly Notices.

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