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The orbits of ancient stars are rapidly rethinking the evolution of the Milky Way



The orbits of ancient stars are rapidly rethinking the evolution of the Milky Way

Representation of the orbit of the star 232121.57-160505.4 in the galactocentric Cartesian frame, color-coded according to time. The white dot represents the current position of the star. The black circular dot and the dotted circular line indicate the position and approximate orbit of the Sun, respectively. Credit: Cordoni et al

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


An investigation into the orbits of the galaxy’s poor metal stars – believed to be some of the oldest in existence – has found that some of them travel in hitherto unpredictable patterns.

“Metal-poor stars – containing less than a thousandth of the iron found in the Sun – are among the rarest objects in the galaxy,” said Professor Gary Da Costa of the Australian ARC Center for Excellence in All Astrophysics in 3 dimensions ( ASTRO 3-D) and the Australian National University.

“We studied 475 of them and found that about 11 percent of the orbit in the almost flat plane, which is the disk of the Milky Way.

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

Previous studies have shown that metal-poor stars are almost extremely limited to the halo and bulge of the Galaxy, but this study reveals a significant number orbiting the disk itself.

The sun also orbits the disk, making it appear as a relatively thin strip-like structure, easily visible from Earth in the night sky. In fact, we see it from the inside.

“Over the last year, our vision of the Milky Way has changed dramatically,” said lead author Giacomo Cordoni of the University of Padua in Italy, who conducted most of the research while recently on an internship at ANU, funded by the European Research GALFOR Project. advice.

“This discovery is not in line with the previous scenario for galaxy formation and adds a new piece to the puzzle, which is the Milky Way. Their orbits are very similar to those of the Sun, although they contain only a small part of its iron. Understanding why they move the way they do is likely to lead to a significant reassessment of the way the Milky Way has evolved over many billions of years. “

The ancient stars were identified using three very high-tech pieces of the kit: ANU’s SkyMapper and 2.3-meter telescopes, as well as the European Space Agency’s Gaia satellite.

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

The results – crushed by researchers from Australia, Italy, Sweden, the United States and Germany – found that the orbits of ancient stars fall into a number of different models, all but one of which are consistent with previous predictions and observations.

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

Others have had uneven and “hesitant” pathways that are thought to be the result of two cataclysmic collisions with smaller galaxies that occurred in the distant past – creating structures known as Gaia Sausage and Gaia Sequoia.

Some stars orbited in retrograde orbit — they were actually orbiting the galaxy — and some, about five percent, seemed to be in the process of exiting the Milky Way as a whole.

And then there were the other 50 or more, 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 choose one, it would be the discovery of this population of extremely poor metal disk stars,” Cordoni said.

“Future scenarios for the formation of our galaxy will have to take this finding into account – which will change our ideas quite drastically.”


Astronomers have identified 18 metal-poor stars in the dwarf galaxy Sagittarius


More info:
Cordoni et al. Explore the halo and very weak metallic thick disk of the Galaxy with SkyMapper and Gaia DR2, Monthly notices of the Royal Astronomical Society (2020). DOI: 10.1093 / mnras / staa3417

Provided by ARC Center of Excellence for All Sky Astrophysics in 3D (ASTRO 3D)

Quote: The orbits of the ancient stars are rapidly rethinking the evolution of the Milky Way (2020, November 16), extracted on November 16, 2020 from https://phys.org/news/2020-11-orbits-ancient-stars-prompt- rethink.html

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