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Metal-poor stars, which contain less than one thousandth of the amount of iron found in the Sun, are some of the rarest objects in the Galaxy. Studying the orbits of these stars has found that some of them travel in previously unforeseen patterns.
According to scientists, theories on how the Milky Way formed should be rewritten following discoveries about the behavior of some of its oldest stars.
In a study by the ARC Center of Excellence for All-Sky Astrophysics in 3D (ASTRO 3D), scientists studied 475 metal-poor stars. 11% of the stars orbit around the nearly flat plane that is the disk of the Milky Way.
Previous studies have suggested that metal-poor stars were limited solely to the halo and bulge of the Galaxy, but this study found a huge number orbiting the disk itself.
The Sun also orbits within the disk, which is why it looks like the relatively thin ribbon-like structure actually visible from Earth in the night sky. We see it from the front.
Lead author Giacomo Cordoni of the University of Padua in Italy stated: “Over the past year our view of the Milky Way has changed dramatically.”
“This discovery is not consistent with the previous Galaxy formation scenario 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 tiny fraction of its iron. Understanding why they move the way they do will likely require a significant reassessment of the development of the Milky Way over many billions of years. “
Scientists identified ancient stars using three pieces of high-tech kit: ANU’s SkyMapper and 2.3-meter telescopes and the European Space Agency’s Gaia satellite.
The result, elaborated by analysts from Australia, Italy, Sweden, the United States and Germany, found that the orbits of ancient stars fell into various examples, all but one of which coordinated forecasts and observations of the past.
As expected, many of the stars generally had spherical circles, clustered around the galaxy’s stellar halo, a structure thought to be 10 billion years old anyway.
Others had erratic and “rickety” paths that were thought to be the result of two catastrophic collisions with smaller galaxies in the distant past – packaging structures known as Gaia Sausage and Gaia Sequoia.
Some stars were orbiting retrograde – practically going in the wrong direction around the Galaxy – and a few, about five percent, appeared to be on the verge of leaving the Milky Way entirely.
And then there were the remaining 50 or so, with orbits aligned with the disk of the Galaxy.
Cords said, “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 metal-poor stars on the disk. Future scenarios for the formation of our Galaxy will have to take this discovery into account, which will change our ideas quite dramatically. “
Journal reference:
- Cordoni et al. Exploring the galaxy’s halo and thick disk as faint as metal with SkyMapper and Gaia DR2, Royal Astronomical Society monthly communications (2020). DOI: 10.1093 / mnras / staa3417
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