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Some of the oldest stars in the Milky Way are not where we’d expect them to be.
One of the ways we categorize stars is by their metallicity. It is the fraction of heavier elements that a star has relative to hydrogen and helium. It is a useful metric because the metallicity of a star is a good measure of its age.
The hydrogen and helium we see in the Universe were created in the early moments of the Big Bang. This is why they are so abundant. Heavier elements such as carbon and iron are created by astrophysical processes such as the fusion of elements in stellar nuclei, or during collisions of white dwarfs and neutron stars.
For this reason, the first stars were only made up of hydrogen and helium. Over time, the abundance of heavier elements gradually increased, so younger stars tend to have higher metallicity.
Since we can determine the metallicity of a star by observing its spectrum, we know the overall metallicity of stars, both in our galaxy and in others. We can therefore group the stars into “populations” of metallicity.
This is done by setting the hydrogen to iron ratio, [Fe/He], on a logarithmic scale defining our Sun as zero point. Thus, the stars of population I have a ratio of at least -1, which means that they have 10% or more of the [Fe/He] report.
(Wikimedia Commons)
Older stars in population II have lower metallicity than population I, and population III (the first generation stars) would have no metallicity at all.
In our galaxy, these populations of stars are distributed from the galactic plane outwards. Younger Population I stars tend to be in the spiral arms of our galaxy, while older Population II stars tend to be above or below the galactic plane. The diffuse outer halo of stars surrounding the Milky Way tend to be the lowest stars in metallicity.
This makes sense since stars are born in dense gas and dust in the galactic plane, especially spiral arms. Over time, the gravitational dance of the stars would allow them to migrate outward. Only the older stars had time to move away from the plane.
But as sky-to-ground surveys and the Gaia spacecraft give us a more detailed view of the Milky Way, it reveals some surprises about our long-standing galactic model. This can be seen in a recent study looking at some of the oldest stars in our galaxy.
Using Australia’s SkyMapper Southern Survey, the team identified 475 stars with a [Fe/He] ratio less than a thousandth of that of our Sun.
We would expect them to be halo stars, but when the team calculated the positions and orbits of these stars using data from Gaia, they found that 11% of they revolve around the galactic plane.
Their orbits are also very circular, similar to the orbit of the Sun. This is surprising and goes against the predictions of current models of galactic evolution.
The great studies of the skies of our galaxy will certainly revolutionize our understanding of the Milky Way. As even these early results show, it is clear that we still have a lot to learn.
The research was published in Monthly notices from the Royal Astronomical Society.
This article was originally published by Universe Today. Read the original article.
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