The oldest stars in the Milky Way move in unexpected ways

Some of the oldest stars in the Milky Way are not where we expect them to be.

One of the ways we classify stars is their metallicity. This is the fraction of heavier elements a star has compared to hydrogen and helium. It’s a useful metric because a star’s metallicity 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 through 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 made only 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 looking at its spectrum, we know the overall metallicity of the stars, both in our own galaxy and in others. We can therefore group the stars into “populations” of metallicity.

This is done by defining the hydrogen / iron ratio, [Fe/He], on a logarithmic scale by setting our Sun as the zero point. Therefore, Population I stars have a ratio of at least -1, which means they have 10 percent or more of the Sun [Fe/He] relationship.

(Wikimedia Commons)

Older stars in Population II have lower metallicity than Population I and Population III (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 within 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 the stars surrounding the Milky Way tends to be the stars with the least metallicity.

This makes sense as stars are born within the dense gas and dust within the galactic plane, especially the spiral arms. Over time, the gravitational dance of the stars would allow them to migrate outward. Only the oldest stars have had time to get away from the plane.

But as surveys of the sky on the ground and the Gaia probe give us a more detailed view of the Milky Way, it is revealing 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 the Australian SkyMapper Southern Survey, the team identified 475 stars with a [Fe/He] ratio less than one 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 Gaia data, they found that 11% of them orbits within 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 large sky reliefs of our galaxy are sure to revolutionize our understanding of the Milky Way. As these first results also show, it is clear that we still have a lot to learn.

The research was published in Royal Astronomical Society Monthly Notices.

This article was originally published by Universe Today. Read the original article.