Some of the Milky Way’s oldest stars are not where we expect them to be.
One of the ways we categorize stars is their metallicity. It is the fraction of heavier elements that a star has compared to hydrogen and helium. It is a useful measurement because a star’s metallicity is a good measure of its age.
Hydrogen and helium we see in the universe were created in the early moments of the Big Bang. That is why they are so abundant. Heavier elements such as carbon and iron are created through astrophysical processes such as fusion of elements in stellar nuclei or during collisions between white dwarfs and neutron stars.
Because of this, the earliest stars were made only of hydrogen and helium. Over time, the surface of heavier elements gradually increased, so younger stars tend to have a higher metallicity.
Since we can determine the metallicity of a star by observing its spectrum, we know the total metallicity of stars, both in our galaxy and in others. We can therefore group stars into the “populations”
This is done by defining the ratio of hydrogen to iron, [Fe/He], on a logarithmic scale that sets our Sun as the zero point. Population I stars thus have a ratio of at least -1, which means that they have 10 percent or more of the sun [Fe/He] relationship.
Older Population II stars have a lower metallicity than Population I, and Population III (first generation stars) would have no metallicity at all.
In our galaxy, these populations of stars from the galactic plane are distributed outward. The youngest 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 stars surrounding the Milky Way tends to be the stars that are lowest in metallicity.
This makes sense since stars are born in the dense gas and dust of the galactic plane, especially the spiral arms. Over time, gravity with stars will allow them to migrate outward. Only older stars have had time to glide away from the plane.
But as Earth-based sky surveys and the Gaia spacecraft give us a more detailed overview of the Milky Way, it reveals surprises about our long-held galactic model. This can be seen in a recent study of some of the oldest stars in our galaxy.
Using Australia’s SkyMapper Southern Survey, the team identified 475 stars with one [Fe/He] the ratio less than a thousandth of our sun.
We expect them to be halo stars, but when the team calculates the positions and orbits of these stars using data from Gaia, they found that 11 percent of them orbit within the galactic plane.
Their orbits are also very circular, similar to the orbit of the sun. This is surprising and contradicts predictions of current galactic evolutionary models.
Major celestial studies 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 is published in Monthly announcements from the Royal Astronomical Society.
This article was originally published by Universe Today. Read the original article.