We categorize stars among other things according to their metallicity. That is the proportion of heavier elements that 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 that we see in the universe were created in the early moments of the Big Bang. That is why they are so numerous. Heavier elements such as carbon and iron arise through astrophysical processes such as the merging of elements in star nuclei or when white dwarfs and neutron stars collide. Because of this, the earliest stars were made up only of hydrogen and helium. Over time, the heavier elements gradually increased in abundance, so younger stars tended to have 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 metallicity populations. For this, the ratio of hydrogen to iron (Fe / He) is defined on a logarithmic scale, with our sun being set as the zero point. Therefore, Population I stars have a ratio of at least -1, which means they have 10% or more of the solar ratio (Fe / He). Older Population II stars have less metallicity than Population I, and Population III (the first generation stars) would have no metallicity at all.
Typical position of the stars in the Milky Way. Photo credit: Wikipedia
In our galaxy, these populations of stars are distributed outward from the galactic plane. The youngest Population I stars are usually in the spiral arms of our galaxy, while older Population II stars are usually above or below the galactic plane. The stars' diffuse outer halo that surrounds the Milky Way is usually the star with the least amount of metallicity.
This makes sense because stars are born in the dense gas and dust in the galactic plane, especially in the spiral arms. In time, the stars' gravitational dance would allow them to move outward. Only older stars had time to move away from the aircraft.
Since ground-based sky surveys and the Gaia spaceship give us a more detailed view of the Milky Way, there are surprises about our long-standing galactic model. This is evident from a recent study that examined some of the oldest stars in our galaxy.
A glow of stars examined by Gaia in the Milky Way. Photo credit: ESA / Gaia / DPAC, A. Khalatyan (AIP) and StarHorse Team
Using the Australian SkyMapper Southern Survey, the team identified 475 stars with a (Fe / He) ratio of less than a thousandth 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 them orbit within the galactic plane. Their orbits are also very circular, similar to the sun's orbit. This is surprising and contradicts predictions of current galactic evolution models.
Large sky measurements of our galaxy will certainly revolutionize our understanding of the Milky Way. As these early results also show, it is clear that we still have a lot to learn.
Reference: Cordoni, G. et al. "Use SkyMapper and Gaia DR2 to explore the halo and the very weak metal thick disk of the Galaxy." Monthly releases from the Royal Astronomical Society (2020).