One that the methods we categorize stars is by your metallicity. The is the fraction of heavier facets a star has contrasted to hydrogen and also helium. It’s a helpful metric since the metallicity that a star is a great measure of its age.

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The hydrogen and helium we see in the cosmos were created in the early moments that the large bang. That’s why they space so plentiful. Heavier aspects such as carbon and iron are produced through astrophysical procedures such together the combination of facets in mainly cores, or during the collisions the white dwarfs and neutron stars. Because of this, the earliest stars to be made only of hydrogen and also helium. With time the abundance of heavier elements gradually increased, so younger stars tend to have actually a higher metallicity.

Since we have the right to determine the metallicity the a star by observing that spectrum, we recognize the all at once metallicity that stars, both in our galaxy and others. We can as such group stars right into metallicity “populations.” This is excellent by specifying the hydrogen come iron ratio, , top top a logarithmic scale setup our sunlight as the zero point. Thus, population I stars have a ratio of at least -1, an interpretation they have 10% or an ext of the Sun’s ratio. Older populace II stars have a lower metallicity than population I, and population III (the very first generation stars) would have actually no metallicity at all.

Typical location of stars in the Milky Way. Credit: Wikipedia

In our galaxy, these populations of stars are dispersed from the galactic aircraft outward. The youngest populace I stars tend to be in ~ the spiral arms of ours galaxy, if older population II stars have tendency to be above or below the galactic plane. The diffuse outer halo the stars surrounding the Milky way tend to be the stars lowest in metallicity.

This makes sense because stars space born in ~ the dense gas and also dust within the galactic plane, specifically the spiral arms. End time, the gravitational run of stars would enable them to move outward. Only older stars have had time come drift away from the plane.

But together ground-based skies surveys and the Gaia spacecraft offer us a more detailed view of the Milky Way, it’s revealing surprises around our long-held galactic model. This have the right to be viewed in a current study looking at few of the oldest stars in our galaxy.

A glow of stars learned by Gaia in ~ the Milky Way. Credit: ESA/Gaia/DPAC, A. Khalatyan(AIP) & StarHorse team

Using Australia’s SkyMapper southerly Survey, the team identified 475 stars v a ratio much less than one-thousandth the of our Sun. We would mean them to it is in halo stars, however when the team calculation the positions and orbits of this stars utilizing data indigenous Gaia they discovered that 11% of lock orbit within the galactic plane. Your orbits room also an extremely circular, comparable to the orbit of the Sun. This is surprising and goes versus predictions of current galactic advancement models.

Large skies surveys of our galaxy are particular to revolutionize our expertise of the Milky Way. As even these early results show, that is clear us still have much come learn.

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Reference: Cordoni, G., et al. “Exploring the Galaxy’s halo and very metal-weak thick disk through SkyMapper and Gaia DR2.” Monthly Notices of the Royal huge Society (2020).