Editing Stellar population (section)

{{short description|Grouping of stars by similar metallicity}}
{{redirect|Star generation|the process by which molecular clouds collapse and form stars|Star formation}}
[[File:Artist's impression of the Milky Way (updated - annotated).jpg|right|thumb|upright=1.4|Artist's conception of the spiral structure of the Milky Way showing Baade's general population categories. The ''blue'' regions in the spiral arms are composed of the younger population&nbsp;I stars, while the ''yellow'' stars in the central bulge are the older population&nbsp;II stars. In reality, many population&nbsp;I stars are also found mixed in with the older population&nbsp;II stars.]]

In [[1944]], [[Walter Baade]] categorized groups of stars within the [[Milky Way]] into '''stellar populations'''.
In the abstract of the article by Baade, he recognizes that [[Jan Oort]] originally conceived this type of classification in [[1926]].<ref name=Baade-1944/>

Baade observed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the central [[Spiral galaxy#Structure#Galactic bulge|galactic bulge]] and within [[globular cluster|globular star clusters]].<ref name=Shapley-1977/> Two main divisions were deemed '''''population{{nbsp}}I''''' and '''''population{{nbsp}}II''' stars'', with another newer, hypothetical division called '''''population{{nbsp}}III''''' added in 1978.

Among the population types, significant differences were found with their individual observed stellar spectra. These were later shown to be very important and were possibly related to star formation, observed [[kinematics]],<ref name=Gibson-etal-2013/> stellar age, and even [[Galaxy formation and evolution|galaxy evolution]] in both [[Spiral galaxy|spiral]] and [[Elliptical galaxy|elliptical]] galaxies. These three simple population classes usefully divided stars by their chemical composition, or ''[[metallicity]]''.<ref name=Kunth-Östlin-2000/><ref name=schonrich2009/><ref name=Gibson-etal-2013/> In [[astrophysics]] nomenclature ''metal'' refers to all elements heavier than [[helium]], including chemical [[non-metals]] such as oxygen.<ref name=metal/>

By definition, each population group shows the trend where lower metal content indicates higher age of stars. Hence, the first stars in the universe (very low metal content) were deemed ''population&nbsp;III'', old stars (low metallicity) as ''population&nbsp;II'', and recent stars (high metallicity) as ''population&nbsp;I''.<ref name=Byant-2005/> The [[Sun]] is considered population&nbsp;I, a recent star with a relatively high 1.4% metallicity.