Stellar population

"Star generation" redirects here. For the process by which molecular clouds collapse and form stars, see Star formation.
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 I stars, while the yellow stars in the central bulge are the older population II stars. In reality, many population I stars are also found mixed in with the older population 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.[1]

Baade observed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the central galactic bulge and within globular star clusters.[2] Two main divisions were defined as Population I star and population II, with another newer, hypothetical division called population 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,[3] stellar age, and even galaxy evolution in both spiral and elliptical galaxies. These three simple population classes usefully divided stars by their chemical composition or metallicity.[4][5][3]

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 III, old stars (low metallicity) as population II, and recent stars (high metallicity) as population I.[6] The Sun is considered population I, a recent star with a relatively high 1.4% metallicity. Note that astrophysics nomenclature considers any element heavier than helium to be a "metal", including chemical non-metals such as oxygen.[7]

  1. ^ Cite error: The named reference Baade-1944 was invoked but never defined (see the help page).
  2. ^ Cite error: The named reference Shapley-1977 was invoked but never defined (see the help page).
  3. ^ a b Cite error: The named reference Gibson-etal-2013 was invoked but never defined (see the help page).
  4. ^ Cite error: The named reference Kunth-Östlin-2000 was invoked but never defined (see the help page).
  5. ^ Cite error: The named reference schonrich2009 was invoked but never defined (see the help page).
  6. ^ Cite error: The named reference Byant-2005 was invoked but never defined (see the help page).
  7. ^ Cite error: The named reference metal was invoked but never defined (see the help page).