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Stellar population
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<p>In <a href="/facts/1944/A6Jvm3T6">1944</a>, <a href="/facts/Walter_Baade/olu54Cof">Walter Baade</a> categorized groups of stars within the <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a> into stellar populations. In the abstract of the article by Baade, he recognizes that <a href="/facts/Jan_Oort/VLCCpHRS">Jan Oort</a> originally conceived this type of classification in <a href="/facts/1926/03cK1qtm">1926</a>. </p><p>Baade observed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the central <a href="/facts/Spiral_galaxy/hRT2dtxJ">galactic bulge</a> and within <a href="/facts/Globular_cluster/FL0EERJU">globular star clusters</a>. Two main divisions were deemed <i>population I</i> and <i>population II stars</i>, with another newer, hypothetical division called <i>population III</i> added in 1978. </p><p>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 <a href="/facts/Kinematics/o7mSyeJA">kinematics</a>, stellar age, and even <a href="/facts/Galaxy_formation_and_evolution/uAPtXxr4">galaxy evolution</a> in both <a href="/facts/Spiral_galaxy/hRT2dtxJ">spiral</a> and <a href="/facts/Elliptical_galaxy/1PUchrj1">elliptical</a> galaxies. These three simple population classes usefully divided stars by their chemical composition, or <i><a href="/facts/Metallicity/nhhPqbYF">metallicity</a></i>. In <a href="/facts/Astrophysics/krFPRWLA">astrophysics</a> nomenclature <i>metal</i> refers to all elements heavier than <a href="/facts/Helium/yRul93TG">helium</a>, including chemical <a href="/facts/Non-metals/0oejKuzK">non-metals</a> such as oxygen. </p><p>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 <i>population III</i>, old stars (low metallicity) as <i>population II</i>, and recent stars (high metallicity) as <i>population I</i>. The <a href="/facts/Sun/CadPCVHh">Sun</a> is considered population I, a recent star with a relatively high 1.4% metallicity. </p>