stellar populations,two broadly contrasting distributions of star types that are characteristic of different parts of a galaxygalaxy,
large aggregation of stars, gas, dust, and usually dark matter, typically containing billions of stars. Recognition that galaxies are independent star systems outside the Milky Way came from a study of the Andromeda Galaxy (1926–29) by Edwin P.
..... Click the link for more information. . Population I stars are young, recently formed stars, whereas population II stars are old and highly evolved. Population II stars are formed early in the history of the galaxy from pure hydrogen with an admixture of primordial helium. Because massive blue-white giants burn their nuclear fuel quickly and therefore have lifetimes of only a few million years, no stars of this type are found in population II. The most luminous population II stars are red giants. Population I stars, of which the sun is typical, are young stars that still lie mostly on the main sequence of the Hertzsprung-Russell diagramHertzsprung-Russell diagram
[for Ejnar Hertzsprung and H. N. Russell], graph showing the luminosity of a star as a function of its surface temperature. The luminosity, or absolute magnitude, increases upwards on the vertical axis; the temperature (or some temperature-dependent
..... Click the link for more information. . The most luminous population I stars are blue giants. Because they are second-generation stars formed from the debris of exploded population II stars, population I stars have a considerable content of heavy elements that were created by nucleosynthesisnucleosynthesis
in astronomy, production of all the chemical elements from the simplest element, hydrogen, by thermonuclear reactions within stars, supernovas, and in the big bang at the beginning of the universe (see nucleus; nuclear energy).
..... Click the link for more information. in the interiors of the earlier stars. Population I and population II stars are both found in the spiral galaxies. Population I stars are located in the disk singly and in galactic, or open, star clustersstar cluster,
a group of stars near each other in space and resembling each other in certain characteristics that suggest a common origin for the group. Stars in the same cluster move at the same rate and in the same direction.
..... Click the link for more information. . They are particularly concentrated in the interstellar dust of the spiral arms, where new stars are continually being formed. The very brightest population I stars are not distributed at random, but are grouped in loose associations of several hundred stars that partake in the general galactic rotation and are believed to have a common origin. Population II stars are found both in the spiral arms and in the gas-free and dust-free regions of the spiral galaxies, i.e., the nucleus and the halo of high-velocity stars and globular clusters that surround the disk of the galaxy. Irregular galaxies are predominantly, or sometimes exclusively, composed of population I stars. Elliptical galaxies, which lack spiral arms, are composed almost entirely of population II stars. The categories population I and population II were first introduced by Walter Baade as a result of his studies of the Andromeda GalaxyAndromeda Galaxy,
cataloged as M31 and NGC 224, the closest large galaxy to the Milky Way and the only one visible to the naked eye in the Northern Hemisphere. It is also known as the Great Nebula in Andromeda. It is 2.
..... Click the link for more information. .
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stellar populationsSee population I, population II.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006