stellar populations, two broadly contrasting distributions of star types that are characteristic of different parts of a galaxy. 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 diagram. 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 nucleosynthesis 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 clusters. 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 Galaxy.
"stellar populations." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (November 21, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/stellar-populations
"stellar populations." The Columbia Encyclopedia, 6th ed.. . Retrieved November 21, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/stellar-populations
Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).
Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.
Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:
Modern Language Association
The Chicago Manual of Style
American Psychological Association
- Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
- In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.