astlhrd - Laboratory 7 Hertzsprung-Russell Diagram...

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48 Laboratory 7 - Hertzsprung-Russell Diagram Materials Used: Scaling transparency, photographs of the Trapezium and Pleiades star clusters, Excel spreadsheet. Objectives: To investigate Hertzsprung-Russell diagrams; to estimate the ages of stars from the HR diagram; to study the evolution of stars and star clusters. Discussion: Hertzsprung-Russell (HR) diagrams are plots of stars by l u m i n o s i t y (intrinsic brightness) versus s p e c t r a l c l a s s (color). The same distribution may be achieved by plotting l u m i n o s i t y or a b s o l u t e m a g n i t u de versus s p e c t r a l c l a s s or c o l o r i n d e x or s u r f a c e t e m p e r a t u r e . HR diagrams are very useful for determining the age of a group of stars. A HR diagram of most random groups of stars will display three groups: red giant stars, main sequence stars and white dwarf stars. Figure 1 is a depiction of a HR diagram. The largest group of stars in any HR diagram is the m a i n s e q u e n c e . The main sequence is common to all HR diagrams and is the longest stage of evolution for any active star. A star on the main sequence derives its energy almost entirely from a nuclear reaction involving the conversion of hydrogen to helium via fusion. A star spends most of its active life on the main sequence. White Dwarfs Red Giants Main Sequence Spectral Class Temperature Blue Red cool hot O B A F G K M Luminosity dim bright Figure 1. Hertzsprung-Russell diagram.
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49 The HR diagram reveals that bright main sequence stars are also very hot. The hotter main sequence stars are also bluer and more massive. The cooler, dimmer main sequence stars are redder and less massive. Although it is not obvious from the HR diagram itself, the hotter, brighter, more massive and bluer a star is the less time it spends on the main sequence. It is important to note that s t a r s d o n o t m o v e u p a n d d o w n t h e m a i n s e q u e n c e . The position of a star on the main sequence is uniquely determined by its mass. When stars leave the main sequence they do so by moving off to the side. High and low mass stars follow very different evolutionary paths upon leaving the main sequence (although both go through a Red Giant phase). Few high mass stars are found on a typical HR diagram anywhere except in the main sequence and red giant regions. The Red Giant 1 region of the HR diagram is populated by a variety of former main sequence stars. When the hydrogen gas is nearly exhausted in the core of a main sequence star the core begins to cool. As the core cools it exerts less pressure against the enormous mass of the star surrounding it and begins to contract. This gravitational compression of the core increases pressure - causing it to heat up once again (just like squeezing a balloon cause the air inside to heat up). Heat
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