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Stellar components can dominate in UV

Stellar components can dominate in UV - Stellar components...

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Stellar components can dominate in UV, visible, or far-IR (through dust emission) depending on the galaxy's history and structure. X-ray dominance and significant radio emission are produced by active nuclei. The stellar content is fundamental to understanding a galaxy's history, given the interplay of stellar populations with interstellar matter and dynamics. Baade recognized a basic distinction between various parts of Local Group galaxies, in whether the brightest individually resolved stars are blue or red . The key papers are ApJ 100, 137 (1944) on M31 and M32, and ApJ 100, 147 (1944) on NGC 147 and 185; these included actual second-generation photographic prints from Mt. Wilson material because printing wasn't capable of preserving the faint stellar images adequately. The differences between these populations may be understood by considering the H- R diagrams of young (less than 10 9 years) and old (say 10 10 years) groups of stars. The young stars will have few red giants, and a main sequence reaching to blue colors and high temperatures, while the older population's brightest members will be red giants and the main sequence will be truncated at cooler temperatures (lower masses, redder colors) as stellar evolution eats away down the mass sequence. This figure (from Baade 1944 on M31/M32, courtesy the AAS) shows the difference.
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The population designations are chosen to make the Sun Population I. The best-studied Pop II
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