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Why is it called a red giant?

  1. Dec 8, 2004 #1
    A star which radiates as a blackbody has an increasing temperature almost it's entire life. If it begins as a huge, hot blue-white star, why is it that it's called a red giant when the hydrogen runs out? If the temperature is higher, it should be radiating mostly in the ultraviolet region of the spectrum, not the other way around, right?

    Would appreciate any answers, thanks!
  2. jcsd
  3. Dec 8, 2004 #2


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    This page will explain it better than I could, but the fact is that as a star enters this phase, it becomes very large, and the radiation becomes less-energetic (shifted toward the red end of the spectrum), thus "red giant".

  4. Dec 8, 2004 #3
    Thanks mate.

    If I understood it correctly, the radiation generated from the proton-fusing shell surrounding the alpha-particle core, has less energy when it reaches the photosphere than when protons are fused in the core.
  5. Dec 8, 2004 #4


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    Yes, the proton-fusing shell gives a new energy source to the star, and this radiative pressure reverses the collapse of the star and causes the outer regions of the star to be pushed out (thus the giant). The radiation loses energy traversing this extra thickness of star-stuff, resulting in longer wavelength emissions (thus the red). When our sun does this, the Earth will have had to suffer a long period of declining radiation (icy planet syndrome), only to be toasted like a marshmallow when the proton-fusing shell lights off and causes the sun to swell.
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