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How does the ratio of the amounts of hydrogen to helium in a star affect what kind of star it is and what kind of light it gives off?

Stars behave like blackbodies. This means that they radiate light at all colors, and that their brightness and color depend only on their temperature. So a hot star appears blue and bright, while a cool star appears red and dim.

The centers of stars are so hot that atoms are split into nuclei and electrons. These free electrons prevent the light produced by the nuclear reactions inside a star from escaping easily. Instead, the light slowly works its way out towards the surface, where we can see it.

A helium atom has two electrons and a nucleus with two protons and two neutrons. A hydrogen atom has one electron and only one proton. Now suppose you replaced one atom of hydrogen in the center of the star wih an atom of helium. You've added more mass than electrons. So now there are less electrons for the amount of mass, making it easier for the light to escape to the surface of the star. Therefore, a helium-rich star is brighter than a hydrogen rich star of the same mass, and also hotter and bluer since stars behave like blackbodies.

Submitted by Deborah (Maryland, USA)
(March 4, 1998)

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA