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 overall brightness as well as their apparent color depend only on their temperature. So a hot star appears blue and bright, while a cool star appears red and relatively dim. Not all objects behave this way. The neon gas in a neon lightbulb radiates only the familiar red color we see, not all colors, and its color doesn't tell us anything about its temperature.
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 diffuses out towards the surface, where we can see it. The light is scattered by the free electrons inside the star. The number of electrons per unit mass (per kilogram, for example) determines the amount of scattering.
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 four times the mass, but only twice as many electrons. So the number of electrons per unit mass has decreased, 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)