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1. Why and How Stars Evolve
Stars are self-luminous gaseous spheres. They shine by generating their own energy and radiating it off into space.
The stars' fuel for energy generation is the stuff they are made of -- hydrogen, helium, carbon, etc. -- which they burn by converting these elements into heavier elements. "Burning" in this context does not refer to the kind of burning we are familiar with, such as the burning of wood or coal, which is chemical burning. It refers to nuclear burning, in which the nuclei of atoms fuse into nuclei of heavier atoms.
Since stars are of finite size, they will eventually use up their nuclear fuel and run out of energy.
Using up their nuclear fuel -- changing nuclei of one kind into nuclei of another kind -- means that stars change and evolve.
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The energy the Sun radiates into space is generated deep in our star's interior by nuclear burning. In contrast, the radiation we receive from the planets and the Moon is not generated by these bodies. It is reflected sunlight. This image of the Sun was taken in X-rays by the Japanese Earth-orbiting spacecraft Yohkoh. Image Credit: Lockheed-Martin Solar and Astrophysics Laboratory/Yohkoh Public Outreach Program. |
