Stars shine by converting chemical elements into heavier elements through a process called nuclear fusion. This process only takes place under incredibly high pressure and at extremely high temperatures. Under these circumstances, the electrons surrounding an atomic nucleus can be stripped away, and the nuclei forced together.
Stars are thought to condense from huge clouds of gas in the vacuum of space. Generally, these clouds of gas can be regarded as debris left over from the Big Bang, when temperatures in space were a good deal hotter than the current, chilly four degrees above absolute zero . As the expanding universe cooled, most of the protons and electrons formed in the Big Bang paired up to form hydrogen atoms. These atoms consist, surprisingly enough, of a single proton being orbited by an electron.
Gradually, under the influence of their mutual gravity, these clouds shrank. Depending upon the mass of the clouds, various interesting things started to happen. One feature of all the contracting clouds was that they got hotter; gravitational contraction crammed the hydrogen atoms into an increasingly smaller volume, and lowering the volume increases pressure and temperature. This caused the clouds to heat up.
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