What keeps the nuclear reactions in a star under control?

"No star lives forever, but some have shone for 10 billion years and will continue to shine for a billion years more. All stars are fired by the same mechanism: nuclear reactions in the core" (Dickinson 1999).

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Nuclear reactions in a star don't stay under control, not forever, so the mechanics of stellar equilibrium are temporary. It wasn't until Albert Einstein came along with his famous equation that people began to know how stars work. What "e equals mc squared" showed was that if mass could be converted to energy, an enormous amount of energy would be produced; enough energy to keep stars burning for a very long time.

Stars form from interstellar molecular gas clouds which undergo gravitational collapse until the core becomes dense (pressure) and hot (temperature) enough--a temperature of 14 million degrees—to ignite the hydrogen (which makes up 90% of the gas cloud). The heat creates pressure which pushes back on the gravity and counteracts it. The pressure comes from both the motion of the particles (mostly) and the photons being emitted. "The ignition is not a chemical reaction like in a car engine -- it's the initiation of nuclear fusion. This is the process in which hydrogen converts into helium with a slight loss of mass and the subsequent creation of energy (via Einstein's equation E = mc², where E is energy, m is the mass lost, and c is the speed of light)" (Kaler, 2004).

Thus, fusion describes the conversion of mass into energy. The conversion of hydrogen to helium is called "hydrogen burning". The mass which is lost is released as energy. "After a tortuous trek lasting up to a million years, the core-generated energy works its way up to the surface and is radiated into space, mostly as light" (Terence Dickinson. The Universe and Beyond. (1999). New York, Firefly Books).

Stars remain stars out of a balance between gravity and pressure. The bigger the star, the higher the temperature in the middle has to be; the hotter the temperature, the faster the nuclear processes happen. "The maximum mass seems to be around 100 to 120 solar masses. If a star more massive than this should form, it would be ultraluminous, shining a million times brighter than the Sun, and would tear itself apart by the pressure of its own powerful radiation. The lower limit of a real star is 0.08 solar mass. Below this limit, the central pressure -- and therefore temperature -is too low to ignite the gas" (Kaler, 2004).

When the star has used up all of the hydrogen at its core, nuclear fusion stops, and the star changes form and eventually "dies."

References

• Dickinson, Terence. (1999). The Universe and Beyond. New York: Firefly Books.
• Kaler, Jim. Ask Astro. (2004). Astronomy, 32, Issue 1