Ask A Scientist
Why do stars burn?
Asked by: Aaron Woodward
School: Maine-Endwell Middle School
Teacher: Kevin Wagstaff
Hobbies/Interests: Collecting rocks
Career Interest: Scientist
Answer from Stephen L. Levy
Assistant Professor of Physics, Binghamton University
Research area: Biophysics, Microfluidics
Interests/hobbies: Tennis, Ninjago
Family: Married with two young boys, Leo and Max
Stars burn in a different way than what we are used to in our experience with fires. In fact, the word "burn" may be confusing. Let’s say that stars generate heat in a different way than when wood burns in a campfire. Let me first explain both processes, and point out the differences.
In a campfire, the molecules that make up the wood break apart and combine with oxygen in the air to make new molecules, primarily water and carbon dioxide. Breaking and reforming the chemical bonds that hold the molecules together causes the water and carbon dioxide molecules to move faster. The faster motion means that the temperature gets hotter. And hot objects glow, emitting "electromagnetic radiation" (in physics-talk).
On the other hand, stars make heat in the absence of oxygen by combining protons (stable, positively charged subatomic particles) together to form heavier nuclei (the positively charges mass within an atom) and gamma rays (electromagnetic radiation of extremely high frequency), in a complicated chain of reactions that depends on the star’s size. This process is called fusion. Again, the heavier nuclei created by the fusion process move faster, heating up the surrounding particles. In that sense, both the chemical reactions in a fire and the nuclear reactions in a star, cause an increase in the temperature by producing particles that are moving faster. The major difference is that molecules in a fire are rearranged based on electrical forces, while protons in a star fuse together based on the nuclear, or strong, force.
You might then wonder why fusion happens inside a star and not in any container of gas. The electrical force between two protons tries to force them apart, like if you try to hold two north poles of a magnet together. But if the protons are moving at a very high rate of speed, they can become so close to one another that the nuclear force takes over, fusing them together. This happens in the interior of a star where the protons are moving very fast, as a result of the gravitational energy released during the star’s formation. Researchers at the National Ignition Facility in California are trying to start a laboratory fusion reaction by hitting a small target of protons with 192 intense laser beams at once. They hope that laboratory ignited fusion will someday provide a safe and unlimited energy source for the world.