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Fusion in the Sun

Masters (er…Matter) of the Universe

Hydrogen is the most common element in the universe since 90% of all matter exists in the form of hydrogen.  Helium constitutes the other 10% of matter, which is created from hydrogen through nuclear fusion reactions in the core of the star.  It is the fuel of the stars as well as the element from which all other elements are created.

Hydrogen contains one proton in the nucleus and one electron, with this it is the simplest atom.  Hydrogen has three isotopes, the common hydrogenwith one proton in the nucleus, deuterium or heavy hydrogen with one proton and one neutron and tritium with one proton and two neutrons; a radioactive gas. 

During the Big Bang hydrogen was created as the first element, after that helium was the result of hydrogen fusion.  All other elements still are created inside the nucleus of the stars (from hydrogen and helium), by nuclear fusion reactions.  These elements are released when the stars explode at the end of their life cycle.


Fusion in Stars

Inside the nucleus of stars, gases are highly compressed by their gravitational force.  Because of this, the density as well as the temperature (more than ten million Kelvin) in the nucleus is so high that nuclear fusion reaction takes place.  Hydrogen, as deuterium and tritium atoms, fuse to helium, herematter disappears and enormous quantities of energy are released (E=mc2). This energy transports from the nucleus to the surface of the star, where it escapes as light and heat (electromagnetic radiation). 

All stars have a steady state period during their life cycle, in which phase they transform hydrogen to helium. This process converts the energy produced by fusion to light (in the form of photons) and heat.

The Random Walk
A photon is generated at the center of a star and makes its way to the surface. It may take up to several million years to get to the surface, and the form of the energy may change from X-ray to visible wavelengths.  Most electromagnetic energy gets out of the Sun in a very round about way which depends on random motion. The path a photon takes is unpredictable but we can gain an understanding through models of how it works.

The photons that we see as sunlight actually took a long time to leave the interior of the Sun. They did what physicists call a random walk. It's kind of like trying to make your way through a crowded subway station. You head off in one direction, get bounced in another direction, then still another, and so on. The problem is that, unlike the person in a subway station, the photons (particles of light) don't know which way they want to go. They are jostled about in scattering collisions with particles (mostly electrons). It's like being blindfolded in a crowded subway station. Eventually you will work your way to the door by chance and leave. But how many steps will it take you to go a certain distance?

When the photon leaves the Sun, it takes 8 minutes to get to Earth at the speed of light. 


A Simplified Fusion Demo



Direct link to video:



It is recommended that you complete this chart for your notebook. It is a not a graded assignment. 

Summarize "Matter in the Universe"        Summarize "Fusion in Stars"     Summarize "Random Walk  "
 Hydrogen and Fusion video summary       
How is the Light Stick Demo similar to fusion in stars?                                                                           How is the Light Stick Demo not similar to fusion in stars?