Nuclear Fusion in Stars

In the previous article, we discussed what nuclear fusion is and learned that this complicated chemical process can only occur under a very extreme set of conditions. To the average person, it may seem that nuclear fusion can only occur in a laboratory setting, where ideal conditions are created and maintained by scientists. However, that is not the case. Believe it or not, nuclear fusion is one of the more common chemical reactions, as it occurs naturally in stars throughout the universe.


Our sun, like all stars, fuses elements together to create denser elements, in doing so creating enough energy to maintain the star’s high temperature. If a certain temperature (100 million Kelvins) is not maintained, the sun will collapse in on itself. Fusion is possible in the sun (and any star for that matter) only because of the star’s immense mass, leading to an astronomically large gravitational field. In our solar system, the gravitational field is responsible for the aligned orbits of the planets. The size of the star causes extremely high pressure and temperature within its core, where large amounts of hydrogen are present (  In these conditions, hydrogen atoms can overcome the electromagnetic repulsion between themselves, and undergo nuclear fusion. The magnitude of the repelling force increases with the electrical charges on the two nuclei. To keep this force small therefore, the interacting nuclei should have the lowest possible charge(or atomic number).

Fusion primarily takes place during the main sequence of a star. The main sequence is the time in which the star turns all of its available hydrogen into helium, which in turn produces light and heat energy. The exact length of the main-sequence stage depends on the star’s mass: the lower the mass of the star, the longer it takes to burn up all its hydrogen, chiefly due to the relationship between mass and gravity. The main sequence length depends on the mass of the star in question. For example, our sun is about halfway through its 10 billion year main sequence. ( During this stage, the star is in hydrostatic equilibrium. This means that the gravity of the star and the opposing thermal pressure created by fusion are pushing against each other equally, creating stability within the star. Hydrostatic equilibrium is the pressure gradient force that also prevents gravity from collapsing theEarth’s atmosphere into a thin, dense shell, while gravity prevents the pressure gradient force from diffusing the atmosphere into space. 

Now that we have talked about what fusion is along with where and why it occurs naturally in the universe, in our next post, we will discuss how this complicated chemical process has already been used (on Earth), so keep on reading!

(pictures from


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