Astro2110 – The Solar System

The sun is the greatest, most massive, hottest thing in our world. But how does it even work?? I’m gonna be totally honest, I’m not a big science guy so I’ve never really had the change to answer this question. But Chapter 14 gives us an in depth explanation of how the sun functions through the process of nuclear fusion.

When the sun was born 4.5 billion years ago from a collapsing cloud of interstellar gas, a contraction of the cloud released enough gravitational potential energy to raise the interior temperature and pressure of the sun. This process of gravitational contraction is similar to how a shrinking gas cloud heats up because gravitational potential energy of the gas particles far from the center of the cloud is converted to thermal energy as gas moves inward. The process continued until the core finally became hot enough to sustain nuclear fusion! At that point, the Sun produced enough energy to give it the stability it has today. Through this process of nuclear fusion, the sun converts mass into energy. This is based off of Einstein’s theory of relativity, which is that mass itself contains more than enough energy to account for billions of years of sunshine, so the sun shines if it can convert some of its mass into thermal energy. The sun was born with enough fusion to last about 10 billion years, so since we are about halfway through that point, in 5 billion years when the sun exhausts its nuclear fuel, the gravitational contractions will begin again and the cycle will repeat.

The actual process of nuclear fusion on an atomic level can be thought of as the combining or fusing of two or more small nuclei into a larger one. This is the opposite of nuclear fission, which is the process of splitting an atomic nucleus. Nuclear fusion happens when nuclei are colliding with enough sufficient energy that they can bind protons and neutrons into an atomic nuclei (so that it can overcome the electromagnetic repulsion between the positively charges nuclei). It’s important for the temperature to be high so that the nuclei collide at very high speeds to fuse. The process of proton-proton fusion is what mostly occurs in the sun. This is when two protons fuse to form a nucleus consisting of one proton and one neutron. The nuclei collides and fuses with a proton, resulting in a nucleus of Helium-3 (a rare helium with 2 protons and one neutron) and a gamma-ray photon. Then, another neutron is added to H3 to make H4, which occurs through the collision of two H3 nuclei. The result is a normal H4 nucleus and 2 protons. This fusion of hydrogen into helium generates energy because helium nucleus has a mass slightly less than the combined mass of four Hydrogen nuclei. Overtime, the total number of independent particles in the solar core gradually decreases and this gradual reduction causes the solar core to shrink. Further, the gradual increase in core temperature and fusion rate keeps the core pressure high enough to counteract the stronger compression of gravity.
For the energy produced by fusion to actually escape the sun’s core, it moves slowly through the radiation zone through randomly bouncing photons, gets scurried upward by convection in the convection zone, and then moves through the photosphere at the top of the convection zone, where density of gas is low enough that photons escape to space. The energy built up from long before in the solar core finally emerges from the sun as thermal radiation, which is how the sun shines!

Video Source: How The Sun Shines