How Does The Sun Produce Energy

by Jerry Coffey on October 15, 2010

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How Does The Sun Produce Energy

Image Credit: NASA

How does the Sun produce energy? The Sun produces energy by the nuclear fusion of hydrogen into helium in its core. What that means is that, since there is a huge amount of hydrogen in the core, these atoms stick together and fuse into a helium atom. This energy is then radiated out from the core and moves across the solar system. It is not enough to just answer the question, so here is a breakdown of how the energy is transferred from the core to the Earth and the other objects in our solar system.

The Sun’s extends from the center to about 0.25 of the solar radius. It has a density of 150 g/cm3 and a temperature of close to 13,600,000 K. Energy is produced by nuclear fusion during a series of steps called the proton-proton(P-P) chain, converting hydrogen to helium. The core is the only part of the Sun that produces an appreciable amount of heat through fusion(99%). The rest of the star is heated by energy that is transferred outward from the core and the layers just outside. The energy must then travel through many layers to the solar photosphere before it escapes into space as Kinetic energy(sunlight in this case). The P-P chain occurs around 9.2×1037 times each second. Fusing hydrogen into helium releases around 0.7% of the fused mass as energy, so the Sun releases energy at the mass-energy conversion rate of 4.26 million metric tons per second.

The next layer of the Sun is the radiative zone. Here solar material is hot and dense enough that thermal radiation is all that is needed to transfer the intense heat outward. There is no thermal convection. The material grows cooler as altitude increases. The temperature gradient is less than the adiabatic lapse rate, so it cannot drive convection. Heat is transferred because ions of hydrogen and helium emit photons that travel a small distance and then are reabsorbed.

Next comes the convective zone. Here the solar plasma is not dense enough or hot enough to transfer the heat of the interior through radiation. Thermal convection occurs here as thermal columns carry hot material to the next layer, the photosphere. Once the material cools off in the photosphere, it plunges back to the base of the convection zone and receives more heat from the top of the radiative zone. At the surface of the Sun, the temperature has dropped to 5,700 K. The turbulent convection of this layer of the Sun causes an effect that produces magnetic north and south poles all over the surface of the Sun.

Lastly, the photosphere, the visible surface of the Sun, is where visible sunlight is free to propagate into space(move into space). The energy then washes across the surface or atmosphere of the bodies in the solar system. Here on Earth the atmosphere filters some of the UV rays but passes a portion of that energy. The energy bounces off of the surface and is then reflected back by the atmosphere. After this bounce the Earth absorbs some of the energy and our planet is heated. How’s that for an answer to ‘how does the Sun produce energy?’

We have written many articles about solar energy for Universe Today. Here’s an article about solar energy, and here’s an article about harvesting solar power from space.

If you’d like more info on the Sun, check out NASA’s Solar System Exploration Guide on the Sun, and here’s a link to the SOHO mission homepage, which has the latest images from the Sun.

We’ve also recorded an episode of Astronomy Cast all about the Sun. Listen here, Episode 30: The Sun, Spots and All.

NASA Sun Earth Day: Solar Energy

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