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'We Don't Planet' Episode 1: What's the Structure of the Sun?

SDO View of the Sun, August 2016
NASA's Solar Dynamics Observatory captured this view of the sun on Aug. 11, 2016.
(Image: © NASA/SDO)

The interior of the sun has layers like a very, very hot onion. In the core, the incredible gravitational pressures overwhelm the usual electric repulsion of protons, shoving hydrogen atoms together to form helium. The newly forged helium atoms have slightly less mass than the original hydrogen, and that little bit of mass gets released as highly energetic bits of light, or photons.

Just outside the fusion core, the massive number of photons dominates the physics: energy from the core is transported outwards directly by the radiation itself — hence the name "radiative zone."

Farther out, however, the density of photons drops and less exotic energy transfers take over. In this convective zone, huge plumes of plasma are heated from below, become buoyant, and rise higher up. Like a vast pot of boiling water, when the plumes reach the surface they cool, increase in density, and slip back down, only for the process to be repeated again. This process breaks the surface of the sun into Earth-sized convection cells, visible through specialized telescopes.

It’s at this surface, the photosphere, where light can travel freely and escape into space, but it’s not the edge of the sun. Beyond this layer sits the corona, a think tenuous plasma that stretches almost twice the radius of the photosphere. The corona is incredibly hot — over 1 million degrees Kelvin — but so thin that you could travel through it without notice. Usually the glow of the corona is observed by the intensity of the photosphere, but it can be seen during solar eclipses, or with the aid of artificial coronagraphs.

