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What Is the Interior Structure of the Sun?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

While we have mainly direct means of learning about the outside of a star, we need to get creative to see past the opaque nature of things below the photosphere, or the surface layer of the Sun. To do this, we need indirect tools to come into play, and one of these is helioseismology. It involves noticing the movement of waves from one side of the Sun to the other, noting Doppler shifts as it passes through different density regions as well as changing temperature conditions (Seeds 124-5, Fossat, Hathaway “Surface”).

Helioseismology

We have two different types of waves we study with helioseismology. One are high frequency waves known as pressure waves or p-waves, the other are low frequency waves known as gravity waves or g-waves. P-waves are mainly generated in the convective zone of the Sun and move very fast through it. They therefore don’t reveal much about the deeper layers of the Sun, but they do readily refract off the photosphere and back towards the inside of the Sun, where their path is altered due to increasing temperatures. G-waves do reveal much about the interior but struggle to make it to the surface. So one type is readily available to us while the other is more difficult to spot (Fossat, Hathaway “Surface”).

Layers of the Sun with the different modes moving throughout it.

Layers of the Sun with the different modes moving throughout it.

One way to divine the presence of g-waves is to see their impact on p-waves. It takes a sound wave about 24 minutes to go through one side of the Sun and back again, so any deviations from this could be a sign that g-waves were encountered along the path. By taking these into account, a profile of g-wave activity can be found. Based on these findings, it points to a core rotating once every week, way faster than either the surface polar or equatorial regions (Fossat).

The Core

It is certainly a fascinating discussion to go over how the Sun fuses material, but that is for another article. Instead, we will use some broad strokes here in out discussion of the core. It is the site of hydrogen fusion into helium, whose by products someday make it to the surface of the Sun as light, electrons, and atomic nuclei. The core operates at about 1.5 million K with a density of about 150 grams per cubic centimeter. The core’s outer boundary is considered to be around 175,000 kilometers from the center because this is where fusion essentially ends. Here, the temperature is half that of the center and the density around 20 grams per cubic centimeter (Hathaway “The Solar”).

Some of the interior layers of the Sun.

Some of the interior layers of the Sun.

The Radiative Zone

The next region is known as the radiative zone and goes from 175,000 kilometers (at 20 grams per cubic centimeter, 7 million K) to about 490,000 kilometers (at 0.2 grams per cubic centimeter, 2 million K) from the center. After material is fused inside the Sun, movement of particles occurs in many complex ways. Photons located at the center are gamma rays and carry so much energy that they are constantly absorbed and emitted by free electrons. These constant changes to the photons means that their arrival at the surface is delayed significantly, radiating slowly outward. How slowly? It can take a million years for a photon to escape this region, that’s how often collisions are occurring (Seeds 128, Hathaway “The Solar”).

The Tachocline

Next up is the tachocline, or the interface layer. This acts as a transition zone from the radiative zone to the next layer, a transition from radiative motion being king to convective motion. It is believed that this layer is what gives rise to the Sun’s dynamo, or what powers the magnetic fields of the star. This is because of the massive changes in notion causing magnetic field lines to become twisted and convoluted (Hathaway “The Solar”)..

Demonstration of the convective nature of the Sun.

Demonstration of the convective nature of the Sun.

The Convective Zone

The final interior layer is the convective zone, going from 200,000 kilometers below (at 2 million K) to the surface (5,700 K). After traveling a certain distance from the center, the temperature decreases to the point where a gamma ray photon becomes 2 X-ray photons, and this process goes on and on until we get roughly 1800 visible photons per single gamma ray photon. At this point, we lack sufficient temperatures to radiate as before and instead begin to convect the material via thermal properties. Here we can get some of the heavier ions to grab onto electrons because of these decreased temperatures, meaning we cannot see as easily here (Seeds 128, Hathaway “The Solar”).

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Something Going on Down Below

Sunspots, portions of the Sun’s surface with magnetic activity causing cooling, were the first hint at something complex going on below the surface of the Sun. These magnetic fields go through the Sun, but the Sun’s different layers rotate at different rates. On top of this, different latitudes of the Sun also rotate at different rates. Regions at the equator complete a revolution every 24 days while the polar regions do the same in 30 days. All this differentiated movement leads to interesting behavior, because it is the very movement of the gas generates magnetic fields. The material is highly ionized, conducting electricity very well. And when such a conductor has rapid movement, the dynamo effect transfers some of that energy back to the magnetic fields (Seeds 134, Sharp, Hathaway “The photosphere”

Using helioseismology, scientists can determine that the bottom of the convective zone is the origination of our magnetic fields, based on reduced motion of the material caused by dragging. And the sunspots correspond to N/S pairs of magnetic fields, with the north pole being in one hemisphere and the south pole in the other. This is essentially forming magnetic loops which extend outside of the Sun’s surface. How it all plays together to inform the 11-year cycle remains a mystery, but there is a leading theory (Seeds 134-5, Hathaway “Photospheric features”).

This would be the Babcock Model, where the magnetic fields all tangle up as they move about the Sun. Free electrons make the gas a conductor, freezing the magnetic field at highly conductive places. These places then can migrate about, and with all the different rotations cause the fields to get complicated rather quickly as well as convecting material. This convection can bring magnetic fields to the surface, where they pop up and are seen as sunspots. After years have passed, the fields can get so complex in their knotting that they switch pairings with their neighbors, causing magnetic polarity to flip in the Sun. After two 11-year cycles, the Sun will be back to its original magnetic field configuration (Seeds 135).

Works Cited

Fossat, Eric. “Gravity Waves Detected in Sun’s Interior Reveal Rapidly Rotating Core.” sci.esa.int. ESA, 01 Aug. 2017. Web. 03 Mar. 2022.

Hathaway, David H. “Photospheric Features.” Solarscience.msfc.nasa.gov. NASA, 11 Aug. 2014. Web. 17 Feb. 2022.

---. “Surface Waves and Helioseismology.” Solarscience.msfc.nasa.gov. NASA, 11 Aug. 2014. Web. 03 Mar. 2022.

---. “The Photosphere.” Solarscience.msfc.nasa.gov. NASA, 11 Aug. 2014. Web. 17 Feb. 2022.

---. “The Solar Interior.” Solarscience.msfc.nasa.gov. NASA, 11 Aug. 2014. Web. 03 Mar. 2022.

---. “The Sunspot Cycle.” Solarscience.msfc.nasa.gov. NASA, 15 Mar. 2017. Web. 28 Feb. 2022.

Seeds, Michael A. Horizons. Tenth Edition, Thomson Brooks/Cole, Belmont, CA. 2008. Print. 124-5, 128, 132-5.

Sharp, Tim. “The sun’s atmosphere: Photosphere, chromosphere, and corona.” Space.com. Future US, Inc., 01 Nov. 2017. Web. 17 Feb. 2022.

“SOHO Confirms 36 Year Old Solar Theory.” sci.esa.int. ESA, 17 Apr. 2008. Web. 03 Mar. 2022

© 2022 Leonard Kelley

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