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The sun is the only star in the solar system. It accounts for almost 99.8 % of the solar system’s mass. It is composed of about 74% hydrogen (which accounts 92% of its volume), 25% helium (7% of volume), and other elements. It has a surface temperature of 5780 Kelvin, giving a white color (the process called scattering makes the sun appear yellow as viewed from the earth’s surface).

It is classified as a yellow star, orbiting the center of the Milky Way galaxy at an orbital speed of 217 km/s. It is estimated that it will complete its orbit in about 225-250 million years.Generally, the sun has five major parts: 1) the core, 2) the radiation zone, 3) the convection zone, 4) the photosphere, and 5) the atmosphere. The first three are the internal structures of the sun; the second, external/surface structures. The energy derived from this fusion escapes to the radiation zone. It is estimated that the solar core extends from its center to about 0.

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2 solar radii. It has a density of 150, 000 kg/m3 (which is almost 150 times the density of water on earth. It has a temperature (which varied depending on the temperature gradient of sections of the core) close to 13, 600, 000 K (which is considered high when compared to its surface temperature). The solar core rotates at a speed still undetermined by astronomers (although approximations were presented). Nuclear fusion happens in the solar core. Hydrogen is fused to form different isotopes of helium. Nuclear fusion though in the core occurs in a series of steps called p-p (proton-proton) chain. In this process, hydrogen atoms combined to form a specific isotope of helium.

This process produces energy (which is in the form of heat). The rate though of fusion depends on density and temperature. A slight different in the rate of fusion may cause the core to release more energy and therefore expand against the weight of other solar layers.

This stimulates the reduction of the fusion rate and thus returns to its normal condition (self-equilibrium). All the energy travel through successive layers of the sun (heat naturally escapes from a dense material). Some heat escapes to the solar photosphere and then to space.The second and third internal structures of the sun are the radiation and convection zones. The radiation zone ranges from 0.

2 to approximately 0.7 solar radii. This zone has a lower temperature and density than the solar core.

It does not allow nuclear fusion to occur in the early and middle stage of a star’s life cycle. Thermal radiation occurs in the radiation zone. This process allows the transfer of intense heat from the core to the outer layers of the sun.

The heat generated in the core passes to the radiation zone and then to the convection zone. Since the surface density and temperature of the convection zone is insufficient to allow thermal radiation, thermal columns serve as the carrier of hot material to the photosphere. Once the material cools down, it goes back to the convection zone, and is reheated. It then shoots to the sun’s photosphere.There are four ways to measure the density, radius, and the temperature of the sun’s interior. The first is called hydrostatic equilibrium. It is the condition where outward pressure balances inward force.

In the case of the sun, the determination of the approximate value of its inward force led to the determination of the outward force (they are not always equal; variations occur and constants are given). The second is called thermal equilibrium. It is the state where the amount of energy generated equals the amount radiated away. In the case of the sun, the determination of the amount of energy radiated resulted in the determination of the approximate temperature gradient (and density) of the sun’s interior.

The third is opacity. It is the resistance of the solar envelope to the flow of photons. The measure of resistance is equal to the measure of density and temperature gradient of the radiation zone.  The last is called energy transport.

It is the process of transporting energy from the core to the photosphere. The rate of energy transport allows astronomers to determine the rate of resistance in the solar layers, and consequently to its specific temperature and density. ReferenceThe interior of the sun. (2007). Retrieved on October 19, 2007 from http:// http://csep10.phys.utk.edu/astr162/lect/sun/interior.html.Related Links:http://zebu.uoregon.edu/~js/ast121/lectures/lec22.htmlhttp://ircamera.as.arizona.edu/NatSci102/lectures/suninterior.htmhttp://www.windows.ucar.edu/tour/link=/sun/solar_interior_new.html&edu=high

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