The Sun

Our source of heat and energy

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Layers of the Sun

The first layer, working our way outward, is the core. The radioactive zone, the the convective zone, then there is the visible surface known as the photosphere,

the chromosphere, and finally the outermost layer, the corona.

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The Core

The core is the source of all the Sun's energy. Fortunately for life on earth, the Sun's energy output is just about constant so we do not see much change in its brightness or the heat it gives off. The Sun's core has a very high temperature, more than 15 million degrees Kelvin, and the material in the core is very tightly packed or dense. It is a combination of these two properties that creates an environment just right for nuclear reactions to occur.

In the core of a star the intense heat destroys the internal structure of an atom and consequently all atoms are broken down into their constituent parts. An atom is constructed of protons, electrons and neutrons. Neutrons have no electric charge and therefore do not interact much with the surrounding medium. As a result neutrons leave the core fairly quickly. The protons, which have positive electric charge, and the electrons, which have negative electric charge, remain in the core and drive the reactions which fuel the Sun. The charge neutral material of protons and electrons that makes up the core is called plasma.

The high temperature provides the protons and electrons with a large amount of thermal energy and as a result they move around quite quickly. This motion, combined with the high density of the plasma, causes the particles to continuously slam into one another creating nuclear reactions. It is the fusion, or slamming together, of particular combinations of particles that provides the energy source of the Sun.

The Radiation

After energy is created in the core and then it needs a way to travel from the solar center to the outer regions. In the radioactive zone, energy generated by nuclear fusion in the core moves outward as electromagnetic radiation. In other words, the energy is conveyed by photons. When the energy reaches the top of the radioactive zone, it begins to move in a different fashion in the convective zone. In the convective zone, heat and energy are carried outward along with matter in swirling flows called convection cells. This motion is similar to the roiling flows seen in a pot of boiling water. The radioactive layer moves differently than the outer layers. Many other stars also have radiative zones. The Sun's radioactive zone extends from the core outward to about 70% of the Sun's radius. In a smaller (than the Sun) star that is cooler than our Sun, the convective zone tends to be larger, extending deeper into the star's interior. Thus the radiative zone tends to be smaller. In very small, cool stars the convective zone may reach all the way to the star's core, and there may be no radiative zone at all. In a larger, than the Sun, star with a higher temperature, the radiative zone tends to be larger and the convective zone smaller. Especially large, hot stars may not have a convective zone at all - their radiative zone may extend all the way from the core to the star's surface.

The Convection Zone

The convection zone is the outer-most layer of the solar interior. It extends from a depth of about 200,000 km right up to the visible surface. At the base of the convection zone the temperature is about 2,000,000° C. This is "cool" enough for the heavier ions (such as carbon, nitrogen, oxygen, calcium, and iron) to hold onto some of their electrons. This makes the material more opaque so that it is harder for radiation to get through. This traps heat that ultimately makes the fluid unstable and it starts to "boil" or convect.

Convection occurs when the temperature gradient (the rate at which the temperature falls with height or radius) gets larger than the adiabatic gradient (the rate at which the temperature would fall if a volume of material were moved higher without adding heat). Where this occurs a volume of material moved upward will be warmer than its surroundings and will continue to rise further. These convective motions carry heat quite rapidly to the surface. The fluid expands and cools as it rises. At the visible surface the temperature has dropped to 5,700 K and the density is only 0.0000002 gm/cm³ (about 1/10,000th the density of air at sea level). The convective motions themselves are visible at the surface as granules and supergranules.

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The photosphere is the visible surface of the Sun that we are most familiar with. Since the Sun is a ball of gas, this is not a solid surface but is actually a layer about 100 km thick (very, very, thin compared to the 700,000 km radius of the Sun)
The photosphere is the visible surface of the Sun that we are most familiar with. Since the Sun is a ball of gas, this is not a solid surface but is actually a layer about 100 km thick (very, very, thin compared to the 700,000 km radius of the Sun).
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At other times, light from the chromosphere is usually too weak to be seen against the brighter photosphere. The third layer of the sun's atmosphere is the corona. It can only be seen during a total solar eclipse as well. The chromosphere is 2000-3000 km thick. It glows faintly relative to the photosphere and can only be seen easily in a total solar eclipse. When it can be seen it is reddish in color. The faint flow of the chromosphere is due to an emission spectrum from hot, low density gases emitting at discrete wavelengths. The discovery of helium noted earlier was from emission lines seen in the chromosphere during an eclipse in 1868. This new element was only found on the Earth in 1895.

The Corona

The corona is an aura of plasma that surrounds the sun and other celestial bodies. The Sun's corona extends millions of kilometers into space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. The glow of the corona is a million times less bright than that of the photosphere, so it can only be seen when the disk of the Sun is blocked off in a total solar eclipse (adjacent image), or by using a special instrument called a coronagraph (or coronameter) that artificially blocks the disk of the Sun so that it can image the regions surrounding the Sun.
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Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They correspond to concentrations of magnetic field flux that inhibit convection and result in reduced surface temperature compared to the surrounding photosphere.


A prominence is a large, bright, gaseous feature extending outward from the Sun's surface, often in a loop shape. Prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the Sun's corona. Prominences can loop hundreds of thousands of miles into space. Prominences are held above the Sun's surface by strong magnetic fields and can last for many months. At some time in their existence, most prominences will erupt, spewing enormous amounts of solar material into space.
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Solar Flares

A solar flare is a magnetic storm on the Sun which appears to be a very bright spot and a gaseous surface eruption. Solar flares release huge amounts of high-energy particles and gases and are tremendously hot (from 3.6 million to 24 million °F). They are ejected thousands of miles from the surface of the Sun.

Solar flares were first observed by in 1859 by Lord Richard C. Carrington. He wrote that as he was watching the sun with a telescope, he saw "two patches of intensely bright and white light" near a huge group of sunspots. Just a few seconds later, the flare has disappeared.

It has been recently discovered that solar flares can cause sunquakes. Sunquakes are violent seismic events on the Sun. When a sunquake occurs, energy is released in seismic waves on the relatively fluid surface of the Sun. These waves radiate in concentric circles from the epicenter of the sunquake. These seismic waves seem to be compression waves (perhaps like "P" waves generated by earthquakes). Sunquakes would rate about 11.3 on the Richter scale. These huge quakes release about 40,000 times more energy than the 1906 San Francisco earthquake.

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he Aurora is an incredible light show caused by collisions between electrically charged particles released from the sun that enter the earth's atmosphere and collide with gases such as oxygen and nitrogen. The lights are seen around the magnetic poles of the northern and southern hemispheres.