How the Sun Releases Energy?

The spectacular picture shows the sun's granular surface, a few surface flares, and an amazing edge prominence called the “Handle”. The sun's surface temperature is 5,800 Kelvin or about 10,000 °F. But... the temperature of its core is 15,700,000 Kelvin! The sun's diameter is 865,000 miles, which is 109 times that of the Earth. Its surface area is approximately 12,000 times that of the Earth. The sun's mass is 333,000 times that of Earth and is about 99.9% of the total mass of our whole solar system. 73% of the sun's mass is hydrogen while 25% is helium. The 2% rest consists of heavier elements: oxygen, carbon, iron and a smattering of other elements.

Our Sun appears orange/ yellow when viewed from Earth, but is actually white due to atmospheric scattering. The sun is about 93 million miles on average from Earth's orbit, which is known as one Astronomical Unit (1 AU). It takes light 8 minutes and 19 seconds to travel this distance. The sun revolves around the galactic center of the Milky Way at a radius of approximately 26,000 light years. It completes its orbit once every 235 million years. The sun's orbital velocity with respect to the Cosmic Microwave Background (CMB) is about 828,000 miles per hour. Because the sun exists in a plasma state and behaves like a heavy fluid (not a solid) it rotates faster at its equator than at its poles. This is known as differential rotation (see Convection Zone below), and is caused by fluid rotational differences in the sun's interior due to steep temperature gradients from its core outwards. The sun's period of rotation is 25.6 days at the equator and 33.5 days at the poles. However, due to our constantly changing vantage point from the Earth as it orbits the sun, the apparent rotation of the sun at its equator is about 28 days. How is solar energy released? This article will explore the composition of the sun.

The Sun's Regions
The sun does not have a definite boundary as rocky planets do. In its outer parts, the density of its gases drops exponentially with increasing distance from its center. Nevertheless, it has a well-defined interior structure. The Sun's radius is measured from its “Core” to the edge of the “Photosphere”. This is the layer above which gases are too cool to radiate light and is, therefore, the surface visible to the naked eye.

The sketch to the right illustrates the various sections of the sun's interior and some exterior features. These are discussed below:

1. Core
2. Radiative Zone
3. Convection Zone
4. Photosphere
5. Chromosphere
6. Corona
7. Sunspots
8. Granules
9. Prominence

Core
The Core of the sun is considered to extend from the center to about 25% of the solar radius. It has a density of about 150 times the density of water. The Core is the only section of the sun that produces heat through fusion. The temperature is 15,700,000 Kelvin! The rest of the sun is heated by energy that is transferred outward from the Core. The energy produced by fusion in the Core must travel through successive layers to the Photosphere before it escapes into space as sunlight.

Radiative Zone
In the Radiative Zone, from 25% to 70% of the solar radius, the Radiative material is hot and dense enough that thermal radiation (not fusion) transfers the intense heat of the Core outward. Heat is transferred by photon radiation. Very hot ions of hydrogen and helium emit photons which are absorbed in only a few millimeters of solar plasma and then are re-emitted again in random directions. This random radiation process takes a very long time for photons to reach the sun's surface as sunlight. Estimates of the “photon travel time” range from 10,000 to 170,000 years! The plasma density drops a hundredfold from the bottom to the top of the Radiative Zone. Between the Radiative Zone and the Convection Zone is a very narrow transition layer called the Tachocline. The Radiative Zone rotates like a normal solid body. The Tachocline is a region between the uniform solid rotation of the Radiative Zone and the conventional fluid rotation of the Convection Zone. The Tachocline's plasma rotation rate changes very rapidly causing an extreme shear - a situation where successive horizontal layers slide past one another.

Convection Zone
The Convection Zone rotates as a normal fluid with “differential rotation”. That is, the plasma at the poles rotates slowly (33.5 days) with the convection current speeds constantly increasing until they reach the plasma equator which rotates much faster (25.6 days). In the Convection layer, from its surface down 30% of the solar radius, the Convection plasma is not dense or hot enough to transfer the heat energy of the interior outward through radiation. As a result, thermal convection occurs as thermal columns carry hot material to the Photosphere surface of the sun. Once the material cools off at the surface, it plunges downward to the base of the Convective Zone, to absorb more heat from the top of the Radiative Zone and then repeats the cycle. These thermal columns in the Convection Zone form an imprint on the surface of the sun and are called Solar Granules. In addition, there are “rivers” of plasma flowing from the poles to the equator deep in the Convection Zone then up to the surface and back to the poles.

Photosphere
The visible surface of the sun is the layer below which the sun becomes opaque. Above the Photosphere, sunlight is free to propagate into space and its energy escapes the sun entirely. The visible light we see is produced as electrons react with hydrogen atoms to produce hydrogen ions. The Photosphere is hundreds of kilometers thick. Because the upper part of the Photosphere is cooler than the lower part, an image of the sun appears brighter in the center than on the edge.A nice 46% of the radiation is in the visible range. Another 49% is in the infrared range which we feel as heat. The remaining 5% is in the ultraviolet range which tans the skin. During early studies of the photosphere, some absorption lines were found in the solar spectrum that did not correspond to any chemicals then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were a new element which he dubbed “helium”, after the Greek Sun God Helios. It was 25 years later that helium was isolated on Earth.

Chromosphere
The Chromosphere is a layer of hot gases about 2,500 kilometers thick. The Chromosphere cannot normally be seen because it is washed out by the overwhelming brightness of the Photosphere. However, the remarkable picture of the Chromosphere on the left was taken by Luc Viatour of France during the 1999 total eclipse of the sun at just the right moment. During eclipses of the sun, the Chromosphere can be seen by the naked eye. The temperature in the Chromosphere “increases” gradually, ranging from 4,000 Kelvin at its bottom to 20,000 Kelvin at the top.

Corona
The Corona is the outer atmosphere of the Sun which is extremely large. In the lower part of the Corona is a thin “Transition Layer” (about 120 miles thick) in which the temperatures rise from 20,000 K at the bottom of the Transition Layer to temperatures of 1,000,000 Kelvin and above! How this happens is a solar mystery. The average temperature of the Corona is 1,000,000 to 2,000,000 Kelvin. However, in the hottest regions, it is 8,000,000 to an unbelievable 20,000,000 Kelvin! The Corona continuously extends into outer space forming the Solar Wind.

Sunspots
Sunspots are temporary phenomena on the surface of the Photosphere that appear as dark spots compared to the surrounding regions. They are caused by intense magnetic activity, which inhibits convection, forming areas of lower surface temperatures. If a Sunspot were isolated from its surrounding Photosphere, it would be brighter than an electric arc. Sunspots expand and contract as they move across the surface of the sun.

They can be as large as 50,000 miles in diameter making the larger ones visible from Earth. To the left are three sunspots in the sun's northern hemisphere as seen on July 7, 2011. 

Granules
Solar Granules are very hot thermal columns formed in the Convection Zone which rise to the surface of the Photosphere, cool down, and then plunge back down to the base of the Convetion Zone, receive more heat from the Radiative Zone, then cycle up and down again.

The grainy appearance of the Photosphere is produced by the tops of these Convection cells. A typical Granule has a diameter of about 600 miles and lasts only 8 to 20 minutes before dissipating. Just below the Photosphere is a layer of "Super Granules", up to 20,000 miles in diameter whose life span is up to 24 hours.

Prominence
Solar Prominences rise up through the Chromosphere from the Photosphere, sometimes reaching altitudes of 100,000 miles. These gigantic plumes of gases, often in a loop shape, are the most spectacular of the solar phenomena. The Prominence at the left was recorded in April 2010.

While the Corona consists of extremely hot ionized gases that do not emit much visible light, Prominences contain much cooler plasma which emits quite a bit of light. The mass within a Prominence is typically on the order of 100 billion tons. A Prominence forms in about a day and if stable can persist in the Corona for several months.

Some Prominences break apart and morph into Coronal Mass Ejections (CMEs). Scientists are currently researching how Prominences and CMEs are formed and ejected. It is believed that they are caused by intense magnetic activity beneath the surface of the Photosphere. 

Coronal Loops
A coronal loop is a loop of magnetic flux filled with plasma anchored at both ends into the Photosphere and extending well into the Corona. Coronal loops come in many sizes. Some are quite small extending only into the Chromosphere, 1,500 miles. Others extend well into the Corona and can be as high as 100,000 miles. 

One can mentally picture that underneath a coronal loop is a huge horseshoe magnet with a north and a south pole. The magnetic flux extends from one pole up into the Corona and then back down into the other pole. However, the coronal loops are dynamic. They can last anywhere from a day to months. They also get pushed around by the flow of the plasma. The number of coronal loops is directly linked to the solar cycle, i.e. when the sun is very active there are many coronal loops and vice versa. Coronal loops are often found with sunspots within their larger footprint.

Coronal loops have a wide variance in their energy and temperatures. Those under one million degrees Kelvin (1 MK) are referred to as cool loops. Those around 1Mk are warm loops and those over 1Mk are called hot loops. The different categories produce radiation of different wavelengths.

How Do the Coronal Magnetic Loop Fields Form? Just beneath the surface of the Photosphere, there are small-scale magnetic fields forming all the time. These local fields form a thick carpet-like surface of revolving plasma currents circulating in small vertical loops as the heat from the radiation zone makes its way to the surface through the convection zone. See the sun cutaway chart below this section showing the revolving vertical loops in the convection zone.

Separately there are also large currents flowing from the poles deep underneath the surface to the equator, then up to the surface and back again to the poles, referred to as the sun's Conveyor Belts. In addition, the convection zone is rotating, but in a “differential” way. Meaning the plasma at the equator is rotating faster than the plasma at the poles thus causing many shears in the east-west flow from the equator to the poles.

So we have three conflicting plasma flows: vertical loops from the bottom of the convection floor to the surface, north-south conveyor belts, and east-west rotational shears. This results in the various magnetic fields becoming tangled and twisted causing the build-up of stored magnetic energy. At some point there is a release of this magnetic energy as the magnetic fields re-configure themselves into a lower energy state and the excess energy is converted into kinetic and thermal energy. This results in an ejection of plasma into the corona as the plasma follows the new magnetic field lines. Some ejections are bigger than others causing the coronal loops to be of different sizes.

At the poles a slightly different phenomenon takes place. Instead of the loops being closed, most of them are open for an extended period of time. In this case, the plasma is shot out into outer space and forms the "fast" Solar Wind. The Solar Wind has two components - the “slow” wind and the “fast” wind. The slow Solar Wind travels about 400 km/sec. whereas the fast wind travels about 750 km/sec. The slow Solar Wind appears to originate from a region around the sun's equatorial belt that is known as the “streamer belt”. The fast Solar Wind originates from “coronal holes” which are funnel-like regions of open field lines at the sun's poles.  

How Exactly Does the Sun Radiate Energy?
First a quick review of how energy traverses the sun's regions. Nuclear fusion takes place in the sun's core. The energy then moves by photon radiation through the radiation zone (no fusion) to the convection zone. The energy in the form of heat then moves by convection to the surface. Convection is the flow of heat through a fluid, in this case, plasma. (Convection does not occur in solids.) Convection takes place in one of two ways: By the random interaction of high energy (heated) particles (Brownian Motion) and by the flow of heated currents in the fluid plasma. Once the energy reaches the sun's surface it is mainly transmitted by rays (photons) and the Solar Wind (particles) to the rest of the heliosphere.
What exactly is fusion? Fusion is a process whereby 4 hydrogen nuclei at very high temperatures and pressure are burned into one helium nucleus plus some other particles and a lot of energy. The 4 hydrogen nuclei weigh more than the single helium nucleus and other particles. The balance of lost weight (mass) is converted into energy. In summary, the fusion process yields a helium nucleus, which contains 2 protons and 2 neutrons, plus 2 electrons plus 2 neutrinos and a lot of energy (photons). Because of Einstein's equation E=mc², a small amount of mass gets multiplied by the square of the speed of light (a very large number) resulting in a huge amount of energy being released from a small amount of mass (similar to a nuclear bomb).

A huge number of neutrinos are also released by the fusion process. However, neutrinos rarely interact with other bits of matter. About a hundred billion neutrinos pass through your fingernails every second and we are completely unaware of them and they do no harm. A solar neutrino passing completely through the earth has about one chance in a thousand billion of being stopped by another particle. However, using 100,000 gallons of special fluids and very delicate sensing devices, neutrinos have been detected and measured. This experimental technique was used to verify the theory of nuclear fusion as the engine of energy in the core of our sun and other stars.

How Long Will the Sun Last?
The sun is estimated by two different methodologies to be 4.6 billion years old. The sun will last about 10 billion years as a “main sequence” star. The main sequence means that by nuclear fusion it converts hydrogen into helium plus some neutrinos and radiation. The sun does not have enough mass to explode like a “super-nova”. In about 5 billion years the sun will enter a “red giant” phase. As the hydrogen in the core is consumed, the core will begin to contract and heat up. “Helium fusion” will then begin and the sun's outer layers will expand significantly. Following the red giant phase, the sun will eject its outer layers forming a “planetary nebula”, which is a large glowing shell of ionized gases. After all the outer layers have been ejected, that which remains will be the extremely hot “white core”. The white core will slowly cool and fade away as a “white dwarf” over many billions of years. This is the typical end of a medium-sized star.