The Sun 1.Size and Structure 1.1.Sun's Properties 1.2.The Solar Interior 1.3.Energy in the Sun 1.4.The Solar Atmosphere 2.How the Sun Works 2.1.What Powers the Sun? 2.1.1.Nuclear Fusion 3.Probing the Sun's Core 3.1.Solar Seismology 4.Magnetism in the Sun 4.1.Sunspots, Chromosphere and Corona, Solar Wind 5.Solar Cycle 5.1.Cause, Changes in the Cycle, Relationship to Terrestrial Climate

Properties of the Sun
 Radius 7 × 108 km Mass 2 × 1030 kg Distance 1.5 × 108 km (1 Astronomical Unit) Surface Temperature 5780 Kelvin (9900 degrees Farenheight) Core Temperature 15 million Kelvin (27 million degrees Farenheight) Power Output 4 × 1026 watts

The Sun is a G2 (on a scale of O, B, A, F, G, K, M - O being the brightest) low mass star in the center of our Solar System measuring a distance of about 150 million kilometers away from us. Astronomers originally used triangulation, a method that accounts the size of an angle to create a triangle from opposite sides of Earth directed to the Sun. With the technology we have now, we can use radar signals to bounce off the Sun and measure the time it takes them to arrive back home. Now that we know the Sun's distance, astronomers can find its radius by angular size (more of that geometry you hate). Suppose you take a compass and widen it to fit an object in front of you, you realize that the further you're away from it, the smaller the angle it creates. Next comes measuring mass with the help of the equation M = v²r/G where G is constant, v equals Earth's orbital velocity, and r is for the value of radius (Earth's distance from the center of its orbital circle). The Sun contains more than 99.8% of the total mass of our Solar System. Surface Temperature is the temperature of the photosphere, the visible surface of the Sun. To measure this, scientists have made a relationship between the color of an object and temperature. The yellowish color means its temperature is around 6000 K. Its core temperature (15 million K) is measured indirectly using calculations such as the radius and surface temperature. The high temperature means that the Sun is mostly gaseous vaporizing its elements. The power output is the measure of how much solar energy we get on Earth. Obviously, the energy will run out because the Sun will use up its hydrogen fuel in about 10 billion years. Remember that energy is never lost or created but conserved in different forms as stated in the Conservation of Energy Theory. When astronomers study the sun, they use special equipment to block harmful rays that's why they say never look directly at the sun. The intense heat of the sun's rays can destroy the retinal cells, causing blindness.
The Sun has a low density outer atmosphere but in the inside is highly dense because of all the weight of the gas above it. This compression causes the atoms to come closer to each other, blocking any light that tries to pass through. It acts as an insulator to store heat and consume less energy to release sunshine. This visible layer is called the photosphere. The core has a density of about 100g/cm³ which heats up because of the small empty volume. The atoms travel faster and collide more often, therefore they exert great outward pressure against the inward gravitational force that tries to crush the Sun. The rates of inward forces and outward forces are balanced and this is what astronomers call hydrostatic balance. It is all related by the ideal gas law where PV = nRT. P is pressure, V is volume, n is the number of moles of a substance, R is a constant, T is temperature. You can see the indirect relationship P has with V. If V increases then P decreases and vice versa. If P increases then T increases and vice versa.
The heat must rise from the core to the surface because it has a property that makes it travel from hot to cold. Think when you boil water that the water evaporates and the heat rises into the warmer air and eventually to the Earth's atmosphere. There are two processes that occur in the Sun that help the heat escape. In the inner, there is radiation where energy moves by photons, but it is so dense that once a photon moves it gets absorbed by an atom then remitted and so on. They are stuck in traffic, which dramatically slows them down, decreasing the speed of light. In fact the light we get now was born in the core a million years earlier! The next process energy goes through is called convection. Matter is heated up which makes it rise to the surface to radiate their heat then when they cool down they sink back down again and so on. When looking at the photosphere, you can see hot specs surrounded by cool specs, called granulation.
Above the photosphere lies the tenuous atmosphere called the chromosphere then even further away is the extremely tenuous atmosphere called the corona that blends in with interplanetary space. The chromosphere decreases in temperature to about 4500 K, but the corona's temperature reaches several million degrees Kelvin. This remains a mystery for astronomers but they believe the Sun's magnetic field plays a role. The corona is so hot is probably because of it's low mass (low density) that the waves that travel from the convection zone rise to the corona and make the particles move much faster with such low mass. The faster they are, the hotter they get and that's why the corona reaches a million degrees Kelvin. The chromosphere contains millions of spicules, thin columns where jets of hot gas spit out. If you took a prism and intercepted the Sun's light, it would display a dark absorption spectrum because of the hot dense gas through cooler gas. A dark absorption spectrum has dark vertical lines because the atoms absorbed the light. Depending on which atoms, affects the pattern of the dark lines. This is how we figure out what elements the Sun is composed of. It turns out that 71% is hydrogen, the fuel for the sun, 27% helium, the by-product of the fusing of the fuel, and 2% heavier elements, by-products of the fusing of helium and others. The corona also contains spicules but they are called coronal holes through which gas may escape from the Sun into space. The corona consists of little energy because of its low density despite the high temperatures. Imagine sparks of fireworks hitting your hand. You hardly feel them because they are so small in size but carry high temperatures.
The Sun's energy escapes the core as light and heat. Where does the energy come from? If you ever heard of Einstein's equation E=mc², then you are half way taught. If mass is large then the energy will too. The letter c is a constant equal to the speed of light, 3.0 × 108. Therefore mass can generate energy. Thermonuclear fusion is the process of combining atoms, specifically in the Sun they are hydrogen atoms. Fusion can take place inside the Sun because its core is so hot. As we mentioned before, the Sun contains about 70% hydrogen, of which about 27% have fused into helium atoms. Two hydrogen atoms cannot just combine because of the electrical repulsion they have against one another. Remember that in electromagnetic forces, like charges repel and unlike charges attract each other. At such a collision, hydrogen can be brought together so that the electrical repulsion between their protons is overwhelmed by another force called nuclear, or strong force. In order to achieve such collisions, temperature must be high to make atoms travel fast enough. Hydrogen is an atom which consists of one proton and one electron. Helium contains 2 protons and 2 electrons. Combining two hydrogen protons make a hydrogen with two protons, a positron (positive electron), a neutrino (neutron with possibly no mass), and energy.

1H + 1H --> 2H + e+ + v + Energy

If you add up the product mass, it will total up to something less than the reactants. This is where Einstein's equation comes in. The missing mass was converted into generated energy. The product 2H is an isotope of hydrogen, meaning it differs in neutrons to the actual hydrogen with no neutrons. The v is the neutrino. In the second step, two different isotopes of hydrogen collide to form a hydrogen with 3 protons plus a high energy photon as a gamma ray (very short wavelength of the electromagnetic spectrum) and again energy from the missing mass.

1H + 2H --> 3H + y + Energy

The third and final step is when two 3H fuse together. They produce a 4H plus two "normal" hydrogen atoms and again energy from missing mass.

3H + 3H --> 4H + 1H + 1H + Energy

Since six hydrogen atoms are required to return a helium atom, however, two come back, therefore to fuse together a helium atom, it requires a net of four hydrogen atoms. Adding all the energy adds up to the total energy released by exploding 100 billion-megaton H-bombs per second! All this for the sunshine we get here on Earth.
Astronomers can study how many neutrinos are made in order to find out the number of reactions that go on in the Sun. Another helpful tool to studying the interior by analyzing waves in the Sun's atmosphere is called solar seismology. The rising and falling surface gas occurs in a regular pattern which can be detected as a Doppler shift of the moving material. They then use computer models of the Sun to predict how the observed surface waves are affected by conditions in the Sun's deep interior.
Sunspots are the most common type of solar magnetic activity. They are cool regions on the Sun measuring to about 4500 K, about 1500 K less from the surface temperature. What makes them cool are the strong magnetic fields. The magnetic field of sunspots is more than a thousand times stronger than the Earth's or the "normal" field of the Sun. Electrons and other charged particles spiral around the field, "frozen" to it. On Earth, our magnetic field drives particles to the poles to create the auroras. The magnetic field slows the ascent of hot gas from the convection zone which therefore starves the sunspots into cool areas. Prominences and flares are magnetic disturbances in the low-density hot gas of the Sun's atmosphere. Prominences are arcs of glowing gas from the chromosphere into the much hotter corona. They are best seen at the edge of the sun during an eclipse. They occur on sunspots because the cool region of sunspots permit inward low pressure and therefore explodes with a prominence because of the unbalance. The hotter regions of the Sun have such high inward pressure that they hold together the gas and prevent it from exploding. These sunspots also give birth to solar flares which are prominences that escape from the Sun's magnetic field. The corona can get so hot that its atoms will also escape into space in what we call the solar wind. When the solar wind hit our Earth, our magnetic field travels them to the poles and you see the beautiful aurora. You might respond that the wind should be attracted to the Sun's gravity but in fact, high temperatures increase pressure because the atoms collide more as proven by the ideal gas law equation, and therefore this outward pressure overcomes gravity.
The Sun rotates at its equator in about 25 Earth days and in the poles 30 days. As you can see, gas near the equator rotates faster than gas near the poles. This is a property of all gaseous bodies such as Jupiter and the other outer planets. The gas near the equator drags the rest behind it. Differential rotation should similarly distort the Sun's magnetic field. The magnetic field winds up as the Sun rotates faster near the equator. The subsurface magnetic field begins to coil up on itself. Once it's in coils, kinks will form and break through the surface and cause prominences to develop. Each kink breaks the surface in pairs: one where it leaves and the other where it enters the subsurface again. Each of the pair have a polarity of which are reversed on the Southern Hemisphere. Sunspot and flare activity change from year to year according to the solar cycle. In this case, when years pass, the polarity is reversed for all prominences. The cycle averages to about 22 years in which on average every 11 years the polarity is exchanged. It therefore takes an average of 22 years to return to its normal configuration. Climatologists found out that there is a possibility that terrestrial climate is affected approximately every 22 years, similar to the Sun's solar cycle. They say the solar wind released from the corona by the magnetic field causes droughts in the Midwestern United States and Canada. The stream steers storms and hence rainfall. Also the ocean is affect by the number of sunspots that appear on the photosphere. Where there is a minimum of spots, the temperature of the ocean is colder than average.

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