The Sun is called Sol in Latin and that is where the term Solar System comes from. It is a typical main-sequence star and is by far the largest and most massive object in our Solar System. The Sun contains 99.8 percent of all the matter in the Solar System with the planet Jupiter taking up most of the rest. The Sun is a population one, GV2 class star and is sometimes referred to as a typical star and that is true in many respects. However the Sun is actually larger than most of the other stars in the same class as the Sun. The Sun is composed of 74 percent hydrogen, 25 percent helium and traces of other elements. The temperature at the Suns core which is considered the inner 20 percent, is approximately 15.6 million degrees Kelvin, the pressure is 250 billion atmospheres and the mass density is more than 150 times that of water. Under such extreme conditions nuclear fusion takes place where by hydrogen atoms are combined to from helium atoms. This reaction releases massive amounts of energy in the form of gamma rays and is responsible for the Suns 386 billion billion megawatt power output. During the course of their journey out to the surface these gamma rays are repeatedly absorbed and re-emitted at lower and lower temperatures. By the time the energy reaches the surface is has been reduced to mostly visible light and is carried through the last part of the way more by convection than radiation. The convection zone is the Suns outer layer down to about 70 percent of the Suns radius. It is an area where thermal convection takes place in the form of great thermal columns. These thermal columns are heated by nuclear fusion taking place in the Suns core and rise up to the Suns surface where they release their energy out into space in the form of sunlight and particles. As the thermal columns discharge their energy they cool and sink back down in the Suns interior where they are reheated and back up to surface again in a great cycle. The tops of these great thermal columns can be seen on the surface of the Sun in the form of what is called solar granulation and supergranulation. The surface layer of the Sun that we can see is called the photosphere and has a temperature of about 5800 degrees Kelvin. Above the photosphere are five layers that compose the Suns atmosphere. They are the temperature minimum, the chromosphere, the transition region, the corona and the heliosphere. The temperature minimum region extends from the photosphere up to 2000 kilometers and has a temperature of about 4000 degrees Kelvin. This is cool enough for molecules such as water and carbon monoxide to exist and they can be detected by their absorption spectra. The chromosphere extends from the top of the temperature minimum region up another 2000 kilometers and is named for the Greek word chromo which means color. The chromosphere can be seen as a flash of color right at the start and the end of a total solar eclipse of the Sun. Strangely enough, the temperature of the chromosphere gradually increases with altitude up to about 100,000 degrees Kelvin at the top. Above the chromosphere is a transition region where the temperature rises rapidly to about one million degrees Kelvin. This temperature increase is caused by what is known as a phase transition of the element helium present in the transition region. The transition region does not have a well defined altitude and is in constant motion. The transition region is not easily seen from Earth but can be observed by space based instruments operating in the far ultraviolet region of the spectrum. After the chromosphere is the corona which is much larger than the previous layers of the Suns atmosphere and extends far out into space. The corona is characterized by solar prominences which are immense clouds of super heated glowing gas that has erupted from the upper chromosphere. The corona can be clearly seen during total eclipses of the Sun and is very spectacular to see. The corona is composed of charged particles that become what we call the solar wind as they radiate outward from the Sun at 450 kilometers per second and are responsible for the aurora borealis. Beyond the corona is the heliosphere which is also know as the magnetosphere. The heliosphere is immensely strong and extends far beyond the orbit of the dwarf planet Pluto. The solar wind travels outward along the heliosphere until it collides with the helipads about 50 astronomical units from the Sun. When observed with the proper filters we can see sunspots on the surface of the Sun. These spots have a lower temperature than the surrounding area and therefore appear dark. Sunspots are areas of intense magnetic power where thermal convection from the interior of the Sun has been inhibited. Sunspots usually form pairs with opposite magnetic polarity and are responsible for solar flares. The number of sunspots varies over the course of an eleven year solar cycle. The Sun has been active for about four and a half billion years and has used up about half of the hydrogen fuel it started with. The Sun will continue to burn for about another five billion years after which it will start to force helium to under go nuclear fusion into heavier elements. This will cause the Sun to swell up in size to the point of consuming the Earth and more as it becomes what is called a red giant. A billion years after becoming a red giant our Sun will finally collapse into a white dwarf. Incredibly, it could then take as much as one trillion years to cool off.
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