The sun is called Sol in Latin, and it is here that the term "solar system" occurs. This is a typical main sequence star and today is the largest and most massive object in our solar system. The sun contains 99.8 percent of all matter in the solar system, and the planet Jupiter occupies most of the rest. The sun is a population, a star of the GV2 class, and is sometimes referred to as a typical star, and this is true in many respects. However, the Sun is actually larger than most other stars of the same class as the Sun.
The sun consists of 74 percent hydrogen, 25 percent helium, and traces of other elements. The temperature in the core of the Sun, which is considered to be internal 20%, is approximately 15.6 million degrees Celsius, the pressure is 250 billion atmospheres, and the mass density is more than 150 times the density of water. In such extreme conditions, nuclear fusion occurs where hydrogen atoms combine with helium atoms. This reaction emits a huge amount of energy in the form of gamma rays and is responsible for the output power of solar energy in the amount of 386 billion billion megawatts. During their flight to the surface, these gamma rays are repeatedly absorbed and reemitted at lower and lower temperatures. By the time energy reaches the surface, it comes down to visible light and is transported through the last part of the path by more convection than radiation.
The convection zone is the outer layer of the sun to about 70% of the radius of the sun. This is the area where thermal convection occurs in the form of large thermal columns. These thermal columns are heated by nuclear fusion occurring in the core of the Sun, and rise to the surface of the Sun, where they release their energy into space in the form of sunlight and particles. As thermal columns unload their energy, they cool and descend back into the interior of the Sun, where they heat up again and rise to the surface again in a large cycle. The tops of these great thermal columns can be seen on the surface of the Sun in the form of so-called solar granulation and supergranulation.
The surface layer of the Sun, which we see, is called the photosphere and has a temperature of about 5800 degrees Kelvin. Above the photosphere are five layers that make up the atmosphere of the sun. These are the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere. The temperature range of the minimum extends from the photosphere to 2,000 kilometers and has a temperature of about 4,000 degrees Kelvin. It is cold enough that molecules, such as water and carbon monoxide, exist, and can be detected by their absorption spectra.
The chromosphere extends from the upper limit of the temperature region to another 2,000 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 beginning and at the end of a total solar eclipse of the sun. Oddly enough, the temperature of the chromosphere gradually increases with altitude up to 100,000 degrees Kelvin at the top.
Above the chromosphere is a transition region where the temperature quickly rises to about a million degrees Kelvin. This increase in temperature is due to the so-called phase transition of elemental helium present in the transition region. The transition region does not have a well-defined height and is in constant motion. The transition region is difficult to see from Earth, but it can be observed with the help of space instruments operating in the far ultraviolet region of the spectrum.
After the chromosphere is a corona, which is much larger than the previous layers of the Sun’s atmosphere, and extends far into space. The corona is characterized by solar protuberances, which are huge clouds of superheated glazing gas that intercepts the upper chromosphere. The crown can be clearly seen during total eclipses of the Sun and very impressive to see. The crown consists of charged particles, which become what we call the solar wind, when they radiate outward from the Sun at a speed of 450 kilometers per second and are responsible for the aurora.
Behind the crown is the heliosphere, which is also known as the magnetosphere. The heliosphere is very strong and extends far beyond the orbit of the dwarf planet Pluto. The solar wind moves through the heliosphere until it collides with helicopter areas of about 50 astronomical units from the sun.
When observed with the appropriate filters, we see sunspots on the surface of the sun. These spots have a lower temperature than the surrounding area, and therefore become dark. Sunspots are areas of intense magnetic force where thermal convection from inside the Sun has been blocked. Sunspots usually form pairs with opposite magnetic polarity and are responsible for solar flares. The number of sunspots varies during the eleven-year solar cycle.
The sun was active for about four and a half billion years and consumed about half of the hydrogen fuel from which it started. The sun will continue to burn for about five billion years, after which it will begin to force helium to switch to nuclear fusion into heavier elements. This will lead to the fact that the Sun grows in size to the point of consumption of the Earth and more, because it becomes what is called the red giant. A billion years after we became a red giant, our Sun will finally collapse into a white dwarf. Incredibly, it may take as many as three trillion years to cool down.
