The Sun

     The Sun, approximately 6 billion years old, is the sole source of light and heat for the maintenance of life on Earth. It is a star, a glowing ball of gas held together by its own gravity and powered by nuclear fusion. Nuclear fusion is a process in which two or more atomic nuclei combine and release energy at its center. The Sun is very similar to most other stars considering its physical and chemical properties, and regardless of when and where the other stars were formed. Our knowledge of the cosmos rest squarely on our understanding of solar physics.

      The Sun has a radius larger than 100 times the earth, and a mass that is greater than 300,000 times the Earth's mass. The surface temperature of the Sun is well above the melting point of any known material. When we mention the Sun's surface, it is not a solid surface, nor does it contain any solid material at all. The Sun has several varying layers to it, including the part of the Sun that emits the radiation we see, the photosphere. After the photosphere comes the chromosphere, the transition zone, and finally the solar corona, the thin hot upper atmosphere. At even greater distances away, the solar corona turns into solar wind, which flows in great speed away from the sun, permeating the entire solar system.

      Below the photosphere is the convection zone, a region where the material of the Sun is in constant convective motion. Below the convection zone lies the radiation zone, where solar energy is transported toward the surface by radiation rather than by convection zones. Finally after that lies the central core, which is roughly 200,000km in radius. This is the site of powerful nuclear fusion which is the key to the enormous energy output of the Sun.

    

     As you can see above, the suns surface is not simply glowing and yellow.  There are many imperfections including the dark spots that can sometimes be seen called sunspots.  Sunspots typically measure about 10,000 km across, about the size of earth.  At any given second the Sun may have hundreds of sunspots, and inversely it could have none at all.  Further inspection of sunspots show an umbra, or dark center, surrounded by a grayish penumbra.  Sunspots appear black, but they are in reality only relatively cooler regions of the photospheric gas.  The temperature of the umbra is around 4500 Kelvin, where the background of their surroundings is 5800 Kelvin.  Sunspots were one of the first indications that the Sun does not rotate as a sold body.  Instead, it spins differentially, which means faster at the equator and slower at the poles.  Sunspots are not steady.  Most change their size and shape, and all come and go.  Some spots may last anywhere from 1 to 100 days, and a large group of spots typically last 50 days.  After centuries of observations, it is clear that there is a distinct sunspot cycle.  The solar cycle is 22 years long, with repeating fluctuating sunspot occurrences throughout the years.  Below is a short video of half of the sunspot cycle.