Stars are one of the most difficult places to observe in the universe. Many experiments have been conducted by scientists to develop theories in order to better understand stellar evolution. One of the great equations that solved many problems about the origins of the sun’s energy was Einstein’s famous equation in physics, E=mc2. This relation tells us how much energy is released if a mass m is completely converted into energy. Arthur Eddington proved that if four hydrogen nuclei, or protons, come together to form a helium nucleus, the mass of the helium nucleus is slightly less than the mass of the four protons. The missing mass must be released as energy according to Einstein’s theory. By the end of the 1930s, the rates of the different nuclear reactions which convert hydrogen into Helium were understood and Eddington’s hypothesis was found to be absolutely correct. The energy source of the sun is the nuclear burning of hydrogen into Helium. 
The second development of the last 15 years has been a completely new way of studying the internal structure of the sun and the stars. We are all familiar with sounds which are produced by different musical instruments. These sounds are made up of sound waves of different frequencies which correspond to the natural frequencies of vibration of the instrument. If the sound waves are measured and studied, it is possible to learn a lot about the physical structure of the instrument. In exactly the same way, the physical structure of the sun can be studied when it resonates at its natural frequencies of vibration. Also, there is another way of studying the internal structure of the sun. The speed of sound is related to the ratio of pressure and density at each point in the sun and it is a convenient way of studying the internal structure of the sun. The speed of sound is also closely related to the square root of the temperature of the material of the sun.
