I was recently reading a on a physics website (http://www.telegraph.co.uk/news/science/) an article revealing the actual reason why the sun is hot. Most people, like I, would think it is because of the energy dissipated from the collision of billions and billions of hydrogen and helium atoms. It seems like straight forward mechanics, the particles collide and dissipate a certain amount of heat energy due to the collision and the addition of the billions of collisions that happen every second makes the sun hot. In actuality, however, the sun is hot because of its immense weight. Its weight creates a gravitational force toward the core of the sun increasing the pressure. The increased pressure is what increases the temperature of the sun. It is, however, still important that the sun is made up of hydrogen and helium because without fusion occurring, the sun would have been initially hot a very long time ago, but then would have rapidly cooled. Instead the sun keeps burning for billions and billions of years due to fusion.