http://www.newsweek.com/nuclear-fusion-temperature-core-sun-1771949

Nuclear fusion has been the talk of the town lately because of some milestone breakthroughs bringing us closer to a world with the possibility of infinite free energy.

Nuclear fusion is the process by which two atoms collide and fuse, forming another element. The process requires temperatures of up to 27 million degrees Fahrenheit (15 million Celsius). To put that into perspective, the surface of the sun is 9,941 degrees F. The core temperature of the sun is also around 27 million degrees F, while the corona, the outer part, is 2 million degrees Fahrenheit.