Nuclear Fusion

For he light nuclei to come together with in a distance of 1 fm (femtometer), we need a temperature of the order of 10^9 K. At that temperature electrons are completely detached from atoms and only nuclei remain. It is called plasma. In Sun, the temperature is 1.5*10^7 K and fusion is taking place. So fusion can take place due to barrier penetration process at 10^7 K.