Summarise the following passage into as few key words and phrases as possible.

The Sun, like all stars, releases energy through nuclear fusion reactions in the core. Two isotopes of Hydrogen (Deuterium and Tritium) are forced together under high temperature and pressure. Nuclear fusion occurs to form Helium and a neutron. A tiny amount of mass is converted into a large amount of energy, which is why stars emit a lot of energy. The heat produced in the fusion process makes the star expand through thermal pressure. Gravitational attraction is pulling the mass of the star inwards. In a stable star, thermal expansion and gravitational attraction are equal so the star remains the same size. This balance can change later in a star’s life when a great increase in thermal pressure can cause it to expand.