Our Sun is presently in the most stable and long-lasting phase of its life.
The Sun - in the prime of its life.
Source: SOHO (ESA & NASA)
A delicate balance has been established with the gas pressure of the star preventing gravity from collapsing the star in on itself. The pressure of the gas is maintained by the energy (heat) produced by the nuclear fusion reactions inside the core of the Sun. But the nuclear fusion reactions can't continue forever.
Eventually all the hydrogen in the core of the Sun will be turned into helium and the nuclear reactions will stop. At this point the star loses its energy source and gravity wins, overcoming the gas pressure of the star to cause the core of the star to start contracting.
The collapsing core generates its own heat which raises the temperature of the outer layers of the star so that nuclear fusion reactions now start outside of the stellar core. These reactions add more heat to the star and to get rid of this heat, the star expands and glows a red colour.
The stellar core continues to contract until it becomes hot enough to begin a new cycle of nuclear fusion reactions. This time helium is converted to carbon and oxygen. The star is now called a red giant and its decline into old age has begun.