What is the fuel that runs fusion in stars?

Stars radiate energy produced due to the fusion reactions
that are going on within them. The fusion reactions in stars start with hydrogen as a
fuel; two atoms of hydrogen fuse together to form a helium
atom.

Stars like the Sun have enough hydrogen to continue
the hydrogen fusion reaction for around a hundred million years. Once a sufficient
amount of hydrogen has been converted to helium there are other reactions that can place
in stars which are dependent on the initial mass of the star. If a star were large
enough when it was formed, the temperature in the star would be high enough to allow the
fusion of helium to result in elements with a higher mass like nitrogen and carbon.
These can undergo fusion themselves and with atoms of lighter elements to produce
heavier elements. The process continues till finally iron is formed as a result of the
fusion reaction. Iron cannot undergo fusion reactions as the energy generated due to
that is less than the energy required for the fusion reaction to start. This makes the
reaction unsustainable.

The fuel for fusion reactions in
stars is initially hydrogen and as heavier elements are formed they can undergo fusion
reactions too until only iron is left in the star.