Of course not. Before the discovery of nuclear fusion though, scientists did not know the source of the Sun's energy. In the nineteenth century, coal was the main fuel used in energy production on Earth. Not knowing the source of the Sun's energy (and some of them perhaps still believing that the moon was made of cheese), they asked: could coal be the source of the Suns's energy?
They could estimate out the mass of the Sun and the power production of the Sun by other means. They estimated the mass of the Sun to be
and the power or luminosity of the Sun to be
1 Kg of coal contains between 13 – 30 MJ of chemical energy.
If the Sun were made of coal it would contain up to
of chemical energy.
Using
we can find the expected lifetime of the Sun.
There are
in a year so the expected lifetime of the Sun is
Another possible source of the Sun's energy of the energy released by gravitational contraction as the Sun is assembled. This turns out to be much smaller than the energy available were the Sun made of coal.