When the supply of helium runs out, the core will contract again, but since the core has more mass, it will become hot and dense enough to fuse carbon into neon. In fact, when the supply of carbon is used up, other fusion reactions occur, until the core is filled with iron atoms.
When the star runs out of nuclear fuel, it comes to the end of its time on the main sequence. If the star is large enough, it can go through a series of less-efficient nuclear reactions to produce internal heat.
When the protostar starts fusing hydrogen, it enters the "main sequence" phase of its life. Stars on the main sequence are those that are fusing hydrogen into helium in their cores. The radiation and heat from this reaction keep the force of gravity from collapsing the star during this phase of the star's life.
A star's life is a constant struggle against the force of gravity. Gravity constantly works to try and cause the star to collapse. The star's core, however is very hot which creates pressure within the gas. This pressure counteracts the force of gravity, putting the star into what is called hydrostatic equilibrium.
This is why these clouds of material are often called stellar nuseries – they are places where many stars form. As the protostar gains mass , its core gets hotter and more dense.
If the core is larger, it will collapse into a black hole. To turn into a neutron star, a star must start with about 7 to 20 times the mass of the Sun before the supernova. Only stars with more than 20 times the mass of the Sun will become black holes. Updated: February 2014.
Chandra X-ray image of supernova remnant Cassiopeia A. The colors show different wavelengths of X-rays being emitted by the matter that has been ejected from the central star. In the center is a neutron star. (Credit: NASA/CSC/SAO)