Stars start out as gas particles in a nebula. Under gravity these particles come together. As the temperature rises the star begins to shine called a protostar. All stars progress through a main sequence. Toward the end of a stars life it will grow to be a supergiant. Depending the the available mass of the star there are a number of outcomes.
Supergiant stars may contain multiple energy sources within shells. Hydrogen in the outer shell burns into the heavier element Helium giving off heat energy and light. The Helium in the inner shell burns into Carbon and Oxygen. The star is getting hotter towards the core. Ultimately the star may contain heavy metals like Iron and Nickle at the core.
Part of the talk on Constellations Stars and Galaxies constellations stars and galaxies .
A wonderful illustrated dictionary of Astronomy terms can be found here.
