An average star is a star predicted or observed to go through the main sequence life cycle: nebula to main sequence star to red giant to white dwarf to black dwarf. Stars with more mass explode into a supernova and collapse into a neutron star or black hole.
Stars are constantly balancing gravity and thermal expansion. Thermal expansion occurs with the fusion of hydrogen atoms into helium atoms in the star. When hydrogen runs out, the star collapses and re-expands into a red giant that burns helium. If the star is at least five times as massive as the sun, this process repeats for heavier elements, such as carbon, silicon and iron. Iron is the last element to be fused; the fusion reaction for iron takes more energy than it releases.
If the star doesn't have enough internal gravity to counter thermal expansion, the first red giant phase causes it to throw off its diffuse outer layers. More mass causes it to collapse. In extreme cases, the gravity of the star creates heat and pressure, in turn causing a supernova. The remaining core of the star converts to superdense neutrons that may further collapse into a black hole. This is an object so dense that it breaks through the fabric of spacetime and has an escape velocity greater than the speed of light.