Most stars fall into a categorization class called the main sequence, also known as dwarf stars. In a standard chart plotting star color against magnitude, known as the Hertzsprung-Russell diagram, the main sequence stars form a coherent curve, unlike the other categories — white dwarfs, subgiants, giants, bright giants, and supergiants. Although not usually included on the chart, black holes, which are gravitationally collapsed stars, might be considered points on the chart with zero luminosity and a spectral signature of 0° K.
The reason why the main sequence stars fall on a predictable curve is because their luminosity and spectral signatures are dictated solely by their mass, which ranges from 0.08 to about 158 solar masses. White dwarfs, stars that have exhausted their nuclear fuel, have similar spectral signatures to main sequence stars, but much less luminosity. This is because they do not fuse elements or have an ongoing source of energy — their luminosity and heat is all left-over. Over the course of billions of years, it is expected that white dwarfs will cool and become black dwarfs, or lifeless star hulks. However, no white dwarf has been around long enough for this to happen yet.
Main sequence stars fall into several categories: brown dwarfs, with only around 0.08 solar masses, are basically oversized Jupiters with weak fusion reactions in their cores; red dwarfs are slightly hotter and more energetic, with greater mass; these are followed by yellow dwarfs, very common stars of which our Sun is an example.
When stars burn up all their nuclear fuel in the form of hydrogen, they start fusing helium. Because old stars begin to build up a solid core of fused material, the powerful gravitational forces on the core's perimeter compress together the gas layers above, accelerating fusion and increase a star's luminosity and size. Through this developmental route, dwarf stars become giants. Depending on their mass, they eventually collapse into white dwarfs, neutron stars, or black holes. The more massive stars cause supernovas, which are huge blasts of energy which escape when fusion ceases in the stellar core and the gas layers rub vigorously against another during the final collapse.
