The Hertzsprung-Russell diagram is a scatter plot that helps illustrate the life cycle of various sizes of stars. It plots four main characteristics: color, absolute magnitude, luminosity and temperature.
The H-R diagram comes in two types: theoretical, which uses absolute magnitude and temperature; and observational, which uses color and luminosity. Many stars on the H-R diagram fall into the main sequence that all dwarf stars such as the sun follow. They start on the upper-left side of the diagram, which is for hot, bright stars, but gradually cool off. Eventually, dwarf stars expand when all their hydrogen is fused into helium. Once stars begin fusing helium, lithium, carbon and heavier elements, they create more heat and overcome their gravitational pull, but they eventually lose the ability to generate heat.
The H-R diagram classifies stars into five types: white dwarfs, main sequence stars, subgiants, giants and supergiants. All stars eventually leave the main sequence, with the exact process depending on their mass. For example, larger stars explode into a supernova, leaving a dense core that collapses into a neutron star or black hole. Smaller stars go red giant, then collapse into a white dwarf, which gradually burns out. White dwarf stars lack luminosity because of their small size, but they generate a lot of heat from previously being the core of a star that went supernova.