 In our segment on star birth nebula, we'll cover how stars form from giant hydrogen clouds collapsing under the force of gravity. They start shining once the pressure and temperature at the core reaches the level needed to support hydrogen fusion. The fusion of hydrogen and helium converts some of the mass into energy. And because E equals MC squared, and C is a very big number, the process generates a great deal of energy. The more hydrogen there is in the collapsing cloud, the more massive the star. The more massive the star, the more intense the pressure in its core. The more intense the pressure, the higher the temperature. The higher the temperature, the greater the star's luminosity. Thus, the diagonal line on the HR diagram represents the main sequence for stars burning hydrogen. The upper left blue and white hot stars are high mass stars, many times more massive than the sun. The middle region yellow and orange stars are closer to the mass of the sun. The lower right stars are cool, low mass stars that are a fraction of the mass of the sun. When a star runs out of hydrogen fuel, the core contracts and gets hotter. This heat expands the outer layers, reducing the density and turning the star red. The star moves off the main sequence and enters the realm of giants or super giants depending on their original mass. When a red giant with the mass less than five times the mass of the sun runs out of the fuel, it explodes and leaves behind a dim hot tiny star called a white dwarf. We'll discuss these processes more in our segment on planetary nebula. We'll cover the end game for more massive stars in our segment on supernova. Now that we understand the meaning of the HR diagram, let's see how we can use it to find out how far away a star is. For that, we need to view light as a particle and examine its spectrum.