 Here's the Hertzsprung-Russell or HR diagram we covered in the Distance Star segment of the How Far Away Is It? video book. The long diagonal line represents the main sequence for stars in hydrostatic equilibrium burning hydrogen. The lower right red stars are cool low mass stars that are a fraction of the mass of the sun. The middle yellow and orange stars are closer to the mass of the sun, and the upper left blue and white stars are the hot high mass stars, many times more massive than the sun. Protostars that are many times the mass of the sun evolve so rapidly that they show up at the high blue end of the main sequence in a short amount of time. For them, there is no T-Tari phase. T-Tari stars would start here on the diagram. As they stabilize, they shrink in size, increase in temperature, start fusing hydrogen in large quantities, and migrate to the main sequence. The more massive the young stellar object, the higher up on the main sequence the eventual star will land. If we start counting the age of a star from the beginning of the cloud collapse that formed it, we would have stars just reaching the main sequence at about 150 to 200 million years old. How long any particular star currently on the main sequence will remain there depends on how much fuel it started with, how fast it is burning that fuel, and how long ago it started to burn. The mass of a star gives us a measure of how much fuel it started with, and its luminosity gives us a measure of how fast it is consuming this fuel. But before we can make the lifetime calculation, we need to understand how much energy we get from hydrogen when it fuses into helium. Here we are looking at the proton-proton fusion chain, the most common fusion reaction in our sun's core. The mass of helium-4 at the end is a bit smaller than the mass of the four protons at the start. The amount of energy generated is determined by Einstein's equation E equals mc squared. We see that the production of each helium nucleus releases only a small amount of energy. But by measurement, we know that the sun produces 3.9 times 10 to the 26 watts. To produce this amount of energy, it would take a tremendous number of these fusion events every second. We calculate that 613 million metric tons of hydrogen fuse to form 609 million metric tons of helium, converting 4 million metric tons of matter into energy every second. To figure out how long it would take for our sun to burn all the hydrogen it started with, we simply divide the available hydrogen by the amount consumed per second. The sun's mass is 2 times 10 to the 30th kilograms. Fusion is only occurring in the core, which represents about 10% of the sun's mass. We see that once hydrogen burning began, our sun would take 10 billion years before it ran out of fuel. That's the total amount of time the sun will be a main sequence star. At the end, it will expand and cool into a red giant star and consume the earth. Astronomers have empirically found that even though the more massive stars have more hydrogen fuel, a corresponding dramatic increase in luminosity shows that they are consuming this fuel faster, much faster. A small change in mass leads to a small change in the core temperature, but a very large change in the luminosity. For example, stars twice the mass of the sun have over 10 times the luminosity and burn out in under 2 billion years. In the other direction, we see that stars with half the mass of the sun have less than the tenth of the luminosity and remain on the main sequence five times longer. At the extremes, we have theta 1 Orionis C in the trabezian cluster at 33 times the mass of the sun, as luminosity is over 200,000 times greater than the sun and it won't last more than a few million years. At the other end, Wolf 359 is just under a tenth of the mass of the sun and will remain a main sequence star almost 400 times longer than the sun. The dramatically shorter time on the main sequence for higher mass stars and extremely extended time for lower mass stars is due to the proton fusion processes sensitivity to temperature. To understand why this is the case, we need to look at the proton-proton activity in the core one level deeper.