Today we moved on to the structure and evolution of stars. We discussed the fusion processes inside stars like the Sun, and I explained how the radiation pressure resulting from fusion (hydrogen into helium, primarily) counteracts the gravitational pressure, and so we have a perfect sphere in equilibrium. Stars spend the majority of their lives (around 90%) on the 'main sequence', after their birth in a gas cloud. A star like the Sun is expected to live around 10 billion years, which is how long it takes for the fuel to run out (once most of the hydrogen has been converted to helium in its core). All stars can categorized according to temperature and luminosity, and when plotted will show a fairly tight correlation on the 'Hertzsprung-Russell diagram'.