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Today's warm-up: How long would it take for a radio signal to go to the Moon and back?
(distance from Earth to Moon = 3.8*10^8 m).
Answer: about 2.5s

Today: 3 categories of stars:
Main Sequence stars - our sun is a main sequence star, a little larger than average (most stars, by sheer number are very small) - powered by Hydrogen Fusion --> creates Helium. The smallest ones may last 200 billion years, while the largest may only last 200 million years.

Red Dwarfs - from about 50% the mass of our sun and smaller - these appear red because their surfaces are cooler than other stars, about 2000C. These stars have the least Hydrogen (fuel) but last the longest because the low pressures inside them results in slow processes of fusion.

Red Giants - as a main sequence star runs low on Hydrogen, the growing Helium core begins to fuse to create bigger atoms. Helium fusion makes the star's core increase in temperature, and begin to overcome the star's gravitational pull (which had been a delicate balance between gravity and thermal expansion...) When this happens, the star expands to a huge size; sometimes 100x the radius of the previous star. Since the thermal E from the core is spread over such a large area, the temperature on the star's surface is lower and it looks red. Eventually the helium core is left as a neutron star. (our star will share this fate in about 5 billion years)