Field
Stars
The diagram that sorted them, and the one quantity that decides a star's whole life.
The diagram that sorted the stars, by plotting two things against each other
Plot brightness against colour for a few thousand stars and they do not scatter. They fall on a narrow band with two islands off it, and explaining that structure is most of stellar astronomy.
Mass decides everything, by a power of three and a half
Two stars of the same mass are almost the same star. Double the mass and the output multiplies by eleven — which is why a modest range of masses produces a colossal range of stars.
Why the biggest stars die first, and take the galaxy with them
A star thirty times the Sun's mass has thirty times the fuel and burns it forty thousand times faster. It lasts a few million years, and everything heavier than iron exists because of it.