Measuring the unreachable
The law that links period to size, and weighs everything
Kepler found that the square of the period goes as the cube of the orbit. Newton found the constant of proportionality, and that constant is a mass — which is how every mass in astronomy has been obtained since.
GravitationNeither body is still, and the wobble is how planets are found
A planet does not orbit its star. Both orbit a point between them, and the star's share of that motion is small, measurable, and the reason thousands of planets are known.
StarlightThe triangle that reaches the stars, and stops
Parallax is the only distance measurement in astronomy that assumes nothing. It is also the only one with a hard ceiling, and everything beyond that ceiling rests on it.
StarlightA scale that runs backwards, multiplies, and works
Brighter stars have smaller magnitudes, and five steps is a factor of a hundred. A scale invented by eye in the second century BC turned out to be logarithmic, because eyes are.
StarlightColour is a thermometer, and it reads across the galaxy
A star's colour gives its surface temperature, from two brightness measurements and no other information. It is the cheapest useful measurement in astronomy.
StarsThe 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.
StarsMass 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.