The Stars: A Celestial Census (Astronomy)

Recall cards on taking a census of the stars. The solar neighborhood: counting the stars within about 21 light-years shows that cool, faint red M dwarfs vastly outnumber luminous stars, and that the bright stars in our sky are bright because they are powerful, not because they are near. Masses: how binary stars and Newton's form of Kepler's third law let astronomers weigh stars, from the smallest true stars near 1/12 of a solar mass, through brown dwarfs, up to about 250 solar masses, and the mass-luminosity relation linking the two. Diameters: the indirect methods (lunar occultations, eclipsing binaries, and the Stefan-Boltzmann law) that reveal sizes from roughly Sun-sized up to the vast red supergiant Betelgeuse. And the Hertzsprung-Russell diagram: the plot of luminosity against temperature on which the main sequence, giants, supergiants, and white dwarfs each take their place.