Transcript: As a way of exploring stellar properties and understanding how stars work, in the early twentieth century two astronomers, the Danish astronomer Ejnar Hertzsprung and the American astronomer and Henry Norris Russell, experimented with plotting spectral class for stars against their luminosity. They saw patterns in the ways stars appeared in this plot which led them towards an idea of how stars work. This is called the H-R diagram or the Hertzprung-Russell diagram, and it’s a key tool of stellar astronomy. In a typical H-R diagram the y-axis is luminosity, which runs from about 106 solar luminosities, or an absolute magnitude of -10, down to about 10-4 solar luminosities, an absolute magnitude of plus 15. The x-axis is temperature, photospheric temperature, or spectral class running from O stars, traditionally plotted on the left side, at temperatures of forty thousand Kelvin down to N stars with temperatures of twenty-five hundred Kelvin.