My favorite part of the course was our study of the Hertzsprung-Russell diagram--the graph of stellar luminosity
versus temperature that groups different types of stars and their developmental stages.
During the early 20th century astronomers Hertzsprung and Russel independently discovered a correlation between the stellar luminosity
(brightness corrected for distance) and spectral class.
But they have had only limited opportunities to observe long-term changes in stellar luminosity
. "No one has seen a star age in this way before," asserts Mart J.H.
Fifteen feet (4 1/2 meters) in diameter, the Atwood Sphere was perforated with 692 holes that mapped the principal stars and relied--like the cosmos of Anaximander--on exterior ambient light for stellar luminosity
. It rotated on an axis inclined 42[degrees] to the floor, the altitude of the north celestial pole at Chicago's latitude.
Based on the relative populations of different stellar luminosity
classes, Stern finds it "statistically likely that all comets in the Oort cloud have been [heated] to 27 kelvins [-246|C] at least once, and that 20 to 40 percent of all Oort comets have experienced at least one episode of surface heating to 50 K [-223|C].' As few as 50 to 100 stars with 10,000 times the sun's luminosity may ever have been through the cloud, he says.
"There must be some kind of deep-rooted instability which drives the stellar luminosity
to become super-Eddington," says Shaviv.