9
The
Period-Luminosity Relationship
•Relationship discovered by Henrietta Leavitt in 1912
–stars in Small Magellanic Cloud
–all at roughly same distance
–but didn’t have absolute M, just apparent M
–need absolute M to get distances
•Calibrated by Harlow Shapley
–If you can find distance (so M-m) for just one nearby Cepheid, you can convert Leavitt’s
“m” scale to the “M” you want.
From our text: Horizons, by Seeds