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.