Cepheid Variable Stars:
John Goodrick (1784) discovered Delta-Cephei.
Henrietta Leavitt (1908)--Period Luminosity Relation:
(overlay). Stars
varying in brightness with period 1-50 days
have linear relation between period and
luminosity.
This gives luminosity and thus distance to stars
even in other galaxies.
The Interstellar Medium:
1. Dust--recycled star material. Light undergoes to changes
in
going through it (Trumpler 1930):
A. reddening--preferential
scattering-blue light (why sky is blue).
B. absorption--this
affects flux and measured distance.
2. Molecular
Clouds--H2 molecules--dense MC are
star
formation regions (stellar nurseries like Orion
Nebula).