Herschel, around 1800; Kapteyn, around 1900; counting stars
-- the 'grindstone' model, a disk about 15 kpc in diameter, the Sun is at the center
[interstellar extinction -- the grindstone model actually counted only nearby stars]
Harlow Shapley, in 1920, determining distribution of globular clusters
(using RR Lyrae stars)
-- the galactic center is in the direction of Sagittarius;
the Sun is about 8 kpc away from the center!
dust, about 10 K - 100 K, radiating in 30 - 300 microns (far infrared)
stars brighter than dust in near infrared (< 10 microns); extinction in this band not severe
dust in the disk, many stars concentrated near the nucleus of the Milky Way
disk: dust; young, metal-rich, population I stars
halo: old, metal-poor, population II stars; 1% of these stars are in globular clusters
central bulge: containing both population I and population II stars
