Milky Way astronomy.com

Road of Milk The Milky Way appears as a great nebula. Stretches across the sky No stars to human eye The name comes from the Latin Via Lactea. The Greek word for milk gives us the word galaxy.

Star Count Galileo saw thousands of stars in the galaxy. No parallax – very distant Herschel used his telescope to map the Milky Way. Plotted individual stars Assumed the sun at the center Thicker in some directions

Disk The band of the Milky Way is the same view a viewer would have sitting inside a disk of stars. Consistent with the Sun towards an edge sun sun top view side view

Nebula Search William Herschel cataloged over 1000 nebulae. Many star clusters Open clusters matched the Milky Way Globular clusters more uniform in space

Halo In the early 20th century Cepheid variables could be used to map the globular clusters. Harlow Shapley’s model Form a sphere – center at center of galaxy The clusters for the galactic halo sun globular clusters

How Big? Shapley’s data also set the size of the galaxy. The clusters extend more than 20,000 parsecs in each direction. 130,000 ly across Sun 2/3 of the way out from center

Where We Are Other galaxies are visible through telescopes. Spiral arms Central bulge We can reconstruct a likely image of the Milky Way. universetoday.com