1. Milky Way
astronomy.com

2. 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.

3. 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

4. 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

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

6. 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

7. 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

8. 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