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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