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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