Transcript Galaxies - Physics

Announcements
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Galaxies
17 November 2006
Today:
• The “spiral nebulae”
• Measuring distances to galaxies
• Rotation rates and “dark matter”
Messier Catalog
• 27 “open” clusters
• 29 globular clusters
• 6 diffuse nebulae
• 4 planetary nebulae
• 1 supernova remnant
• 2 small groups of stars
• 40 other fuzzy things in
which no individual stars are
visible
The Discovery of Spiral Structure
Lord Rosse, 1845
What are the spiral and elliptical nebulae?
• No individual stars are visible, even in largest
telescopes
• Some have spiral structure, as if spinning rapidly
• Visible amount of rotation over a few decades?
• Continuous spectra
• Hypothesis 1: Swirling clouds of fluid,
possibly forming new solar systems
• Hypothesis 2: “Island universes,” similar
to our own Milky Way star system
Island Universes Confirmed!
Edwin Hubble discovers Cepheid variable stars in the
Andromeda Nebula, 1923
Estimated distance: 1 million light-years (actually 2 million)
Edwin Hubble, 1889 - 1953
Hubble Space Telescope can detect Cepheids
out to 60 million light-years (Virgo Cluster)
Beyond 50 million light-years, the best
standard candles are type-I supernovas
Hubble Deep Field