Announcements• 4th test is finished!

• Please pick up graded work

• Homework 12 due Monday (requires internet use)

• Second project is due in two weeks

• Please reread lyrics to Monty Python galaxy song; document the inconsistency for a free Milky Way bar!

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

Finding the Andromeda Galaxy

Finding the Andromeda Galaxy

Finding the Andromeda Galaxy

Finding the Andromeda Galaxy

The Andromeda Galaxy

The Andromeda Galaxy

The Andromeda Galaxy

The Magellanic Clouds

Hubble Space Telescope can detect Cepheids out to 60 million light-years (Virgo Cluster)

Virgo Cluster

NGC 253

Whirlpool Galaxy (M51)

M83

M104 (Sombrero Galaxy)

Edge-on galaxies

Barred spiral galaxies

Colliding Galaxies

Galaxy with dust lane

Group of galaxies in Leo

Beyond 50 million light-years, the best standard candles are type-I supernovas

Coma Cluster

Coma Cluster

Hubble Deep Field