1755: Philosopher Immanuel Kant proposes the universe is filled with great wheels of stars - ‘island universes’1845: William Parsons identifies ‘spiral nebulae’

The Great Debate (1920):

Heber D. Curtis argued that the universe is composed of many galaxies like our own

Harlow Shapley argued that the "spiral nebulae" were nearby gas clouds, and that the Universe was composed of only one big Galaxy.

1924: Edwin Hubble measures distances to the brightest spiral nebulae- Supergiant stars in the spiral nebulae are so faint that they must be far outside the Milky Way

Andromeda Galaxy

Large and Small Magellanic Clouds

Large Magellanic Cloud

Large Magellanic Cloud

Small Magellanic Cloud

Pinwheel Galaxy

Shapes of Galaxies

Spiral Galaxies,....

...elliptical galaxies,...

... and irregular galaxies

NGC4414

M87

NGC1365

M110

NGC2953

M81

NGC 1350

The Hubble ‘Tuning Fork’ Diagram

The Hubble Deep Field

Galaxy Clusters and Large Scale Structure

Virgo Cluster

Hercules Cluster

Local Cluster

• A few hundred billion galaxies can be seen with current telescopes• Sloan digital sky survey has mapped out the distribution of •galaxies in certain parts of the sky

Olber’s Paradox (1826)Why is the night sky dark?

Olber’s Paradox (1926)Why is the night sky dark?

Olber’s ParadoxWhy is the night sky dark?

• The universe has an edge• The stars and galaxies ‘run out’ at some distance from us• The universe has a beginning

First suggested by Edgar Alllen Poe in 1848