GALAXIES

Types of GalaxiesSpirals (77%) Ellipticals (20%) Irregulars (3%)

National Optical Astronomy Observatory images

Galaxy types: Spirals

a: NASA/Hubble Space Institute; b: Robert Gendler; c: Anglo-Australian Observatory

• spiral disk shape with arms

• central nuclear bulge

Spiral galaxies – side view

Spirals – top view

Spiral arms don’t wind up

Ripples in rotating water/gas/dust form spiral arms

Galaxy Types: Barred Spirals

• central bulge is elongated, like a bar

The Milky Way

Galaxy Types: Elliptical

(a) NASA; (b) RoyalObservatory, Edinburgh

• no spiral shape or arms

Elliptical Galaxy Variations

a: J. D. Wray, McDonald Observatory; b, c: 1999 Princeton University Press/Zsolt Frei and James E. Gunn

• neither spiral nor elliptical

• white & dusty

Galaxy Type: Irregular

Edwin Hubble(1889-1953)

Milton Humason

(1891-1972)

Hubble’s Classification System

Cosmic Distance Ladder

Hubble’s Law:

Velocity of Distant Galaxies Increases with Distance

Closer Galaxies move away from us

Farther Galaxies move away from us faster

Hubble’s Constant yields Age of Universe

Hubble’s Constant Tells Age:

H0 = Velocity/Distance = 1 / Time

Hubble’s Law:

Velocity = H0 x Distance

The Local Group

Neighborhood Superclusters

Large Scale Structure in the Universe

Colliding Galaxies

Gravitational Lenses and Dark Matter