Typical galaxy. [A] 100 billion stars, like city – people born & die Measure rate of star birth – ~1 per year.  only 14 billion stars ?? : LESS than known number We INFER that the rate of star birth was GREATER in the past. [B] Also : stars dying  pollution/enrichment by elements. gradual buildup of elements ---- expect oldest stars to have few elements Next few slides show this : (NEXT SLIDE)

Our Galaxy Earth = 100 nm = virus Sun = 10 μm = cell Earth orbit = ¼ cm = pin head Solar system = 20 cm = saucer Nearest star = 250 m = lawn Jewel Box Cluster Naked eye stars Eagle Nebula Crab Nebula Solar system Galaxy Center

Halo: No ionization nebulae, no blue stars  no star formation Disk: Ionization nebulae, blue stars  star formation

Much of star formation in disk happens in spiral arms Ionization Nebulae Blue Stars Gas Clouds Whirlpool Galaxy

Spiral arms are waves of star formation Gas clouds get squeezed as they move into spiral arms Squeezing of clouds triggers star formation Young stars flow out of spiral arms

Mass within Sun’s orbit: 1.0 x 1011 MSun Total mass: ~1012 MSun

We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H

Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter

Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass

Our Options Dark matter really exists, and we are observing the effects of its gravitational attraction Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter

Hubble Ultra Deep Field

Hubble Ultra Deep Field Elliptical Galaxy Elliptical Galaxy Irregular Galaxies Spiral Galaxy

NGC 3310 Spiral Galaxy

NGC 1365 Barred Spiral Galaxy

Barred Spiral Galaxy: Has a bar of stars across the bulge

Elliptical Galaxy: All spheroidal component, virtually no disk component

Lenticular Galaxy: Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical)

Irregular Galaxy

Interacting galaxies

Hubble’s galaxy classes Spheroid Dominates Disk Dominates

Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies)

Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies)