Our Galaxy Earth Sun Earth orbit Solar system
Our Galaxy Earth Sun Earth orbit Solar system Nearest star = = = 100 nm 10 μm ¼ cm 20 cm 250 m = virus = cell = pin head = saucer = lawn Jewel Box Cluster Naked eye stars Eagle Nebula Galaxy Center Crab Nebula Solar system
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 1. Gas clouds get squeezed as they move into spiral arms 2. Squeezing of clouds triggers star formation 3. 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 1. Dark matter really exists, and we are observing the effects of its gravitational attraction 2. 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 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
Spheroid Dominates Hubble’s galaxy classes 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)
- Slides: 25