Transcript Lec12
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 EllipticalGalaxy Galaxy Elliptical 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)