Transcript Lec11
What does our galaxy look like? The Milky Way galaxy appears in our sky as a faint band of light We see our galaxy edge-on Primary features: disk, bulge, halo, globular clusters Halo: No ionization nebulae, no blue stars no star formation Disk: Ionization nebulae, blue stars star formation If we could view the Milky Way from above the disk, we would see its spiral arms How do stars orbit in our galaxy? Stars in the disk all orbit in the same direction with a little up-and-down motion Orbits of stars in the bulge and halo have random orientations Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit: 1.0 x 1011 MSun 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 Saturn’s Rings: Natural Color 15km/s < 1cm/s 15km/s Unseen Influences Dark Matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate Mass within Sun’s orbit: 1.0 x 1011 MSun Total mass: ~1012 MSun What is the evidence for dark matter in galaxies? Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter Contents of Universe • “Normal” Matter: ~ 4.4% – Normal Matter inside stars: – Normal Matter outside stars: • Dark Matter: • Dark Energy ~ 0.6% ~ 3.8% ~ 25% ~ 71% We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass All three methods of measuring cluster mass indicate similar amounts of dark matter Clusters contain large amounts of Xray emitting hot gas Temperature of hot gas (particle motions) tells us cluster mass: 85% dark matter 13% hot gas 2% stars 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 How dark is it? … not as bright as a star. Two Basic Options • Ordinary Dark Matter (MACHOS) – Massive Compact Halo Objects: dead or failed stars in halos of galaxies • Extraordinary Dark Matter (WIMPS) – Weakly Interacting Massive Particles: mysterious neutrino-like particles Halo stars formed first as gravity caused cloud to contract Remaining gas settled into spinning disk Detailed studies: Halo stars formed in clumps that later merged 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 NGC 3310 Spiral Galaxy NGC 5194 Spiral Galaxy NGC 1365 Barred Spiral Galaxy Interacting galaxies