1	Astro 1050 Mon., Nov. 11, 2002 Today: Chapter 13, Galaxies
2	Homework #8 Q 1: If the inner accretion disk around a black hole has a temperature of 1 million K, at what wavelength will it radiate the most energy? Use Wien’s Law: λpeak = 3 million nm K/T = 3 million nm/1 million λpeak = 3 nm Q 2: Which of the below sequences shows objects with increasing densities? Density is mass/volume, so... Red Giant -- White Dwarf -- Neutron Star -- Black Hole Q 3: The black hole in the center of our Milky Way galaxy seems to be about 2.6 million times the mass of the sun. What is the radius of its event horizon? Rs = 2GM/c² Rs = 7.8 million km, which is about 11 times the solar radius and 5% of an AU. Q 4: Assume a white dwarf and neutron star have the same temperatures. White dwarfs are hard to spot because they are small (about the size of Earth). Neutron stars are smaller. How much fainter are they? Use Steffan-Boltzmann Law, or just the Scaling law for stars: L = 4πR²T⁴ L_WD/L_NS = (R_WD/R_NS)2 = (radius earth=6000 km/10 km)² L_WD/L_NS = (600 x 600) = 360000 times (round to 400000) Q 5: Why the "millisecond pulsars“? Because the neutron star: 1. Is spinning hundreds of times per second.
3	Chapter 13: Galaxies Family of Galaxies Classification Properties of Galaxies Distance; The Hubble Law Size and Luminosity Mass (including Dark Matter) Evolution of Galaxies Clusters Mergers
4	The Hubble Deep Field
5	“Tuning Fork” Diagram
6	Types of Galaxies (pg. 254-255) Spirals Sa Sb Sc (large nuclei Þ small nuclei) (little gas,dust Þ lots of gas, dust) SBa SBb SBc (as above, with BARS) Ellipticals E0 E1 E2 E3 E4 E5 E6 E7 (spherical) (highly elliptical) Irregulars
7	Spiral types The nuclear bulge is population II (old objects) So the Sa – Sc sequence is consistent with little gas Þ more gas
8	Elliptical Galaxy: M87
9	Irregular Galaxies: Magellanic Clouds
10	Properties of Galaxies Distance Use Cepheid Variables for close objects Other objects for which Absolute Magnitude is known: Supernova Planetary nebula in certain emission lines Use “Hubble Law” for more distant objects (Correlation of distance with radial velocity) Diameter and Luminosity Obtain from angular size and magnitude, combined with distance Mass Rotation curves Velocity dispersion 90 to 99% of mass is “dark matter”
11	The Hubble Law using galaxies with visible Cepheid variables.
12	Hubble Law Example v_r = H₀ d with H₀=0.5 (mile/hr)/mile
13	Hubble Law Example v_r = H₀ d with H₀=0.5 (mile/hr)/mile
14	Hubble Law Example v_r = H₀ d with H₀=0.5 (mile/hr)/mile
15	Hubble Law Example using relative v_r and relative d
16	The Hubble Law using secondary distance indicators H_o = 72 ±8 km/s/Mpc
17	The Local Group of Galaxies Galaxies live in clusters Rich clusters: thousands of galaxies Poor clusters: Fewer than a thousand
18	The Coma Cluster
19	Evolution of Galaxies Galaxies live in clusters Rich clusters: thousands of galaxies Poor clusters: Few than a thousand Fundamental difference between stars and galaxies: Stars live isolated lives: They are much smaller than distance between them They virtually never collide Galaxies are not isolated They are only slightly smaller than the distances between them The can (and do) collide, and interact with gas within clusters
20	Effects of Collisions Stars pass “through” each other, but orbits around galaxy disrupted Gas clouds collide Gas stripped away from stars Collisions cause bursts of star formation Ellipticals may be those galaxies which have suffered collisions Spirals may be those galaxies which have not suffered collisions
21	Interacting Galaxies: Cartwheel
22	Interacting Galaxies: The Antennae
23	The Hubble Deep Field
24	Evidence for “Hierarchical” Galaxy Formation from the Hubble Deep Field
25	Chapter 13: Galaxies Family of Galaxies Classification Properties of Galaxies Distance; The Hubble Law Size and Luminosity Mass (including Dark Matter) Evolution of Galaxies Clusters Mergers