Astro 1050 Mon., Nov. 11, 2002
| Today: Chapter 13, Galaxies | |
| 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 | ||||
| LWD/LNS = (RWD/RNS)2 = (radius earth=6000 km/10 km)2 | ||||
| LWD/LNS = (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. | ||||
| Family of Galaxies | ||
| Classification | ||
| Properties of Galaxies | ||
| Distance; The Hubble Law | ||
| Size and Luminosity | ||
| Mass (including Dark Matter) | ||
| Evolution of Galaxies | ||
| Clusters | ||
| Mergers | ||
Types of Galaxies (pg. 254-255)
| Spirals | ||
|
Sa Sb Sc (large nuclei Þ small nuclei) (little gas,dust Þ lots of gas, dust) |
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| SBa SBb SBc (as above, with BARS) | ||
| Ellipticals | ||
| E0
E1 E2 E3 E4 E5 E6 E7 (spherical) (highly elliptical) |
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| Irregulars | ||
| The nuclear bulge is population II (old objects) | |
| So the Sa – Sc sequence is consistent
with little gas Þ more gas |
Irregular Galaxies: Magellanic Clouds
| 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” |
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The Hubble Law using galaxies with visible Cepheid variables.
Hubble Law Example
vr = H0 d
with H0=0.5 (mile/hr)/mile
Hubble Law Example
vr = H0 d
with H0=0.5 (mile/hr)/mile
Hubble Law Example
vr = H0 d
with H0=0.5 (mile/hr)/mile
Hubble Law Example using relative vr and relative d
The Hubble Law using
secondary distance indicators
Ho = 72 ±8
km/s/Mpc
| Galaxies live in clusters | ||
| Rich clusters: thousands of galaxies | ||
| Poor clusters: Fewer than a thousand | ||
| 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 | |||
| 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 | ||
Interacting Galaxies: Cartwheel
Interacting Galaxies: The Antennae
Evidence for “Hierarchical” Galaxy Formation from the Hubble Deep Field
| Family of Galaxies | ||
| Classification | ||
| Properties of Galaxies | ||
| Distance; The Hubble Law | ||
| Size and Luminosity | ||
| Mass (including Dark Matter) | ||
| Evolution of Galaxies | ||
| Clusters | ||
| Mergers | ||