15.1 Island of Stars
Hubble Deep Field
- Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away
Galaxies and Cosmology
- A galaxy's age, its distance, and the age of the universe are all closely related
- The study of galaxies is thus intimately connected with cosmology-the study of the structure and evolution of the universe
- Elliptical Galaxy
- Spiral Galaxy
- Irregular Galaxies
- Disk Component: Stars of all gasses, many gas clouds
- Spheroidal Component: Bulge and halo, old stars, few gas clouds
- Barred Spiral Galaxy: Has a bar of stars across the bulge
- Lenticular Galaxy: Has a disk like a spiral galaxy but much less dusty gas
- Spiral Galaxies are often found in groups
- Elliptical galaxies are more common in huge clusters of galaxies
15.2 Distances of Galaxies
- Brightness above does not provide enough info to measure distance
Step 1: Determine size of solar system using radar
Step 2: Determine distances of stars out to a few hundred light-year using parallax
- Luminosity passing thru each sphere is the same
- Divide luminosity by area to get brightness
- The relationship between apparent brightness and luminosity depends on distance
- We can determine a star's distance if we know its luminosity and can measure apparent brightness
Step 3: Apparent brightness of star cluster's main sequence tells us its distance
Knowing a star cluster's distance, we can determine the luminosity of each type of star within it
The ligh curve of this Cepheid variable star shows that its brightness alternately rises and falls over a 50 day period
Step 4: Because the period of a Cepheid variable star tells us it luminosity, we can use these...
Step 5: Apparent brightness of a white dwarf supernova tells us the distance to its galaxy
- Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles
- Hubble also knew that the spectral features of virtually all galaxies are red-shifted, the're all moving away from us
- By measuring distances to galaxies, Hubble found that red-shift and distance are related in a special way
- Red-shift of a galaxy tells us its distance...
- We measure galaxy distances using a chain of interdependent techniques
- Hubble's constant tells us the age of the universe because it relates velocities and distances of all galaxies
- The universe has no center and no edge
Cosmological Principle
- The universe looks about the same no matter where you are within it
- Matter is evenly distributed on very large scales in the universe
- Not proved but consistent with all observations to date
- Distances between faraway galaxies change while light travels
15.3 Galaxy Evolution
- Deep observations show us very distant galaxies as they were much earlier...
- Matter originally filled all the space almost uniformly
- Gravity of denser regions pulled in surrounding matter
- Denser regions contracted, forming protogalactic clouds
- H and He gases in these clouds formed the first stars
- Supernova explosions from the first stars kept much of the gas from forming stars
- Leftover gas settled into a spinning disk
- Conservation of angular momentum
- Spin-Initial angular momentum of protogalactic cloud could determine the size of the resulting disk
- Density- Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk
Distant Red Ellipticals
- The collisions we observe nearby trigger bursts of star formation
- Collisions may explain why elliptical galaxies tend to be found where galaxies are closure together
15.4 Quasars and Other Active Galactic Nuclei
- If the center of a galaxy is unusually bright, we call it an active galactic nucleus
- The highly red-shifted spectra of quasars indicate large distances
- Variability shows that all this energy comes from a region smaller than the solar system
- Galaxies around quasars sometimes appear disturbed by collisions
- Radio galaxies contain active nuclei shooting out vast jets of plasma, which emit radio waves coming from electrons moving at near light speed
- The lobes of radio galaxies can extend over hundreds of millions...
- The mass of a galaxy's central black hole is closely related to the mass of its bulge
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