15.1 Island of Stars
What are the three major types of galaxies?
- 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 studies of galaxies in thus intimately connected with cosmology - the study of the structure and evolution of the universe.
- Astronomers classify galaxies into three major categories:
- Spiral Galaxies: halo, bulge, disk, globular clusters, spiral arms
- Disk Component: stars of all ages, many gas clouds.
- Blue-white color indicates ongoing star formation.
- Spheroidal Component: bulge and halo, old stars, few gas clouds.
- Red-yellow color indicates older star population.
- Barred Spiral Galaxy: has a bar of stars across the bulge.
- Disk Component: stars of all ages, many gas clouds.
- Lenticular Galaxies: Has a disk like a spiral galaxy but is much less dusty (intermediate step between spiral galaxies and elliptical galaxies).
- Elliptical Galaxies
- Irregular Galaxies: neither spiral nor elliptical.
- Spiral Galaxies: halo, bulge, disk, globular clusters, spiral arms
- Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies per group).
- Elliptical galaxies are much more common in huge clusters of galaxies (hundreds of thousands of galaxies).
How do we measure the distance of galaxies?
- Brightness alone does not provide enough information to measure distance.
- Step 1: Determine size of solar system using radar.
- Step 2: Determine distance of stars out to a few hundred light-years using parallax.
- Luminosity passing through each sphere is the same. Divide luminosity by area to get brightness.
- Area of sphere: 4 π (distance)^2
- The relationship between apparent brightness and luminosity depends on distance.
- Luminosity passing through each sphere is the same. Divide luminosity by area to get brightness.
- Step 3: Apparent brightness of star cluster's main sequence tell us its distance.
- Knowing a star cluster's distance, we can determine the luminosity of each type of star within it.
- Cepheid variable stars are very luminous.
- Cepheid Variable Stars
- The light curve of this Cepheid variable star shows that its brightness alternately rises and falls over a 50-day period.
- Cepheid variable stars with longer periods have greater luminosities.
- White dwarf supernovae can also be used as standard candles.
- Apparent brightness of a white dwarf supernova tell us the distance to its galaxy (up to 10 billion light-years).
- The Puzzle of "Spiral Nebulae"
- Before Hubble, some scientists argued that spiral nebulae were entire galaxies like our Milky Way.
- Hubble settled the debate my 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 redshifted: they're all moving away from us.
- By measuring distance to galaxies, Hubble found that redshift and distance are related in a special way.
- Before Hubble, some scientists argued that spiral nebulae were entire galaxies like our Milky Way.
- Hubble's Law: Velocity = H0 X distance
- We measure galaxy distances using a chain of interdependent techniques.
- Hubble's constant tell us the age of the universe because it relates velocities and distances of all galaxies.
- The expansion rate appears to be the same everywhere in space.
- One example of something that expands but has no center or edge is a balloon.
- 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.
- No center or edges
- Not proven, but consistent with all observations to date.
- Distances between far away galaxies change while light travels.
- Astronomers think in terms of look-back time rather than distance.
- Expansion stretches photon wavelengths, causing a cosmological redshift directly related to look-back time.
15.3 Galaxy Evolution
How do we observe the life histories of galaxies?
- Deep observations show us very distance galaxies as they were much earlier.
- Our best models for galaxy formation assure that:
- Matter originally filled all of space almost uniformly.
- Gravity of denser regions pulled in surrouding matter.
- Denser regions contracted, forming protogalactic clouds.
- H and He gases in these clouds formed the first fars.
- Supernova explosion from the first stars kept much of the gas from forming stars.
- Leftover gas settled into a spinning disk.
- Conservation of angular momentum
- Conditions in a Protogalactic Cloud
- Spin: Initial angular momentum of protogalactic cloud could determine the size of the resulting disk.
- Density: Elliptical galaxies could come from dense protogalatic clouds that were able to cool and form stars before gas settled into a disk.
- Distant Red Ellipticals
- Observations of some distance red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe.
- We must also consider the effects of collisions.
- Collisions were much more likely early in time because galaxies were closer together.
- The collisions we observe nearby trigger bursts of star formation.
- Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical.
- Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together.
- Giant elliptical galaxies at the center of clusters seem to have consumed a number of smaller galaxies.
15.4 Quasars and Other Active Galactic Nuclei
What are quasars?
- If the center of a galaxy is unusually bright, we call it an active galactic nucleus.
- Quasars are the most luminous examples.
- The highly redshifted spectra of quasars indicate large distances.
- From brightness and distance, we find that luminosities of some quasars are > 10^2 LSun.
- Variability shows that all energy comes from a region smaller than the solar system.
- Quasars are the most luminous examples.
- Galaxies around quasars sometimes appear disturbed by quasars.
- Quasars powerfully radiate energy over a very wide range of wavelengths, indicating that they contain matter with a wide range of temperature.
- Radio galaxies contain active nuclei shooting out vast jets of plasma, which emit radio waves coming from electrons moving at nearly light speed.
- The lobes of radio galaxies can extend over hundreds of millions of light-years.
- An active galactic nucleus can shoot out blobs of plasma moving nearly at the speed of light.
- Radio galaxies don't appear as quasars because dusty gas clouds block our view of their accretion disk.
- Characteristics of Active Galaxies
- Luminosity can be enormous.
- Energy From a Black Hole
- The gravitational potential energy of matter falling into a black hole turns into kinetic energy.
- Friction in the accretion disk turns kinetic energy into thermal energy.
- Jets are thought to come from the twisting of magnetic field in the inner part of the accretion disk.
- Orbits of stars at the center of the Milky Way indicate a black hole with a mass of 4 million MSun.
- Orbital speed and distance of gas orbiting center of M87
- Black Holes in Galaxies
- The mass of a galaxy's central black hole is closely related to the mass of its bulge.
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