Section 3 review

Stellar Populations

Pop I: These are the current generation of stars (like our Sun). They have 0.1 to 2% "metals" (everything other than H).
Pop II: These are the previous generation of stars. They have 0.01 to 0.1% "metals."
Pop III: These are the first stars. They formed from only H and He.

Type I: Exploding white dwarfs.

Type II: Exploding massive stars.

Really a collapse and rebound.
Not a fixed mass. Any mass greater than about 8 times that of our Sun will undergo a Type II supernova.
Spectra show H.

Remember that stars do not form alone- they form in groups. But only globular clusters are gravitationally bound.

Consists of 3 parts:
1. Bulge- Is at the center. The bulge consists of mostly old, red stars (Pop II). Our bulge is about 18,000 light years across.
2. Disk- The disk of the Milky Way is 100,000 light years across but only 900 light years thick. The Sun is about 25,000 light years from the center. The disk contains all the young, blue (Pop I) stars in spiral galaxies as the disk also has all the gas and dust.
3. Halo- the halo is a spherical shell that surrounds the disk. It contains the dark matter and globular clusters: large groups of stars (Pop II) that formed before the disk of our galaxy.
Contains over 200 billion stars.
Our Sun orbits the center of our galaxy once every 250 million years. It has already gone around over 18 times.
Contains about 500 billion solar masses of material.

Makes up 90% of the matter in our galaxy.
Only interacts through gravity (it doesn't give off light, or form stars, or kill stars, or do anything else that could allow us to spot it).
What it's not:
1. Stars, planets, moons, rocks, or other normal "stuff."
What it could be:
1. Hot Dark Matter: Lighter (i.e. electron mass or less) particles that travel quickly through the galaxy. Smoothly distributed over larger scales.
2. Cold Dark Matter: Fairly massive (i.e. proton mass) particles that travel slowly through the galaxy. Can have a patchy distribution in the galaxy.
How do we know it exists? By measuring the rotation rate of stars, gas, and/or globular clusters at the edge of the galaxy.

Be sure you can draw and label a Hubble Tuning Fork
Remember Hubble's Tuning fork diagram? If not, look at the notes.

Elliptical galaxies.
1. Round galaxies where stars orbit in random directions.
2. Old red stars
3. Relatively gas free- no new star formation.
4. E0s are very round, E7s are somewhat flat.
S0 Galaxies. A blend between ellipticals and spirals.
1. Large bulge
2. Bulge contains old red stars and is gas free.
3. Small disk- but no spiral structure.
4. Disk contains dust and younger blue stars.
Spiral Galaxies.
1. Contain a bulge, disk and halo with stars as outlined above.
2. Sa galaxies have a large bulge and tightly wound spiral arms.
3. Sc galaxies have a small bulge and arms that are loosely wound.
Barred Spiral Galaxis- same as other spirals, but with a bar crossing the bulge.
Irregular galaxies.
1. Not ellipticals- stars tend to orbit in a preferred direction.
2. May have some disk- but usually a bit messy.
3. Very dusty and gas rich.
4. Contains young and blue stars.

Be sure you can identify galaxies!

Local Group:
1. Contains about 38 galaxies.
2. Has only 3 large spiral galaxies.
Clusters can contain up to thousands of galaxies, all gravitationally bound together.
1. Contain mostly elliptical galaxies.
2. Collisions between galaxies break up spirals and make elliptical galaxies.
3. On the largest scales, superclusters, or clusters of clusters, exist.

In the 1920s, Hubble noticed all galaxies are moving away from the Milky Way (except those in our local group).
The further away galaxies are, the faster they are moving away from us.
This led to Hubble's law: H_o=v/D.
Can also use D=v/H_o to get distance once you know H_o.
Best value is H_o=72 km/s/Mpc
Hubble's constant also contains the age of the Universe.
Implies an age for the Universe of 14 billion years.

Stages of the Big Bang: (should know the order, but the times are not important. Concentrate on the evidence in the next section)
1. 10^-45 seconds: Gravity separates from other 3 forces.
2. 10^-35 seconds: Inflation. The Universe expands at an exponential rate.
3. 10^-11 seconds: all forces separate from each other. Universe is energy, quarks, electrons, positrons, neutrons- some protons and neutrons.
4. 10^-6 seconds: Neutrons and Protons can no longer be formed.
5. 1 second: electrons and positrons can no longer be made from photons. Also the begining of the Deuterium bottleneck.
6. 100 seconds: Deuterium bottleneck over. Nucleosynthesis begins.
7. 3 minutes: Nucleosynthesis ends. Universe is 75% H, 24% He and 1% ³ He and Li.
8. 300,000 years: Recombination occurs. Electrons and protons form neutral atoms. Universe becomes transparent.
Evidence for the Big Bang: (THIS IS IMPORTANT TO KNOW)
1. Expanding Universe: Galaxies are not moving randomly, but uniformaly expanding.
2. Nucleosynthesis: says that once the Universe was hot and dense enough to be a nuclear furnace.
3. Cosmic Microwave Background: The UV photons that escaped after recombination have been expanded down to microwave wavelengths (3K)
4. Fluctuations in the CMB: 1 part in 100,000: need to be small, or the Universe would be too lumpy. Yet need to be there, or the Universe would be a smooth gas and could not form stars and such.

Be sure to look at the summary slides of the lecture powerpoints.

Open: Gravity loses and the Universe continues to expand forever. Over time, galaxies lose stars, stars decay into particles, black holes evaporate. It's a cold, dark death.
Closed: Gravity wins. There's enough mass in the Universe that eventually it will reach a maximum size, and then fall back. It will end in the Big Crunch, or Gnab Gib (big bang in reverse).
Critical case: Gravity is sufficient to stop the expansion, but not close the Universe. Another cold dark death.

The Drake Equation: N=Rxf_sxf_pxn_exf_lxf_ixf_cxL (yes, the x means times.)
Be sure you can describe how the factors will be determined: by astronomy, biology, or sociology.
What do the most recent exoplanet findings mean?
Be able to make a reasonable statement about life in the galaxy based on facts.
The Seager Equation: N = N_*^.F_Q^. F_HZ^.F_o^._L^. F_S

Be sure you know roughly:

The contents of a solar system, a galaxy and a universe.
The sizes and scales of each.
The difference between Jovian and Terrestrial planets.
Why stars change and evolve.
What is the use of the HR diagram?
What is the use of the Hubble tuning fork?
Our Sun: is slightly more massive than average; will evolve into a white dwarf; will eventually consume Mercury and Venus when it becomes a red giant and AGB star; is unusual in that it is not in a binary; is the most massive object in our solar system.
Why is it that if we can measure 2 things, we always plot them against each other?
Roughly the distances and ages of objects in the universe.