1. Galaxies and the Universe
Chapter 30 Notes
Galaxies and the Universe

2. The Milky Way Galaxy
It is difficult to see the size and shape of the Milky Way from our perspective (inside it)
Every star we can see is part of the Milky Way

3. Milky Way: Size and Shape
Variable stars in globular clusters help to determine the size and shape of the galaxy
From the side, it looks something like a fried egg

4. Milky Way Galaxy: Spiral Arms
4 major arms, several minor arms
About 100 billion stars in the disk
Galaxy halo: older, dimmer stars
Center: supermassive black hole

5. Population I and II Stars
Population I: Younger stars like the Sun found mostly in the disk and spiral arms; about 2% heavy elements
Population II: Older stars found in the halo or nuclear bulge; minimal amounts of elements heavier than helium

6. The Milky Way Galaxy The spiral arms do not rotate as a unit
Spiral density wave hypothesis: traffic jam analogy

7. Other Galaxies
First one (Andromeda) discovered in 1924 by Edwin Hubble
Mass of other galaxies is dominated by mysterious subatomic particles called dark matter
The Milky Way is part of a group of about 40 other galaxies: the Local Group

8. Types of Galaxies
Hubble’s “tuning fork” diagram

9. The Expanding Universe
Another discovery by Hubble (1929) made by measuring the “redshift” of galaxies:
All distant galaxies are moving away from Earth
The farther away a galaxy is, the faster it is moving away
The entire universe is expanding
Extrapolating backward: at one time, all matter in the universe was compressed together

10. The Big Bang
Not an explosion—an expansion of the universe from a single point called the singularity
Further evidence: “fossil” radiation (cosmic background radiation)

11. Possibilities for the Future of the Universe
Flat: The expansion of the universe will slow down and stop
Closed: The universe will begin to contract
Open: The universe will continue to expand forever

12. Dark Energy/Dark Matter
Dark matter accounts for a large proportion of the mass of the universe
Astronomers estimate the amount of dark energy based on rate of expansion