Chapter 30 Notes

Galaxies and the Universe

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

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

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

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

The Milky Way Galaxy

• The spiral arms do not rotate as a unit

• Spiral density wave hypothesis: traffic jam analogy

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

Types of Galaxies

• Hubble’s “tuning fork” diagram

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

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)

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

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