Galaxies and the Universe 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