Chapter 30 Notes Galaxies and the Universe The

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