1. Ch. 31 – Galaxies & the Universe
Have you seen the Milky Way at night?
The Sun is one of over 100 billion stars that make up the Milky Way.
The Sun is on the outer part of the Milky Way, so you are looking at the galaxy from the inside of it.
This makes it hard to put size, distance, and the Earth’s location within it into perspective.
Variable stars are used to help us determine the distance of the stars.
A variable star that pulsates in brightness because of the expansion and contraction of its outer layers.

2. RR Lyrae variables pulsate between 1.5 hours to a day on average.
Cepheid variables pulsate between 1 and 100 days.
This helps astronomers determine a stars luminosity, absolute magnitude, apparent magnitude, and distance.
By using radio waves astronomers have been able to determine the shape of the Milky Way.
The center has a bulge and the halo (spherical region) circles it.
The disklike shape with a central bulge also has spiral arms that extend out.
Fig (pg. 835)

3. There may be over 100 billion stars in the Milky Way.
As much as 90% of the galaxies mass may be in the halo.
The oldest objects occur in the halo.
Most of the younger stars are in the spiral arms.
Two locations in the Milky Way
Population I stars are in the disk and arms.
Population II stars are in the halo and bulge.
The halo and bulge must of formed first.

4. Other Galaxies in the Universe
The Milky Way is only one of billions of galaxies in the universe.
Edwin Hubble was one of the first astronomers to discover other galaxies.
He proved that Andromeda Nebula was actually a galaxy  Andromeda Galaxy
Fig (pg. 839)
Galaxy Classification
1. Spiral Galaxies
A. Normal Spirals – pg. 840
B. Barred Spirals – pg. 840
2. Elliptical Galaxies
A. Round to Elongates Galaxies (E0 to E7)
Pg. 841

5. Astronomers have figured out that the mass from a galaxy is much larger than the visible mass of the galaxy.
This suggest that most of the mass in a galaxy is invisible or dark matter.
This dark matter is still unknown.
Clusters of galaxies are organized into even larger groups called superclusters.
Edwin Hubble (1929) discovered that most galaxies have redshifts (moving away from Earth). The redshift is dependent on its distance from Earth.
The farther away a galaxy is, the faster it is moving away. The universe is expanding.

6. Cosmology
Cosmology – the study of the universe, its current nature, and its origin and evolution.
Big Bang theory – the theory behind the point at which the universe began and has been expanding ever since.
The steady-state theory – proposes that the universe looks the same on large scales to all observers, and that it has always looked that way.
Each theory predicts what the universe should look like. Today, the evidence weighs in favor of the Big Bang.

7. Astronomers have found that the rate of expansion is speeding up!
In the Big Bang model, there is a competition between the outward momentum of the expansion of the universe and the inward force of gravity as the matter in the universe acts to slow the expansion. Fig (pg. 849)
Astronomers have found that the rate of expansion is speeding up!
It appears that the rate of expansion slowed for a while but is now gathering speed.
If the expansion constant is known then the age of the universe can be determined.
Based on the best value for the expansion rate, calculated from Hubble Space Telescope, the age of the universe is hypothesized to be about 13.5 billion years old.