# Chapter 20 Galaxies (Really, Galaxies and the Expansion of the Universe)

Various Types of Galaxies
Spiral Galaxies:
Spirals with (M83 , NGC1365 , NGC4650 )
versus without M33 (Triangulum) a bar
Viewing angle can effect appearances: Example = Hickson Compact Group

Elliptical Galaxies: M87

Lenticular Galaxies: M102

Irregular Galaxies: Large Magellanic Cloud

Sizes range from "dwarf" to "giant"

How would you sort these galaxies ?
Hubble's classification scheme

"Groups" and "Clusters" of Galaxies
Small number of galaxies -> "group"
Examples: the Local Group,  Hickson Compact Group
100s to 1000s of galaxies -> "cluster"
Example: Abell 1689

The universe contains a lot of galaxies, at least 80 billion!
Hubble Deep Field Image
Zoom to Hubble Deep Field

Distances to galaxies
See near and far galaxies (Hubble Deep Field Image)
How do you find the distance to a galaxy?
1st, find Sun-Earth distance
Then, use parallax -> find distances to nearby stars
Treat stars as "Standard candles"
Use Main sequence fitting for star clusters
Use Cepheid variable stars' period - luminosity relationship
Example: Cepheids in M100 56 million ly away
Treat White dwarf supernova explosions as standard candles
Use spiral galaxies' rotation rate vs luminosity relationship
called Tully-Fisher relationship
Summary: calibrate – The Distance Chain

Hubble's Law
Far away galaxies are moving away from us,
Examples
redshifted spectrum
The farther away, the faster they move away
data
Hubble's Law:     v = H0 d          thus,          d = v/H0
H0 is the Hubble constant = 72km/sec/Mpc = 22 km/sec/Mly
Explanation of Hubble's Law:    The

Finite speed of light means we look back into the earlier universe

Lookback time

Cosmological horizon – since the universe has a beginning