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:
d = v/H0
H0 is the Hubble constant = 72km/sec/Mpc
Explanation of Hubble's Law: The universe is expanding.
Expansion of space spreads out (clusters of) galaxies
stretches space -> redshifts light
Movie of 2 ways to look at expansion
Finite speed of light means we look back into the earlier universe
Lookback time
Cosmological horizon – since the universe has a beginning
Classifying Galaxies How would you sort these galaxies into 3 groups? A fourth group: Lenticular galaxies Sizes range from "dwarf" to "giant" Spiral Galaxies: Spirals with (M83 , NGC1365 , NGC4650 ) vs without a bar Viewing angle can effect appearances: Example = Hickson Compact Group Elliptical Galaxies Irregular Galaxies Lenticular 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
