Sep 20 – 2B03 20/09/2011 9:25:00 AM
Two main types of galaxies:
o Our milky war
o Think, disk-like (similar to a frizbee) [2d]
o Have spiral arms
o Stars are being formed today
o Compared to a frizbee, they are 3d. Similar to a balloon.
o Similar, as well, to a cloud of bees.
Within galaxy, stars move around like bees flying about.
o Not much rotation and star formation
Most galaxies are one or the other
Galxies don’t live in isolation -> they have nearby neighbours.
They live in galaxy grouos.
o Feel each other’s gravitational forces.
Our milky way have tiny galaxies orbiting around us, as well as
have other large galaxies around us (Andromeda)
Some live in isolation, but most common is in groups
Compact (galaxy) groups – distances between galaxies are smaller than
Really feeling eachother’s forces.
Two spirals colliding (galaxies) -> may create an ellicptical galaxy.
Two ellicptical galaxies colliding will create a larger elliptical.
These are when galxies collidge.
Cluster vs group
Different in magnitude :
o Group has like 30-50 (no rigid boundary)
o Cluster may have like 1000 (much more) Biggest galaxies are the ones in the centre of the cluster (we think it is a
result of, over time, colliding and merging with other galaxies)
Gravitational lensing -> background galaxies, their light that is, is stretched
out by the other galaxies. We will come back to this.
Spinning of spiral galaxies – left on its own will stay spiral – the spiral means
that it is stable.
When interacted with others galaxies, it will change shape.
Galaxy clusters - biggest places in the universe
(Two kinds of clusters:)
o Made up of stars, and also made up of galaxies.
Most of the mass we see is invisible to the key
Most of the mass of a galaxy is in its dark matter -> invisible.
o (we were speaking about the edge of the universe; edge is