1. Introduction to Galaxies Types of Galaxies The Milky Way Dark Matter
April 2, 2003
Introduction to Galaxies
Types of Galaxies
The Milky Way
Dark Matter

2. Messier Objects 1784 – Charles Messier Identified 103 objects in the
he was a comet hunter
Identified 103 objects in the
sky which were not stars
these were fuzzy objects
he identified them so they would not be mistaken for comets
These were actually galaxies, globular clusters and such
far away
Now a very useful list of interesting objects for amateur astronomers to look at
M1 – Crab Nebular

3. Looking at Distant Objects
Objects look different depending on how they are viewed
We are unable to “walk around” an object which is millions of lightyears away
So we have to try to interpret what we see
compare to objects oriented differently

4. Looking at the Dark Sky
A picture of a dark patch of the sky using the Hubble Space Telescope

5. Galaxies Galaxies are large collections of stars
millions and billions of stars
The Milky Way is our own galaxy
There are hundreds of billions of galaxies in the Universe
Millions to hundreds of billions of stars in each galaxy

6. Types of Galaxies Three main types of galaxies Spiral Elliptical
based on shape
Spiral
rotating disk
(with arms)
Elliptical
oval disk of stars
more chaotic motion
Irregular
none of the above

7. Active Galactic Nuclei
Some galaxies have supermassive black holes at the center
If material is falling into the black hole, enormous amounts
of energy are released
accretion disk
Can shine with a luminosity
of million Suns!
Quasars are a type of AGN

8. Think of walking around before a football game
Galaxy Collisions
Occasionally galaxies collide
don’t actually slam into each other
Passage of one galaxy through/near another causes major “stirring”
due to gravity
Causes new activity
star formation, AGNs,…
Think of walking around before a football game

9. Looking at the Milky Way
Viewed as a bright band of stars across the sky
Galactic center appears in the southern part of the sky (from the northern hemisphere
Much of the Milky Way is blocked by dust
dark band through the middle of the Milky Way
But we can study it in longer wavelengths
e.g. radio waves

10. Spiral Galaxies Spiral arms are
a natural consequence of some rotations
If gas/dust in a disk are rotating with the same speed, stuff further out will take longer to go around
it will lag behind

11. Pieces of a Spiral Galaxy
Disk
Arms
Bulge
barred?
black hole
Halo
Globular clusters

12. Globular Clusters Groups of old stars in the galactic halo
may have around 500,000 stars
may be around 15 lightyears across
Formed before the galaxy
or at very early stages
Orbiting the galaxy
No new star formation going on
Useful for studying the distribution of material in the galaxy
from gravity

13. Rotation Speeds of Galaxies

14. Rotation Speeds of Galaxies
For spiral galaxies, stars farther out should be moving slower
But we see them all moving with the same speed!
Dark
Matter!
must have an explanation

15. What is Dark Matter? Dark matter is material we can’t see
We have various evidence the Universe has lots of matter we can’t see
rotation speeds of galaxies
movement of global clusters around galaxies
Two primary theories
MACHOs – MAssive Compact Halo Objects
planets or brown dwarfs or low-mass black holes
unlikely, people are looking, have seen a few, but not enough
WIMPs – Weakly Interacting Massive Particles
new type of elementary particle

16. A Great Question
The nature of dark matter is one of the great questions of astronomy
Could have very important implications on our understanding of the Universe
something is out there which we do not understand
could radically change how we think about many things
Pay attention to this issue in the coming decades…

17. Studying Star Clusters
Clusters of stars formed at the same time of the same materials
Studying them tells us about the life of stars
plot where stars fall on H-R diagram
Looking at many clusters tells us how stars leave main sequence

18. Summary Galaxies are large collections of stars
three types/shapes
hundreds of billions of galaxies
Spiral galaxies have structure
Milky Way is a spiral galaxy
Globular clusters
Dark matter
from rotational velocities of stars in galaxies