1. Galaxies

2. Background Billions of them in the universe
Only largest telescopes can gather the light that travels from them to Earth
Hubble Deep Field
Relatively nearby galaxies
10 billion LY away

5. Discovery William Parsons 1845 built a telescope 72 in. diameter
“spiral nebulae” great spirals of stars
Foggy swirls of dust and gas

6. December 30, 1924 Mt. Wilson observatory Edwin Hubble
Discovery of cepheid variables
Supergiants are faint, very distant
Hence, they must exist outside of the Milky Way

7. Spiral

8. Types of Galaxies
Spiral
Elliptical
Irregular

10. Barred Spiral

11. Elliptical

13. Irregular

15. Dark Matter We estimate the mass of stars and galaxies from luminosity
Measured masses are often too large…something must be missing
The mass that is not visible is called “dark matter”
What we can see is only the central, luminous parts of much larger structures.
X-ray images reveal more evidence

16. X-ray images
Galaxy clusters filled with very hot, low-density gas (not the dark matter)
Something must hold the gas there (gravity)
What is creating the gravity is the dark matter.

17. 90-99% of all matter in the universe is dark
Could be low-luminosity white dwarfs and brown dwarfs
We have found them, but not enough to account for all the missing mass!
Could be a yet discovered type of matter.

18. The Life of a Galaxy How do they form?
There is NO answer to this question!
How do they evolve?
Interactions, collisions, and mergers with other galaxies

19. Colliding Galaxies Should happen often (they are large and in motion)
There are hundreds that do this

21. “The mice”

22. They don’t need to collide for their gravitational fields to affect one another
Can last hundreds to millions of years
The results are ring galaxies and galactic cannibalism (a small galaxy is pulled apart by a larger one)
The Milky Way is cannibalizing the Magellanic clouds!

25. Galaxy Clusters A few to a few thousand members
We (the Milky Way) are a member of a cluster containing 36+ galaxies
Rich clusters- over 1000 (Virgo cluster)
Poor clusters- less than 1000 (the Local Group, of which we are a member)

26. Virgo Cluster

27. The Local Group

28. Superclusters Clusters of galaxies that are associated with each other
Local group is part of the local supercluster
Includes the local group, the Virgo cluster, and others
So faint and far away, they are difficult to see, even with the best telescopes

29. Evolution Elliptical galaxies formed from the merger of spirals
Spirals have not experienced collisions with large galaxies since their formation