1. H205 Cosmic Origins APOD Today: Galaxy Evolution (Ch. 21)
EP 4 & Reflection 1 due

2. Find the Supernova in Messier 51
Whirlpool Galaxy
~10 Mpc (31 MLY)
A supernova is an exploding star

3. Two Types of Supernovae
Type II – Massive stars that run out of nuclear fuel, collapse, and explode
Type Ia – Carbon-oxygen white dwarfs that exceed the white dwarf mass limit

4. What type of SN is SN 1994i in M51?

5. Are Type Ia SN really “standard candles???”
Do all Type Ia supernovae have the same intrinsic brightness?
If all arise from CO white dwarfs that explode at the white dwarf mass limit, should they all be the same?

6. What is the peak magnitude of SN 1994S?

7. Galaxy Evolution: looking back through time
How do we observe the life histories of galaxies?
How did galaxies form?

8. Deep observations show us very distant galaxies as they were much earlier in time
(Old light from young galaxies)

9. Why do galaxies differ? Initial conditions when the galaxies formed
Collisions and mergers

10. Initial conditions when galaxies formed
Fast-spinning protogalactic clouds might form spirals
Dense protogalactic clouds might cool and form stars before gas settled into a disk, producing ellipticals

11. We must also consider the effects of collisions
Computer simulation showing the interactions and eventual merger of galaxies in a compact group over roughly 4 billion years. (Joshua Barnes)

12. Collisions/Mergers Change Galaxies
Distant clusters of galaxies have a higher fraction of spirals than nearby clusters
Distant clusters contain more galaxies within a given volume
Distant galaxies show more signs of disturbance by neighboring galaxies (odd shapes, bent arms, twisted disks)

13. Many of the galaxies we see at great distances (and early times) indeed look violently disturbed

14. Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical

15. Shells of stars observed around some elliptical galaxies are probably the remains of past collisions

16. Evidence for Galaxy Collisions
Galaxies are relatively close together
Computer simulations match images of galaxies
Ring galaxies
Starbursts
Stars not affected

17. Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together

18. Groups, Clusters, and Superclusters of Galaxies
Coma
Centaurus
Galaxy Clusters
galaxies aren’t the biggest structures in the Universe
gravity holds clusters together
Perseus
Hercules

19. The Great Wall of Galaxies
The largest structure yet found in the Universe
280 x 700 million light years in size
less than 20 million light years thick
diagonally across the northern sky
similar structure across the southern sky
are they related?
View of galaxies obscured
by the disk of the Milky Way

20. Bubbles and Voids about 300 million LY in size
In addition to super clusters and “walls,” astronomers also find huge empty regions with few or no galaxies.
about 300 million LY in size
fossils from the early universe
regions with little or no dark matter?

21. For Next Week Chapter 21 – More Galaxy Evolution EP 5
Delay 2nd Reflection until April 22