1. Galaxies
Astronomy 315
Professor Lee Carkner
Lecture 19

2. Upcoming Extra Credit Events
Thursday, April 27, 7pm, Olin Auditorium
Andrew Knoll
“Are We Alone in the Universe?”
Thursday, May 4, 7pm, Olin Auditorium
Kjell Lundquist
“Stars Above, Stars Below and Stars Within: On Tycho Brahe, Uraniborg and a Garden Reconstruction”
Saturday, May 6, Planetarium
Planetarium Open House
Will receive 10 points extra credit on the observing project for each you go to (up to a maximum of 20 points)
See me there to sign in

3. The Size of the Universe
Separate star systems like the Milky Way or just nebula in our galaxy?
In the 1920’s Edwin Hubble used the new 100 inch Mt. Wilson telescope to view Cepheids in “spiral nebula” and found they were too distant to be in our galaxy

4. Types of Galaxies A visual inspection reveals 3 types: Spiral
Elliptical
Composed of older stars
Irregular
Look like altered spirals

5. Spiral Galaxies Spiral galaxies are similar to the Milky Way
Spiral arms contain gas and dust and young stars
We see a lot of spirals (~80% of bright galaxies)
Most smaller than the Milky Way

6. Classifying Spiral Galaxies
Spiral galaxies are classified based on two properties:
From this Hubble produced 3 categories: Sa Sb Sc
Loosely wound arms, small bulge

7. M31, The Andromeda Galaxy (Sb)

8. M83 (Sc)

9. M100 (Sc)

10. M51, The Whirlpool Galaxy (Sc)

11. NGC Edge-on Spiral

12. Other Spirals
Many spiral galaxies show a bar of material across the nucleus (barred spiral)
Some galaxies have disks and bulges, but no spiral arms (called S0 or lenticular)
Why do spirals look the way they do?
Bars may form naturally, unless suppressed by a large dark matter halo

13. M95 -- Barred Spiral (SBb)

14. NGC 1300 -- Barred Spiral (SBc)

15. M S0 Spiral

16. Elliptical Galaxies Elliptical galaxies have almost no structure
Classified by how elongated they look from our point of view
Have almost no gas, dust or young stars
Have a wide range in size

17. M87 -- Giant Elliptical (E1)

18. M59 -- E5 Elliptical

19. Irregular Galaxies Some galaxies have no discernable regular shape
Often show evidence of star formation
Distortion might be due to:
Galaxy collisions

20. The Large Magellanic Cloud

21. Hubble’s Tuning Fork
Hubble categorized the galaxies and then placed them on a diagram
As you go from left to right in the diagram you roughly increase in gas, dust, number of young stars and star formation rates

22. Hubble’s Tuning Fork Diagram

23. Galactic Collisions Galaxies should collide fairly often
What happens when they collide?
May trigger wave of star formation
One galaxy may merge with another (galactic cannibalism)

24. Evolution of Galaxies
It is not completely clear how galaxies evolve, but there is growing evidence for this basic picture
Burst uses up all gas and dust and star formation stops (Elliptical)

25. Next Time
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