1. Galaxies

2. Galaxies Huge (>1000’s ly across) Gravitationally bound
Collections of gas, stars, planets, ice & dust.

3. Galaxies Different shapes & sizes.
Dwarf containing tens of millions (107) of stars,
Supergiants containing up to a trillion stars (1012).
> 100 billion (1011 ) galaxies in the observable universe.

4. Galaxies Ecosystems of the Universe Gas Stars (& vice versa)
No star formation between galaxies (intergalactic space)

5. Edwin Hubble In 1924, he divided galaxies into different
“classes” based on
their appearance.

6. Galaxy Types

7. Hubble’s Classification Scheme

8. Elliptical Galaxies ~ 50% of all galaxies They are gas poor
little star formation
smooth featureless light distribution & appear elliptically shaped in
found near the center of rich galaxy clusters.

9. Elliptical Classification Scheme
Identified by the letter E
0 -7
O is the most spherical, 7 is the most elliptical

10. by amount of flattening
Elliptical Examples
0-7 determined
by amount of flattening

11. Lenticular Galaxies* About 20% of all galaxies
Between elliptical & spiral
Have central bulge but no arms
Aging, little star formation
* Hubble added later

12. Spiral Galaxies ~ 20% of all galaxies* They are gas rich
But 70% of visible galaxies due to their brightness.
They are gas rich
Lots of star formation
More isolated

13. Spiral Classification Scheme
Identified by the letter S
B is added if barred spiral

14. Spiral Examples: Sa very tightly wound spiral arms
bright central bulge
Ex: Messier 81
“Bode’s Galaxy”

15. Spiral Examples: Sb Ex: M51 “Whirpool Galaxy”
less tightly wound spiral arms
fainter central bulge.

16. Spiral Examples: Sc loosely wound spiral arms
smaller, fainter central bulge.
Ex: NGC 6946
New general catalogue of galaxies and nebulae 1886 originally but updated since, most recent 2009

17. Barred Spiral Examples
classified in the same way as non-barred spiral galaxies.

18. Irregular Galaxies about 5% No symmetry
Lots of young, blue stars and interstellar material
Smaller than most spirals and elliptical galaxies

19. Irregular Galaxies
Large & Small Magellenic “Clouds”
LMC, SMC