1. The Milky Way Galaxy

2. The Milky Way Galaxy
A galaxy is a collection of stars, gas, and dust bound together by gravity.
Before we look at other galaxies, let’s look at our own….

3. The Development of an HII Region--
HII regions are clouds with ionized hydrogen

4. Our galaxy is filled with interstellar gas and dust, which makes it hard to see through our galaxy
Light hits the dust and can produce
1. Dark nebula (if dust blocks/scatters lots of light)
2. Emission nebula (if dust is close to stars--reddish)
3. Reflection nebula (if dust is far from stars--bluish)

5. Dark Nebula, Emission Nebula, Reflection Nebula

6. What can you see through dust?
Longer waves pass through easier
Lets infrared waves through gas and dust clouds (and 21 cm radiation)
Previous experiments about size and shape of the Milky Way were off because they didn’t account for interstellar gas and dust.

7. Herschel’s first estimates of the Milky Way by counting stars …notice it isn’t very accurate

8. Where are we? How do we know?
We initially look at distributions of globular clusters to see where they’re centered
We look at 21 cm radiation to map the milky way

9. Where are we? How do we know?
Plot the globular cluster locations (just x and y coordinates)
After you plot them, mark a star (*) where you think the center of the galaxy is
(where is the center of the clusters?)

10. Harlow Shapley’s estimates of milky way by plotting globular clusters

11. The Distribution of Globular Clusters in the Galaxy
Sun is 28,000 ly from the center
about 150 Globular Clusters in Milky Way
globular clusters are smaller clusters of stars found in halo around galaxy
globular clusters contain redder, older stars
Spiral arms contain younger, bluer stars

12. The Shape of the Milky Way

13. The Shape of the Milky Way

14. The Shape of the Milky Way

15. 21 cm Observation of Spiral Structure

16. Spiral Arms Near the Sun --plot of bright, young stars

17. What does the galaxy look like?
(label these on your notes page)
Q: Can we really be looking at it from a top-down view?
A: NO!!! Can’t leave the galaxy…it’s too big.

18. The Milky Way ~3,000 light years thick ~100,000 light years across
We are ~28,000 ly from the center
Contains ~200 billion stars
Total mass = 1x 1012 solar masses
spiral structure
Supermassive black hole of 2 1/2 million solar masses at center

19. The Milky Way Q: What is dark matter?
Missing Mass = 90% (dark matter)
Q: What is dark matter?
A: there’s your nobel prize because we don’t know.
Q: How do we know there is dark matter?
A: From rotation rates…outside stars are spinning just about as fast as inside stars

20. Where would I look to look toward the center of the galaxy?
The center lies in the direction of the constellation Sagittarius (Sgr A*), but very close to the border of both neighbor constellations Scorpius and Ophiuchus.
Note…this is NOT the center of the universe, just our galaxy.

21. What do we see? (a panoramic view)