1. The Milky Way
Our Galactic Home

3. Goals Structure of our Galaxy. Its size and shape.
How do stars and things move through it?
The Galactic Center.

4. The Milky Way Stars Dust Gaseous Nebulae Open Clusters
Globular Clusters
Pulsars
Black Holes
How do they all fit together to make our galaxy?

5. Optical emission from stars and nebulae

6. Near-Infrared emission from stars – copyright E. L. Wright and COBE

7. Far-Infrared dust emission – copyright E. L. Wright and COBE

8. Radio emission from neutral hydrogen – copyright J. Dickey

9. X-ray emission from hot gas – copyright S. Digel and ROSAT

10. Gamma-ray emission from pulsars and black holes – copyright NASA

11. Where are We? Not at the center of the Milky Way. Where is the center?
Globular Clusters point the way.
M10 – copyright Credner and Kohle

12. You Are Here

13. Near-Infrared stellar emission – copyright E. L. Wright and COBE

14. Galactic Distances
How do we know the distance to stars and clusters in our galaxy?
Stellar parallax:
Parallax of nearby stars relative to background stars.
Good out to ~500 pc.
What about the background stars?
How far are they?

15. Standard Candles “Standard Candles”
If we know how bright something looks,
And we know how bright it should be,
Result  Distance
We do this everyday with size.

16. Spectroscopic Parallax
If you know how luminous a star REALLY is and how bright it looks from Earth, you can determine how far away it must be to look that faint.
For any star in the sky, we KNOW:
Apparent Magnitude (m)
Spectral Type (O, B, A, F, G, K, M)
Luminosity Class (Main Sequence, Giant, etc…). These are denoted by a roman numeral (V, III, I,…).
Combine spectral type and luminosity class to get absolute magnitude (M).
m – M give you distance.

17. Example Deneb is A2Ia star m = 1.25 A2  Blue star Ia  Supergiant
Distance = 1000 pc

18. Standard Candles Other “Standard Candles” Variable stars.
Stars that change in luminosity.
RR Lyra stars
Cepheid variables

19. Variable Stars For RR Lyrae stars: For Cepheid variables:
Average luminosity is a standard candle
Always ~ 100 x Sun
For Cepheid variables:
Pulsation period is proportional to average luminosity
Observe the period  find the luminosity
Good to 15 Mpc!

20. 30 kpc
8.5kpc
500 pc

21. Rotation … Objects in the disk, rotate in the disk.
Nebulae
Open clusters
Young stars
Objects in the halo, swarm in a halo.
Old stars
Globular clusters

22. The Sun’s “age” We define our age by trips around the Sun.
How many trips of Sun around Milky Way?
R = 8.5 kpc
V = 220km/s
P = 2.5x108 yrs
20 trips. R V

23. The Heart of the Galaxy
Because of all the dust in the Galaxy, we can’t see its center in visible light.
Can use IR and radio to pierce the dust.

24. 200 pc
5 pc
Sagittarius A* - Sgr A*

25. Supermassive Black Hole
Infrared images of stars in the Galactic Center over 8 years.
The “+” is the radio source Sgr A*
Conclusion: Must be over one million solar masses within less than 1/5 of a light year!
Event Horizon ~ 0.02 AU!
Probably in the centers of all spiral galaxies.
Copyright Eckart & Genzel

26. Homework #11 Read: Bennett Ch20.1 - 20.3: Do Ch20: Problems 2, 3, 12
Math folks substitute problem 20 for 12.