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?
Mass and Dark Matter.
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 stellar emission – 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? We aren’t at the center of the Milky Way.
Where is the center then?
Globular Clusters point the way.
M10 – copyright Credner and Kohle

12. You Are Here

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

15. Galactic Distances
How do we know the distance to stars and clusters in our galaxy?
Trigonometric parallax good out to 100 pc.
We believe galaxy is ~30 kpc wide.
How do we know?

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).
From Lecture 7B: 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 “Standard Candles”
If we know how bright something should be, and we know how bright it looks  Distance
Variable stars.
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!

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. … and Formation Picture the formation of the Sun:
Spherical cloud
Condenses to disk
Planets in a plane
Oort cloud sphere.
Perhaps the same with the galaxy?

23. Missing Mass From variable stars we know distances.
From Doppler shift we know rotation velocity.
Use Kepler’s Third Law (again) to get mass of the Milky Way. More than what we see.
M = 1011 x Msun

24. Dark Matter What causes the mass to keep on increasing?
Don’t see anything there. Thus  “dark” matter.
Brown dwarfs
Planets
White dwarfs
Strange matter?
Use gravitational lensing (last lecture) to look for these “dark” objects.

26. 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.

27. 200 pc
5 pc
Sagittarius A* - Sgr A*

28. Stellar Motion
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!
Supermassive Black Hole!
Event Horizon < 0.05 AU!
Probably in the centers of all spiral galaxies.
Copyright Eckart & Genzel