1. Milky Way Galaxy 1
Dr. Bill Pezzaglia
Updated: Nov 25, 2012

2. Milky Way 2
Star counts
B. Core and Arms
C. Galaxy Rotation

3. A1a. Milky Way 4

4. A1b. Milky Way: Galactic Equator5

5. 2a. Galileo Galilei (1564-1642) 6 Galileo was one of the
very first scientists to
do experiments to
understand Nature
He was the first
astronomer to use a
telescope (in 1610)
to study the sky.
Sees that the Milky Way is made of stars

6. A1c Thomas Wright 7
Milky Way is thin shell of stars, which explains why we see a band across the sky.

7. A1c. 1784 Herschel’s Telescope
20 year study of 2400 regions of sky (maps over 90,000 stars)

8. A1c. The Galactic Equator9
1784 Herschel’s Star Gauging
Counts stars in 683 regions
Estimates universe is disk shaped
Diameter is 5 times thickness
Sun appears to be at the center

9. A1c. Kapteyn Universe 9
1897 measured that stars move (rotate around universe)
(Parsec=3.26 light years=200,000 x distance to sun)

10. Magnitude Distance Relation10
Magnitude Distance Relation
Objects further away look fainter
If the star is too far away, it will be fainter than our limiting magnitude and we won’t see it.
Assume at 10 parsecs an average star has (absolute) magnitude of M=+2.5
Every factor of 10 in distance it gets 5 magnitude fainter
Distance (parsecs)
magnitude 10
2.5 32 5
100
7.5
316
1000
12.5
3162 15

11. Space Penetrating Power11
Space Penetrating Power
Limiting Magnitude
Distance (parsecs)
2.5 10 5 32
7.5
100
316
12.5
1000 15
3162
Turn this idea around. From the limiting magnitude of our telescope, we can estimate how “deep” we are penetrating into the galaxy.
We need to see deeper than 500 parsecs to be able to see the thickness of the Milky Way.

12. Counting Stars 12
The number of stars “N” seen in a field of view of “” as a function of the “depth” we see into space (“space penetrating power) assuming constant density :
If density is constant, expect plot of log of star count vs log of distance to have a slope of 3

13. Stars vs Magnitude 13
Since magnitude is proportional to 5 times log of distance, expect plot of log count vs magnitude to be a line with slope of 0.6 :
This assumes density of stars is constant.
Assume we live in a BIG spherical ball of stars. Count ALL the stars we can see for entire sky,

14. 3a Number of Stars by Magnitude14
There are only about 15 bright (first magnitude and brighter) stars
There are only about 8000 stars visible to naked eye
There are much more stars with higher magnitude!

15. 3b Number of Stars by Magnitude15
All visible stars up to m=+8.5
If a ball of stars expect slope of 0.6
If a thin disk of stars expect slope of 0.4
Our data is smack dab in the middle of the two.

16. Sample in Milky Way 16
If we sample a region of the sky along the Milky Way, we get a slope close to what we expect for living in a “disk” of stars.

17. Sample far from Milky Way17
If we sample a region of the sky perpendicular to the Milky Way, we get a much lower slope (closer to ¼ ) which implies we are seeing “out of the disk”.

18. Galactic Longitude 18
View looking down on disk of galaxy

19. Galactic Latitude 19
Side view of galaxy

20. Galactic Coordinate of Some Stars20

21. What we should see 21
Space penetration power “D” is furthest distance can see with our telescope
As galactic latitude “” increases, we hit the border of disk of stars, so see less stars.
Least number of stars seen at galactic pole

22. Observational Data: as a Bar Graph22

23. Data: Line Graph 23
The data can be fitted with a nice exponential decaying formula. In fact it’s a very good fit (perfect would have an Rsquared value of 1).

24. Thickness of Galaxy 24
Calculate it from the ratio of counts of stars at pole and at equator (assume D=1000 pc) N0 Np

25. 2c. Interstellar Reddening17
Note NGC3603 (left) is more red than NGC3576 (right) because it is twice as far away. Short wavelength Blue light is absorbed more than Red

26. Extinction of Light 16 There is a lot of gas and dust in the galaxy
This absorbs light (1 magnitude per 1000 parsecs)
Makes stars look fainter
Hence we think they are further away than they really are. Causes us to overestimate distances
First measurements of the Milky Way (1920s) was hence 10x bigger than it really is due to this error!

27. Corrected Space Penetration18
Corrected Space Penetration

28. Globular Cluster of Stars28

29. A2a. Shapley Core ( ) 29
Postulates globular clusters orbit galactic core
More in direction of Sagittarius
Estimates core is 15kpc away from sun (error: its 9 kpc)
Overestimated distances, because did not know about absorption of light by galactic dust

30. A1c. Robert Trumpler 30
1930 (Lick Observatory) shows that there is dust in the galaxy which absorbs light.
Hence, clusters appear fainter, and more distant than they actually are.
Shapley’s size of universe is 40% too big!

31. A2b. The Central Bulge 31
Central Bulge is 4kpc in size, with a small 5 pc bright radio source “Sagittarius A”, also bright in the IR (see below)

32. A2c. Black Hole? 32
In the center of the 5 pc Nucleus is an X-Ray source smaller than 100 AU
Recent measurements of orbits of stars around this core imply that there is a 2.6 million solar mass black hole!

33. A3a. Mapping Spiral Arms (1960)33
1944, Hendrik van de Hulst predicted Neutral Hydrogen gas will emit a 21 cm “spin flip” spectral line
1951 First Observed with radio telescope
1960 Used to map spiral arms of our galaxy

34. A3b. Our place in the Galaxy34

35. A3c. Rotations of Galaxies35
Spiral Galaxies Rotate Slowly
Sun takes 226 million years to go around (220 km/sec or 1 AU in 8 days)
The rotation speed can be measured by the Doppler effect on the 21 cm radio line

36. A3d. The “Winding Dilemma”36
Outer stars move slower.
Why haven’t the spiral arms wound up and disappeared a long time ago?

37. A3e. The “Winding Dilemma”37

38. A3f. Density Wave Theory 38 Bertil Lindblad
1925 Shows stars further from center of galaxy should move slower due to weaker gravity
1927 Jan Oort proves this with observations
1940 Lindblad Proposes “density wave theory” to explain spiral arms (resolve the winding paradox)

39. A3f. Density Wave Theory 39
A compression wave through the galaxy causes stellar birth; the bright short-lived O,B stars show the crest of the wave.

40. A3f. Density Wave Theory 40

41. A3f. Emission Neb in M51 41 This shows stellar formation
In in the spiral arms (where
Density waves bunch up matter

42. A3f. Emission Nebulae 42 Red is ionized hydrogen gas
Emission nebulae are where stars
have recently formed.

43. Addenda: Rotation of Galaxies43
The way galaxies should rotate
B. Galaxies rotating too fast
C. Theory of “dark matter”

44. A4a. Rotation Curves Assuming most of mass of galaxy is in the core44
Assuming most of mass of galaxy is in the core
Velocity of a Star predicted by Newton’s Gravity:
V2/R = GM/R2
Or: V  1/R

45. A4b. Rotating Rong? 45
1980 Vera Rubin shows rotation curves of galaxies are nearly constant!
Implies a lot of “missing” (dark) matter surrounds galaxies.
Pivotal Paper:
Rotational Properties of 21 Sc Galaxies with a Large Range of Luminosities and Radii from NGC 4605 (R=4kpc) to UGC 2885 (R=122kpc)," Astrophys. J. 238: 471 (1980), V.C. Rubin, W. K. Ford, Jr. and N. Thonnard.

46. A4c. What IS Dark Matter? 46
MACHOs (Massive Compact Halo Objects) were looked for:
White Dwarfs
Brown Dwarfs
Black Holes
But its not enough!
WIMPs (Weakly Interacting Massive Particles): Must propose exotic things like a neutrino, but with BIG mass (10 to 10,000x that of proton). Even though 96% of the universe is made of it, not a single piece of it is in this room.
Or maybe there is something wrong with our theory of gravity?

47. REFERENCES 47
B Carroll and D. Ostlie, “An Introduction to Modern Astrophysics” (Addison-Wesley, 1996), Chapter 22