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Milky Way Galaxy

Dr. Bill Pezzaglia

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Updated: Nov 25, 2012

Milky Way 2

A. Star counts

B. Core and Arms

C. Galaxy Rotation

A1a. Milky Way 4 A1b. Milky Way: Galactic Equator 5

2a. GalileoGalilei(1564-1642)

Galileo was one of thevery first scientists todo experiments tounderstand Nature

He was the firstastronomer to use a telescope (in 1610)to study the sky.

Sees that the Milky Way is made of stars

6 A1c. 1750 Thomas Wright 7

Milky Way is thin shell of stars, which explains why we see a band across the sky.

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A1c. 1784 Herschel’s Telescope 8

20 year study of 2400 regions of sky (maps over 90,000 stars)

A1c. The Galactic Equator 9

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

A1c. Kapteyn Universe 9

1897 measured that stars move (rotate around universe)

(Parsec=3.26 light years=200,000 x distance to sun)

Magnitude Distance Relation10

• 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 10

1000 12.5

3162 15 D/105Logm M

Space Penetrating Power11

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.

Limiting Magnitude

Distance (parsecs)

2.5 10

5 32

7.5 100

10 316

12.5 1000

15 3162Mmm m/51010D

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 :

)(3)(

VN2

121 DLogLogNLog

If density is constant, expect plot of log of star count vs log of distance to have a slope of 3

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Stars vs Magnitude 13

Since magnitude is proportional to 5 times log of distance, expect plot of log count vsmagnitude to be a line with slope of 0.6 :

)()( 53 DLogbNLog

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,

3a Number of Stars by Magnitude 14

•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!

3b Number of Stars by Magnitude 15

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

y = 0.4914x + 0.9204R² = 0.9958

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1

2

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5

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-2 0 2 4 6 8 10

Lo

g(C

um

ula

tive

co

un

t)

Limiting Magnitude

Stellar Counts

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.

Samples in Milky Way

y = 0.442x - 2.9778

R2 = 0.9935

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0

Limiting Magnitude

Lo

g o

f C

ou

nt

(un

kno

wn

fie

ld s

ize)

Sample far from Milky Way 17

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”.

Star Counts Centered on Arcturus from Starry Night Program

y = 0.2442x - 1.2763

R2 = 0.9647

-0.5

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10 12 14 16

Limiting Magnitude

Lo

g o

f S

tar

Co

un

ts p

er 9

0' f

ield

Galactic Longitude 18

View looking down on disk of galaxy

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Galactic Latitude 19

Side view of galaxy

Galactic Coordinate of Some Stars 20

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

Observational Data: as a Bar Graph 22

Galactic Star Counts

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13

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2.3 2.95

1

0

5

10

15

20

25

30

7.5 22.5 37.5 52.5 67.5 82.5

Galactic Latitude

Ave

rag

e N

um

ber

of

Sta

rs p

er F

ield

Data: Line Graph 23

Galactic Star Counts

y = 35.33e-0.04x

R2 = 0.94

0

5

10

15

20

25

30

0 20 40 60 80 100

Galactic Latitude

Ave

rag

e N

um

ber

of

Sta

rs p

er F

ield

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).

Thickness of Galaxy 24

Calculate it from the ratio of counts of stars at pole and at equator (assume D=1000 pc)

Galactic Star Counts

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13

8

2.3 2.95

1

0

5

10

15

20

25

30

7.5 22.5 37.5 52.5 67.5 82.5

Galactic Latitude

Ave

rag

e N

um

ber

of

Sta

rs p

er F

ield

NpN0

parsecs65828

1)1000(2

N2DT

3

3

0

p

N

5

2c. Interstellar Reddening

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

17Extinction of Light

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•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!

Corrected Space Penetration18

Limiting Magnitude vs Space Penetration Power

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

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22.0

24.0

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Distance in Parsecs

Ma

gn

itu

de

absorption 1 mag/kpc

no absorption

Globular Cluster of Stars 28

A2a. Shapley Core (1914-17) 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

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!

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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)

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!

http://en.wikipedia.org/wiki/File:A_Black_Hole%E2%80%99s_Dinner_is_Fast_Approaching_-_Part_2.ogv

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

A3b. Our place in the Galaxy 34

A3c. Rotations of Galaxies 35

•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

A3d. The “Winding Dilemma” 36

Outer stars move slower.

Why haven’t the spiral arms wound up and disappeared a long time ago?

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A3e. The “Winding Dilemma” 37

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)

A3f. Density Wave Theory 38

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

A3f. Density Wave Theory 39 A3f. Density Wave Theory 40

A3f. Emission Neb in M51 41

This shows stellar formationIn in the spiral arms (whereDensity waves bunch up matter

A3f. Emission Nebulae 42

Red is ionized hydrogen gasEmission nebulae are where starshave recently formed.

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A4a. Rotation Curves 43

•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 R

A4b. Rotating Rong? 44

•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.

A4c. What IS Dark Matter? 45

•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?

REFERENCES 46

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

http://en.wikipedia.org/wiki/Supermassive_black_hole