The Milky Way1. Attempts to survey

1. Herschel2. Milky Way Structure

1. Thin & Thick disks2. Milky way stats

3. Motion of Stars1. Measuring the rotation curve

4. Dark Matter5. The Galactic Center

1. If we were closer...2. Supermassive Blackhole

Attempts to survey

HerschelIncorrect assumptions lead to this model of the galaxy, where our sun wasroughly near the center. We can't see all the stars in our galaxy due tointerstellar dust.

Globular ClusterGlobular Cluster

A dense cluster of old stars.

Fig.1Herschel's 1785 map of thegalaxy.

doi: 10.1098/rstl.1785.0012 Phil. Trans. R. Soc. Lond. 1785 vol. 75213-266

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Variable StarsCepheids

Absolute magnitude is related to period:

where is the absolute magnitude in the band, measured over one complete period.

Fig.2Messier 68, a globular cluster.Might have 10 stars within a small (30pc) diameter.

ESA/Hubble & NASA

6

Fig.3Brightness of some stars wasfound to be periodic.

Fig.4

Ryden & Peterson Fig 17.7

Fig.5

Periods of 25 Variable Stars in theSmall Magellanic Cloud. Leavitt,Henrietta S.; Pickering, Edward C. ;Harvard College Observatory Circular,vol. 173, pp.1-3

Fig.6"A straight line can readily bedrawn..."

Periods of 25 Variable Stars in theSmall Magellanic Cloud. Leavitt,Henrietta S.; Pickering, Edward C. ;Harvard College Observatory Circular,vol. 173, pp.1-3

= −2.76 log( )− 4.16M̄v

P

10 days(1)

M̄V V

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With this relation, if we know and , then we can get distance, .

Obtaining this relationship requires knowing the distance to at least a few Cepheids. Parallax can get us a few.

Shapley's MapThe realization was that we are not in the center of the globular clusterdistribution. The center is 'over there'. Shapley didn't account for dust, so hisMilky way was about twice as big.

Milky Way Structure

False-color image of the near-infrared sky as seen by the DIRBE. Data at 1.25,2.2, and 3.5 µm wavelengths are represented respectively as blue, green andred colors. The image is presented in Galactic coordinates, with the plane ofthe Milky Way Galaxy horizontal across the middle and the Galactic center atthe center. The dominant sources of light at these wavelengths are stars withinour Galaxy. The image shows both the thin disk and central bulge populationsof stars in our spiral galaxy. Our Sun, much closer to us than any other star,lies in the disk (which is why the disk appears edge-on to us) at a distance of

M m d

M = m− 5 log( )d

10 pc(2)

Fig.7

Astrophysical Journal, 48, 154-181(1918)

Fig.8

image by: NASA/Adler/U.Chicago/Wesleyan/JPL-Caltech

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about 28,000 light years from the center. The image is redder in directionswhere there is more dust between the stars absorbing starlight from distantstars. This absorption is so strong at visible wavelengths that the central partof the Milky Way cannot be seen. DIRBE data will facilitate studies of the

content, energetics and large scale structure of the Galaxy, as well as the nature and distribution of dust within theSolar System. The data also will be studied for evidence of a faint, uniform infrared background, the residual radiationfrom the first stars and galaxies formed following the Big Bang.

Thin & Thick disksThin disk:

M: 6 × 10 M☉

form:

[Fe/H]: -0.5 to +0.3

Thick disk:

M: 0.2-0.4 × 10 M☉

form:

[Fe/H]: -2.2 to -0.5

MetallicityThe ratio of Fe to H in the spectra of stars can be used to quantify a star.

This is called the metallicity. ( and are the number of Iron and Hydrogen atoms atoms.) Stars with the sameproportion as the sun, will have values of [Fe/H] = 0. More metal rich stars have higher positive values. Less metal richstars have negative values.

In general, younger stars will have higher metallicity than older stars.

Milky way stats200 Billion StarsType: Barred_spiral_galaxyDiameter: 100–180 kly (31–55 kpc)Thickness of thin stellar disk: 2 kly (0.6 kpc)Oldest known star ≥ 13.7 GyrWhole thing is moving at 600 km per second w.r.t the extragalactic frame of reference

And our place in it:

Fig.9

NASA, GSFC, COBE

Fig.10Schematic showing the mainfeatures: two disks and two sphericalelements: bulge and halo.

10

e−z/h

h ∼ 0.350 kpc

10

e−z/h

h ∼ 1 kpc

[Fe/H] ≡ [ ]log10( /NFe NH )star( /NFe NH )sun

(3)

NFe NH

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Distance to center: 26.4 ± 1.0 kly (8.09 ± 0.31 kpc)Sun's galactic rotation period: 240 Myr

Motion of Stars

Radial velocity: . Measured using doppler shift of the star's absorption lines.

Tangential velocity: . Measured using proper motion, and distance .

(in the small angle limit)

Space motion is the resultant of the radial and tangential velocities:

sun proper motion (μ)

star

distance

radial vel

tangential velspacevel

Fig.11

Fig.12

Ryden & Peterson, Fig 19.10

vr

Fig.13

Ryden & Peterson, Fig 19.11

vt μ d

μ=vt

d

Fig.14Space motion

v = +v2r v2t− −−−−−√

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Measuring the rotation curveWe would like to know the distance . Let's say we can't measure , but weknow and .

If we look along the line , chances are likely there will be several gas clouds.Each with a different radial velocity. This difference is noted in the doppler shiftof the light coming from some atomic process. For example, moleculescollide with CO molecules and create a radio emission withe wavelengths of2.6 and 1.3 mm, as measured in the lab. Deviations from these speeds areindicative of doppler shifts due to relative velocities.

We can measure these different velocities for the different gas clouds along theline of sight. The fastest will likely be closest to . (this is a limitation of thismeasurement.)

Dark Matter

Ryden & Peterson, Fig 19.12

Sun

Star/Cloud

GalacticCenter

R0

R

Rmin

d

l

Fig.15Geometry for measuring the Rvalue for a star/cloud.

R d

R0 l

velocity

faster

slower

Sun

Star/Cloud

GalacticCenter

R0

R

Rmin

d

l

Fig.16

d

H2

Rmin

Fig.17

Astrophysical Journal, Part 1 (ISSN0004-637X), vol. 295, Aug. 15, 1985, p.422-428, 431-436.

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There's more stuff there that we just can't see, so it's called dark...

Astrophycisists and Cosmology folks are still figuring out what it is.

The leading candidate is WIMPS: Weakly Interacting Massive Particle. These are hypothetical particles that don'tfollow the same rules as the others in the standard model of particles physics.

The Galactic CenterCan't see it too well

If we were closer...Let's move the solar system to half a parsec away from the galactic center.

The nearest star would be ~ 1000 AU awayThe night sky would have 10 stars brighter than SiriusThe total starlight would be ~ 200 times brighter than the moonStars might collide!

Based on the orbital parameters, we can get the semi-major axis of S2:

Fig.18

6

Fig.19Chandra X-ray Image ofSagittarius A*

NASA/CXC/Caltech/M.Muno et al.

= = 1.4 × mS2rp 14

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Based on Kepler's Third law, and a period of 15.24 ± 0.36 yr:

Supermassive Blackhole

Fig.20The orbits of S0-2 (black) andS0-102 (red). The data points and thebest fits are shown. Both stars orbitclockwise

http://science.sciencemag.org/content/338/6103/84

= = 1.4 × maS2rp

1 − e1014

M = ≃ 7 × kg ≃ 3.5 ×4π2a3

S2

GP 21036 106M

Fig.21Accretion Disk

ESA, NASA, and Felix Mirabel (FrenchAtomic Energy Commission andInstitute for Astronomy and SpacePhysics/Conicet of Argentina)

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