9B The Milky Way Our Galactic Home. 9B 9B Goals Structure of our Galaxy. Its size and shape. How do stars and things move through it? Mass and Dark Matter.

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The Milky WayThe Milky Way

Our Galactic Home

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GoalsGoals

• Structure of our Galaxy.• Its size and shape.• How do stars and things move through

it?• Mass and Dark Matter.• The Galactic Center.

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The Milky Way

• Stars• Dust• Gaseous Nebulae• Open Clusters• Globular Clusters• Pulsars• Black Holes• How do they all fit together to make

our galaxy?

9B9BOptical emission from stars and nebulae

9B9BNear-Infrared stellar emission – copyright E. L. Wright and COBE

9B9BFar-Infrared dust emission – copyright E. L. Wright and COBE

9B9BRadio emission from neutral hydrogen – copyright J. Dickey

9B9BX-ray emission from hot gas – copyright S. Digel and ROSAT

9B9BGamma-ray emission from pulsars and black holes – copyright NASA

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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

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You Are Here

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9B9BNear-Infrared stellar emission – copyright E. L. Wright and COBE

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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?

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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.

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Example• Deneb is A2Ia star

– m = 1.25 – A2 Blue star– Ia Supergiant– M = -8.8

10pc

distancelog5 10Mm

Distance = 1000 pc

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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

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Variable Stars• For RR Lyrae stars:

– 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!

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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

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… 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?

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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.

• M = 1011 x Msun32 aP

32

2 4a

GMP

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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.

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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.

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200 pc

5 pc

Sagittarius A* - Sgr A*

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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