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The Milky WayThe Milky Way

More than just a candy bar.

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GoalsGoals

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

it?• 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?

1515Optical emission from stars and nebulae

1515Near-Infrared emission from stars – copyright E. L. Wright and COBE

1515Far-Infrared dust emission – copyright E. L. Wright and COBE

1515Radio emission from neutral hydrogen – copyright J. Dickey

1515X-ray emission from hot gas – copyright S. Digel and ROSAT

1515Gamma-ray emission from pulsars and black holes – copyright NASA

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Where are We?• Not at the center of

the Milky Way.• Where is the

center?• Globular Clusters

point the way.

M10 – copyright Credner and Kohle

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You Are Here

1515Near-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?

• Stellar parallax:– Parallax of nearby stars relative to

background stars.– Good out to ~500 pc.

• What about the background stars?• How far are they?

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Standard Candles• “Standard Candles”• If we know how bright something looks,• And we know how bright it should be,• Result Distance• We do this everyday with size.

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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).

• m – M give you distance.

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Example • Deneb is A2Ia starm = 1.25 A2 Blue starIa SupergiantM = -8.8

10pc

distancelog5 10Mm

Distance = 1000 pc

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Standard Candles• Other “Standard Candles”• Variable stars.• Stars that change in luminosity.

– 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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30 kpc

500 pc

8.5kpc

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Variables in Clusters

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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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The Sun’s “age”

• We define our age by trips around the Sun.

• How many trips of Sun around Milky Way?

R = 8.5 kpcV = 220km/sP = 2.5x108 yrs

• 20 trips.

RV

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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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Supermassive Black Hole

• 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!

• Event Horizon ~ 0.02 AU!

• Probably in the centers of all spiral galaxies.

Copyright Eckart & Genzel

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Homework #15Homework #15

• Read: Bennett Ch20.1 - 20.3:• Do Ch20:

– Problems 3, 12