9B9B

The Milky WayThe Milky Way

Our Galactic Home

9B9B

9B9B

GoalsGoals

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

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

9B9B

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

9B9B

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

9B9B

You Are Here

9B9B

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

9B9B

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?

9B9B

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.

9B9B

Example• Deneb is A2Ia star

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

10pc

distancelog5 10Mm

Distance = 1000 pc

9B9B

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

9B9B

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!

9B9B

9B9B

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

9B9B

… 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?

9B9B

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

9B9B

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.

9B9B

9B9B

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.

9B9B

200 pc

5 pc

Sagittarius A* - Sgr A*

9B9B

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