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The Milky Way




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.

Slide 17



Deneb is A2Ia star

m = 1.25

A2  Blue star

Ia  Supergiant

M = -8.8

Distance = 1000 pc

Slide 18

Standard Candles

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

Slide 19

Variable Stars

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!

Slide 20

Variable Stars

Slide 21

Rotation …

Rotation …

Objects in the disk, rotate in the disk.


Open clusters

Young stars

Objects in the halo, swarm in a halo.

Old stars

Globular clusters

Slide 22

… and Formation

… 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?

Slide 23

Missing Mass

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 Msun

Slide 24

Dark Matter

Dark Matter

What causes the mass to keep on increasing?

Don’t see anything there. Thus  “dark” matter.

Brown dwarfs


White dwarfs

Strange matter?

Use gravitational lensing (last lecture) to look for these “dark” objects.

Slide 25

Dark Matter

Slide 26

The Heart of the Galaxy

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.

Slide 27

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