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The Milky Way
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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

Example

Example

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

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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!

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Variable Stars

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Rotation …

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

… 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

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

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Dark Matter

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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Dark Matter

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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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