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.

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Example

Deneb is A2Ia star

m = 1.25

A2  Blue star

Ia  Supergiant

M = -8.8

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 Msun

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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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