Finding the Milky Way in the Galaxy

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Use known luminosity to determine distance

Flux versus luminosity relation

We can figure out the luminosity of a pulsating star by timing the pulsations. Since, we can measure its flux, we can then find the distance to the star.

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Size of Milky Way

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Luminosities of components

With distances, can do star counts, correct for dust absorption and estimate luminosities

Disk: 19109 L

Bulge: 2109 L

Halo: 2109 L

Total: 23109 L

Average star is dimmer than the Sun, total number stars is ~200 billion.

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Review Questions

What are globular clusters? How are they distributed in the Galaxy?

What are Galactic coordinates?

Why do some stars pulsate? Why are pulsating stars useful in measuring distances?

What is the size and shape of the Milky Way?

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Orbits of stars in the Milky Way

Stellar orbits in disk and halo

Finding the mass from the orbit

Mass of the Milky Way

Rotation curves

Dark matter

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What keeps the planets in orbit around the Sun?

The force of gravity from the Sun

To orbit, a planet at a particular distance from the Sun must have a particular orbital speed.

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Orbits of stars in the Milky Way

The orbit of a star is determined by the total mass lying inside the orbit

By measuring the speed of the star’s orbit and its distance from the center, we can figure out the total mass lying inside the orbit of the star

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Stellar Orbits in the Galaxy

Stars in the disk all orbit the Galactic center:

in the same direction

in the same plane (like planets do)

they “bobble” up and down

this is due to gravitational pull from the disk

this gives the disk its thickness

Stars in the bulge and halo all orbit the Galactic center:

in different directions

at various inclinations to the disk

they have higher velocities

they are not slowed by disk as they plunge through it

nearby example: Barnard’s Star

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