The Milky Way Galaxy

Astronomy 315Professor Lee

CarknerLecture 17

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Via Lactia

The band forms a complete ring We seem to be in the center of a disk of

stars

Clouds of gas and dust block our view so it

is hard to see beyond our local region

Our View of the Milky Way

The Milky Way from Outside

Discovering The Galaxy In the early part of the century Harlow Shapley

found the distance to globular clusters using Cepheid variables

Globular clusters

Unlike stars in the disk, we can see distant globulars Cepheid variables

If we can find luminosity and flux we can get distance (F = L/4d2)

The Center

Shapley found the distance to the globulars and plotted their positions

He found:

We are not at the center of the galaxy

and the disk extends out much further than we can easily see

Changing Views of the Galaxy

How Do We Learn About The Milky Way?

Optical observations

Radio observations

Infrared observations

Observing other galaxies Since we are in the middle of the Milky Way

we can’t get an overview of it

Structure of the Milky Way

Disk

Nucleus

Halo Spherical distribution of old stars and

globular clusters around disk and bulge

Edge-on and Face-on

The Disk The disk is very thin

Younger stars and star forming regions near the center, older stars above and below

Disk exhibits differential rotation (inner parts rotating faster than outer)

Differential Galactic Rotation

Mass and Orbits Finding the properties of a star’s orbit

allows us to find the mass internal to the orbit

M = a3/P2

M = mass (in solar masses) P = a =

You must use the correct units!

Spiral Structure We know that other galaxies have spiral

structure, but it is harder to see the Milky Way’s

We find spiral arms by tracing:

They are not uniformly distributed but

are found in a loose spiral structure How do spiral arms form?

Local Spiral Arms

Density Waves Spiral arms are like traffic jams

This can trigger star formation in the arms

The clouds eventually move out the other side

The spiral arm material changes, only the pattern stays the same

Density Wave

At the Core

The nucleus is the hardest part of the galaxy to observe due to all the gas and dust

One, Sagittarius A* (Sgr A*), may be the center of the galactic core What is it?

Properties of the Core

Stars near the core are very close together and moving very fast

Sgr A* is emitting enormous amounts of energy

A black hole

Massive Black Holes Sgr A* does not move and may have jets and an

accretion disk

Can get mass from orbit of star S2: Period = Semimajor axis =

Formed from stars and clouds of material falling into the center

Our own is very hard to observe

The Halo The halo are stars orbiting in a large

sphere around the galaxy The halo is composed of old stars

Halo stars -- Population II -- metal poor Disk stars -- Population I -- metal rich

Halo stars formed formed early from relatively unprocessed material

Globular Clusters

Size: Shape: Contents:

Globulars are in elliptical orbits around the galactic center

Unlike open clusters in the disk, globular clusters are very tightly gravitationally bound

History of the Milky Way How did the galaxy form?

Basic theory has Milky way forming from smaller protogalaxies Gas stripped out to form disk

Remains of protogalaxies form halo

Next Time

Quiz #2 Covers lectures 10-16