The Milky Way Galaxy

Roger Smith, CTIO

The Sun is one of about 200 billion stars in the Milky Way Galaxy
The Milky Way is a spiral galaxy, with a flat disk, central bulge and bar, and spherical halo
The disk contains stars, gas, and dust.
The halo contains stars and globular clusters

First a little History:

The first efforts to map the structure of the Milky Way through star counts concluded that we were in the center of the Milky Way, because extinction of dust was neglected

William Herschel's Picture of the Milky Way (18th century):

Harlow Shapley (Harvard Observatory) used RR Lyrae Stars in Globular Clusters to map the halo of the Milky Way in the early part of the 20th century:
He realized that the Sun is not in the center of the Milky Way
.

The modern view of the structure of the Milky Way uses IR and radio maps to sort out the structure of the disk, and map out the spiral arms through the dust.

Artist's Conception of the Milky Way:

Here's the actual structure of the spiral arms of the Milky Way:

If you could fly out of the Milky Way and look back on it, it might look something like this:

Amazing Fact #1: Stars are forming today out of the molecular clouds in the disk.

Star formation is triggered by spiral density waves, which gives a spiral disk its characteristic "arms".

Amazing Fact #2: Despite the appearance of galaxies in these pictures, the Milky Way is mostly empty space.

A few analogies:
If the Sun were the size of a baseball, the density of the stars in our galaxy would be comparable to scattering fifty baseballs across the United States, so that there would be one star per state.

If the distance from home plate to the pitcher's mound were equal to the distance from Earth to the Sun, the next star would be 800 miles away.

Amazing Fact #3: About 90% of the Mass in our Galaxy is something besides stars, planets, gas or dust.

How do we know the mass of the Milky Way?

We use the Doppler shift to measure the velocities of stars and gas clouds in the disk of the Milky Way, and in the Halo.
We find that the disk is rotating. The Sun orbits the center of the Galaxy once every 220 million years. So since the Sun was formed, it's been around the Galaxy about 20 times.
The stars in the halo and globular clusters are not rotating, but are in more radial orbits.
From the velocities of all these objects, we can compute what mass the Galaxy must have to keep everything gravitationally bound (ie. to keep everything from flying away). The answer is:

HUGE -- one trillion solar masses.
And the "edge" of the Milky Way is probably 200,000 parsecs from the center -- far beyond the edge of the visible stars.

So 90% of the Mass of the Milky Way is not the stuff we see (stars, gas clouds, dust) then what is it?

"DARK MATTER"

Neutrinos? (probably not)
MACHOS? (Massive Compact Halo Objects, planets, bricks, brown dwarfs)
WIMPS? (Weakly Interacting Massive Particles -- particles which interact through the weak force)

We really have no idea.

AMAZING FACT #4: At the center of the Milky Way is a BLACK HOLE which has a mass of about 2 million solar masses.

**Sagitarrias A* - the radio source at the very center of the galaxy. The Black Hole probably "swallows" a star like the Sun every 5000 years or so.**

High Resolution IR pictures of the Galactic Center since 1992 show the motion of stars around the Black Hole.

ANIMATION

Another amazing fact:

Galaxies are merging with the Milky Way today: e.g. the Sagittarius Dwarf