Introduction to Astronomy

Structure of the Milky Way

Since we are a part of the Milky Way galaxy, it is difficult to figure out what it looks like. It is much like trying to figure out what a house looks like from the outside, if you've never been out of the kitchen.

The Center

In 1915, this guy named Shapley (no story here...) decided to try to figure out where we are by mapping out the positions of globular clusters. A globular cluster is a group of hundreds of thousands of stars, all tightly bound together in a ball 20-100 pc across. They have a very distinctive appearance, and so are easy to find.

He discovered that 20% of the clusters lay in the direction of the constellation Sagittarius, and occupied only about 2% of the sky.
He estimated the distances to the globular clusters using RR Lyrae stars, which are all of about the same luminosity (so they can be used as standard candls).
These globular clusters are centered on a point about 15 kpc from the Sun. Shapley reasoned that if the clusters were distributed evenly about the Galaxy (and there's no reason to think they shouldn't be), then the center of the Galaxy lies in the direction of Sagittarius, about 15 kpc away.
Shapley didn't know about dust, so he overestimated the distance to the center of the Galaxy by a factor of two. The center is actually about 8.5 kpc away.

The Shape

We think that the Galaxy is a spiral galaxy, and thus composed of three parts:

The Disk: Most of the stars in the Galaxy are located in a disk about 2 kpc thick, and about 40 kpc across. The disk is composed primarily of hot, young stars, and contains lots of dust and gas. (This is totally logical, and exactly what you'd expect, why?)
The Bulge: The bulge lies in the center of the Galaxy. It is about 6 kpc across, and extends above and below the disk. There is very little gas or dust, and therefore mostly old stars (Why?).
The Halo: The halo is primarily made up of globular clusters, and is roughly spherical in shape. It is centered on the center of the galaxy, and is about 30-40 kpc in radius. Again, there is very little dust and gas---the halo is primarily composed of old stars. Interestingly, most of these stars are low-metallicity stars, which means that they have even fewer metals than the Sun.

A diagram of the Milky Way Galaxy as seen from the side looks like this:

The edge of the Galaxy is ill-defined. It is hard to say where the influence of our Galaxy ends. There are many distant globular clusters, and even a few nearby dwarf galaxies, that seem to orbit the Milky Way. Whether these are considered part of the Milky Way or not is largely subjective.

Rotation

The Sun, and everything else in the Galaxy, is in orbit around the center. The Sun's velocity is 220 km/s. The Sun's galactic radius is 8.5 kpc. If we assume that the Sun goes around the center in a circle, we can easily calculate the amount of time that takes:

P=2·pi·R/v

For the Sun, this comes out to about 240 million years. This is about 5% the age of the Sun, or 2% the age of the Milky Way! This means that the Sun has been around the center of the Galaxy 20 times already!

We can also calculate the amount of mass inside the galactic radius of the Sun, using Newton's law of gravity and a little algebra:

M=v²·d/G = 1.9×10⁴¹kg = 100 billion M_Sun

We can find the galactic rotation velocities and galactic radii for a lot of stars (since we know where we are, we can use the information relative to us, and do a little geometry to find out where things are relative to the center...). If we plot rotation velocity vs. galactic radius, this will tell us the distribution of the mass. This is called a Rotation Curve. Out to about 20 kpc, the Milky Way's rotation curve is flat! Compare this with a solid object, like a record, or with a bunch of disconnected objects, like the solar system. These three types of rotation curves are shown below:

This flat rotation curve implies that the mass of the Milky Way increases as you get further from the center of the Galaxy!

We can estimate the mass of the entire Galaxy from these measurements to be about 6×10¹¹ M_Sun. However, we can't see this much stuff! This is where the idea of dark matter comes from. There's a bunch of stuff out there that we just can't see, even though we know it must be there because it gravitationally interacts with the things we can see.

There are a couple of guesses at the moment about what dark matter might be:

Weakly Interacting Massive Particles (WIMPs): These are things like neutrinos, that have some mass, but barely interact with the rest of the Universe.
MAssive Compact Halo Objects (MACHOs): These are things like Jupiter. Perhaps there are a zillion of these out in the halo. These would be hard to see, because they don't shine on their own. But we might be able to find them via gravitational lensing... Chris Stubbs, here at UW is leading a project to look for these objects...
Black Holes: If there are many of these around that are pretty small, and far from most mass, we'd be hard-put to find them.

Other galaxies also have this dark matter problem, it's not just ours being weird! Also, galaxy clusters have this problem...

Spiral Structure

How do we know about the spiral structure?

Optical Observations: show hot, young stars, and HII regions in 3 big clumps. Out past the nearby spiral arms, our view is obscured by dust.
Radio Observations: look at the velocities of 21 cm clouds of neutral Hydrogen. This research is difficult, and controversial, but seems to support the idea.
Probably the strongest argument is a logical one. We see only three kinds of galaxies "out there": Ellipticals, Spirals, and Irregulars. Ellipticals contain only old stars, and have no disk. Irregulars contain mainly hot young stars, and have no disk. Since we have a disk, and both old and young stars, we are probably a spiral galaxy, since only spiral galaxies have been observed to have all these features.