Chapter 24: The Milky Way Galaxy

Our solar system is located in the Milky Way Galaxy. Our Sun is one of hundreds of billions of stars in the Milky Way. And the Milky Way is just one of billions of galaxies in the universe. In thinking about the Milky Way we move to distance scales that are enormous, and when we discuss other galaxies in the following chapter, the distances becomes absolutely stupendous!

24.1 The Architecture of the Galaxy

What does our Galaxy look like?

Trying to determine how the stars in our neighborhood are organized was first attempted by William Herschel in the late 18th century. He did this by measuring the direction and distance to stars, and looking at the resulting distribution. This is a bit like standing on a football field with the marching band, and trying to figure out what letter you spell out. You would figure out where the other band members are standing, and mark them on a sheet of paper. Then when you had enough people marked, you could figure out what letter you had.

Herschel did the same for nearby stars and concluded (correctly) that the Galaxy is flat and round, like a disk or wheel, and (incorrectly) that the Sun is at the center. He could tell that the Galaxy is rather flat, since there were few stars in the "up" and "down" directions and many in the "left", "right", "forward", and "backward" directions. Similarly, on the football field, you would not see any band members above your head or below your feet (I hope!).

One problem in determining the real size of the Galaxy is that the dust in the center of the Milky Way keeps us from seeing the center and anything on the other side in visible light. Harlow Shapley got around this problem by measuring the directions and distances to globular clusters. He used the RR Lyrae and Cepheids to measure the distances. The globular clusters are distributed outside the center of the Galaxy, especially above and below. He found that they were not centered on the Sun, but rather on a point in the direction of Sagittarius.

Overview of the Galaxy

Measurements in the infrared and radio spectrum have helped astronomers fill in the details of the shape of the Galaxy. Infrared and radio waves can pass through the dust in the center of the Galaxy without much attenuation.

The Milky Way is a thin rotating disk, about 100,000 LY in diameter and only 1,000 LY thick. As shown in Figure 24.6, the stars are not distributed uniformly in the disk, but instead are concentrated in spiral arms. The Sun is located about halfway out on one of these arms. From the side, we see the disk, the nuclear bulge formed at the center, and globular cluster distributed around the Galaxy.

Understanding the structure of our Galaxy is a major accomplishment. Astronomers were aided by the existance of other galaxies with similar shapes. We'll discuss the shapes of galaxies more in the coming chapter.

24.2 Spiral Structure of the Galaxy

The Milky Way has four principal spiral arms and smaller spurs. The Sun is located in one of the spurs called the Orion arm. We can see and map out three of the four arms. The fourth arm is on the other side of the center and therefore drowned out by the radiation from the center.

How the spiral arms formed and are maintained is still an issue.

24.3 Stellar Populations in the Galaxy

Two Kinds of Stars

Observations of the stars in the Galaxy reveal two distinct types, inventively called population I and population II stars. These types of stars differ in their locations in the galaxy and their chemical compositions. Population I stars are found in the disk, especially the spiral arms, and their composition includes heavy elements. Population II stars are not correllated with the spiral arms, and lack heavy elements. The conclusion is that the population II stars are old stars, made early in the history of the galaxy before heavy elements became available. They (or their now dead brothers and sisters) presumably created the heavy elements that are now found in the population I stars. The population I stars are younger, presumably formed later, perhaps initiated by supernova explosions of the heaviest population II stars.

24.4 The Mass of the Galaxy

Now that we can define what's in the Galaxy, we'd like to measure its mass. This is done the same way we determine the mass of the Sun or planets, by measuring the orbits and using Kepler's law. The Sun is moving in an orbit around the center of the Galaxy with a speed of 220 km/s. It takes the Sun about 225 million years to orbit the Galaxy. Therefore, in its 4.5 billion year life, the Sun has made about 20 revolutions.

We can use not only the Sun, but many other stars to determine the mass in the Galaxy. Actually, each only determines the mass that lies within its orbit around the Galaxy. The orbital radius and period for the Sun about the galactic center tells us that the mass inside the orbit of the Sun is about 1011 Msun (100 billion times the mass of the Sun).

We can continue doing this for other objects that orbit the galaxy: stars, clusters of stars, and even clouds of gas. One would think that far from the center of the Galaxy, where the orbits enclose all the visible stars, we should get the final value for the mass of the Galaxy. Surprisingly, this isn't the case. As we use more distant objects to measure the mass of the Galaxy, the total mass we get keeps increasing. It's as if there's more "stuff" beyond the luminous edges of the Galaxy.

A graph like Figure 24.14 is used to demonstrate what is happening. This graph shows the orbital speed of objects at different distances from the center of the Galaxy. As we move far from the center, we expect that the orbital speed will decrease, as shown by the blue line. This is what happens for the planets -- Mercury, the planet closest to the Sun has the highest speed, and the speed decreases as we move out to Venus, Earth, Mars, ... But in the Milky Way, this isn't the case. The red line in the graph is the actual measured orbital speeds for distant objects, and it increases with distance. (The reason for the complicated shape at small distances is that, unlike the solar system, the mass in the Galaxy is distributed over many stars, and not concentrated in one big object at the center of the Galaxy.) The conclusion is that something is creating more gravity at large distances from the center of the Galaxy than we can account for from the luminous matter.

There are two possibilities:

  1. The law of gravity is wrong in that region of the Galaxy.
  2. There is additional non-luminous mass in that region of the Galaxy.
Except for this measurement, there is no other evidence that gravity behaves differently in that region. Therefore, the most simple and obvious explanation is that there is additional non-luminous mass, especially in the outer parts of the Galaxy. Where does the non-luminous mass come from?

We call the non-luminous mass dark matter. There are a number of possibilities for the composition of the dark matter.

Experiments are underway to test these and other hypotheses.

How much dark matter is out there? Is it really worth worrying about? Well, the measurements in our Galaxy and other galaxies indicate that about 90% of the mass in the universe is dark matter! This is astounding! It is possible that, not only are we not the center of the Galaxy, the stuff we and our planet are made from may not even be the primary component of the universe!

24.5 The Nucleus of the Galaxy

What is at the center of our Galaxy? Dust obscures our view of the center, and the region is densely packed with stars, making observations doubly difficult. Yet, there is mounting evidence that a massive black hole lies at the center of the Galaxy. The object has a mass greater than 2.6 million Msun, and a diameter smaller than Jupiter's orbit.

Further evidence for this object being a black hole is the observation of black hole candidates at the centers of other galaxies.

24.6 The Formation of the Galaxy

Basically skip this section.

© Robert Harr 2001