Chapter 23. Our Location in the Milky Way Galaxy

23.1 Introduction

Have you seen the Milky Way?

The goals of this module: After completing this exercise, you should be able to:

1. Describe the components and stellar populations of the Milky Way Galaxy.
2. Explain how we know the location of the Solar System in the Milky Way Galaxy.
3. Give the modern dimensions of the Milky Way Galaxy.

In this module you will explore:

1. The methods used to determine our location in the Milky Way Galaxy.
2. The main parts of the Milky Way Galaxy.

Why you are doing it: Stars aren't evenly scattered over the sky - certain regions have more stars and dust than others. The image above shows such a region that stretches from horizon to horizon! The hazy, milky band that is called the "Milky Way" can only be seen from a dark location, so many people haven't actually "seen" the Milky Way. Astronomers first began to question our location in what appeared to be a sea of stars in the eighteenth century. In this activity, you will explore how astronomers began to explore our surroundings in space and eventually determined our location in the vast collection of stars that we recognize today as our home galaxy, the Milky Way.

23.2 Background

During the eighteenth century, the brother and sister English astronomers William and Caroline Herschel began an observing program designed to determine our location in the collection of stars that is today known as the Milky Way Galaxy. They developed a systematic program of counting stars in 683 different regions of the sky. While William looked through the telescope and counted stars, Caroline recorded the star counts. They noticed that certain directions had many more stars. Their data revealed an interesting pattern.

For simplicity, imagine a clock with the observer located in the center where the clock hands meet. If we look in the 12 o'clock direction, we see relatively few stars. To see what the Herschels observed, start the animation.

Summary

The pattern that the Herschels discovered was represented in this drawing that William included in his 1785 paper. This map of the Milky Way led them to conclude that the Solar System was located near the center of a thin disk that looked like the lens of a magnifying glass. Since the views were similar in the opposing directions, the Herschels believed that the Sun must be located near the center of this disk of stars.

Herschel's Milky Way map.

23.3 Locating Galactic Center

The next breakthrough in determining our true location inside the Milky Way occurred when Harlow Shapley began studying RR Lyrae stars in globular star clusters. By watching these regular variable stars during an evening, he measured the changes in brightness over time and determined each RR Lyrae star's period of variation. The video below was taken of the globular star cluster M3 during an evening. Can you spot the RR Lyrae variables? Watch the video.

Commonly found in globular clusters, RR Lyrae stars are low mass giants that are burning helium in their cores. Once identified and the period of variation is determined, Shapley used a period-luminosity relation that he had calibrated to determine the distance to each globular star cluster containing RR Lyrae stars. Shapley combined the positions of 93 globular clusters in the sky with their distances to develop a three-dimensional distribution map. To see what Shapely discovered, use the animation below. First explore how the number of globular star clusters appears from the center of the galaxy as you change your angle of view by sliding the angle scale. You must take the cursor off the angle scale to see the view. Next, change your location to the Sun and examine how the number of globular star clusters changes as you vary your angle of view.

23.4 A Dusty Problem

In 1916, Harlow Shapley concluded that the Solar System was located 16,000 parsecs from galactic center. Based on the sphere defined by the globular star clusters, he estimated the galaxy's diameter as more than 100,000 parsecs. These values are about twice the modern values. So what was the problem?

The problem can be seen in the view of the Milky Way shown above (left). Where the stars and gas are concentrated, there are also clouds of dark dust present. This concentration of dust between the stars, interstellar dust, prevents astronomers from seeing objects behind it using optical telescopes. This problem isn't unique to our galaxy; other galaxies like NGC 891 appearing in the image above (right) also show evidence of dust clouds in their disks.

The dust that lies in our own Milky Way Galaxy affects even stars that we can see through the dust with optical telescopes. Look carefully at the two views displayed on the right of the Horsehead Nebula in Orion.

The bright clouds are glowing gas, and the dark swirls are dust. Notice that you can see some faint stars through the dust in the Hubble Space Telescope close-up image. Compare these faint stars to the stars that are brightest in the ground-based image.

23.5 Modern View of the Milky Way Galaxy

A schematic edge-on view of the Milky Way Galaxy

It is possible to calculate the distances to a particular kind of variable star called Cepheids. In addition to the interstellar dust problem, it took decades for astronomers to realize that there were two different kinds of Cepheid variable stars. There are separate period-luminosity relationships for each kind, and many distances calculated for these stars had to be re-done. Since some of the Cepheid distances had been used to calibrate the distances to RR Lyrae stars, Shapley's globular star cluster distances also had to be re-calculated.

With modern distances, astronomers have determined that the globular star clusters define a sphere with diameter of 50,000 parsecs or 50 kiloparsecs (kpc). Globular star clusters reside within this sphere, which is called the halo of the galaxy. The majority of stars, gas, and dust in the galaxy define the disk, which also has a diameter of 50 kpc. The location of the Solar System is 8 kpc from galactic center or nucleus. The disk of the galaxy is relatively thin, but near the galactic nucleus, there is a central bulge of stars, gas, and dust. The figure below shows a cartoon view of our galaxy with its parts labeled.

Stars within our galaxy have different chemical compositions and are classified as either Population I (metal-rich, younger stars like our sun) or Population II (metal-poor, older stars like globular cluster stars).

Since all stars are mostly hydrogen and helium, 'metal-rich' means that these stars have at least 2% elements other than hydrogen and helium, while ‘metal-poor’ stars are nearly pure hydrogen and helium. These different types of stars are not found everywhere in the Galaxy.

Population II stars make up the halo, moving in high velocity orbits around the galactic center. Population I stars orbit within the disk of the Galaxy, and the young, massive stars give the disk a bluish color. The central bulge contains a mixture of Population I and II stars. The older, cooler Population II stars mixed with the Population I stars in the central bulge tend to make this region appear more yellow in contrast to the bluish disk.

Our Milky Way Galaxy is a magnificent collection of stars, gas, and dust. The Sun is one of 200 billion stars, occupying a position about 1/3 of the way out from the center in the disk. This modern representation of our place in the Milky Way Galaxy is a twentieth century discovery.

23.6 Quick Check

Indepth Activity: Our Location in the Milky Way Galaxy