Activity 5:
How Big is Big? How Far is Far?

In this activity, you will relate the size of the galaxy to familiar objects. You will also learn about the relative distances to stars, globular clusters, and galaxies.

Materials
chalkboard, chalk

Background
Astronomical distances are almost impossible to comprehend. Even though astronomers talk about millions of light-years, it is difficult to gain any real understanding of how far these distances are. Comparing astronomical distances to known and more comprehensible distances and sizes may help.

Milky Way Galaxy

Procedure
Imagine you are outside at night, looking up at the sky. What can you see? Before you read the next paragraph, make a list of the objects that would be visible to you.

Your list may include planets, the Moon, comets, and meteors. These are all part of our solar system. But you can also see stars, which are outside of our solar system. If you know where to look, it is easy to see open star clusters (such as the Pleiades) and globular clusters. If you observe the sky from a really dark spot, you can see our own galaxy, the Milky Way. The most distant object you can see easily is the great galaxy in Andromeda, M31. It is about 2 million light-years away. How far is that?

From Earth, we can only see about 6,000 individual stars. This is a tiny fraction of the stars in our galaxy. On the chalkboard, make the tiniest dot you can with the chalk. This dot probably contains a few thousand grains of chalk, so it represents the 6,000 stars we can see from the Earth. Now, draw the Milky Way on the chalkboard (copy the galaxy in the picture), and shade it in with the side of the chalk. From a distance, you cannot see the individual chalk grains because they are too small. Imagine that each grain of chalk represents a star, and possibly a solar system like our own. How many grains of chalk do you think there are in your picture of the Milky Way?

The Milky Way Galaxy and the Andromeda Galaxy each contain several hundred billion stars. Although we cannot see the individual stars in distant galaxies with our unaided eye, we can see the collected light from the stars in these galaxies. The farther away an object is, the more stars it must contain to be visible to our unaided eye. We can see stars that are nearby. Open clusters contain hundreds of stars, so they can be seen from a greater distance. Globular clusters contain hundreds of thousands of stars, so they can be seen at even greater distances. Finally, galaxies are the most distant objects we can see with our eyes. But galaxies are not the largest objects in the universe!

Additional Activities
Explore the following questions:

  1. Find out what "objects" in our universe are the largest. Why are we unable to "see" these objects with our eyes?
  2. Quasars are not the largest objects in the universe, since some appear to be inside galaxies. However, they are the most distant objects we can see with telescopes. How can this be? Find out what characteristic of quasars enables them to be seen from such great distances. How far away are the most distant quasars?