Activity: Demonstrating the Distances of Stars

Students will -

1. study how stars may appear to be equidistant from the Earth but are in actuality not,
2. use a model to demonstrate the arrangement of stars,
3. study the importance of the parallax effect in determining distance.

When early astronomers observed the sky, they imagined the vast array of stars embedded in a giant "crystalline sphere" that encompassed the Earth and the Sun. They believed that the planets resided on a smaller transparent sphere concentric to the one on which the stars moved.

Only over time did astronomers realize that as the Earth moved around the Sun, some stars shifted in position relative to the background of others. They realized that this shifting occurred because of a parallax effect when the Earth was on opposite sides of the Sun.

In this activity, students will use a piece of black paper to create a sample star field. They will transform this two-dimensional star field into a three-dimensional star field by shining light from an overhead projector across the room.

20-30 minutes

High School and Middle School (Extensions for advanced students are given below.)

• Black construction paper
• Single-hole punch
• Pencil or pen
• Pin
• String, cut into 2-meter lengths
• Thumbtacks
• One-inch Styrofoam balls
• Tape
• Large black sheet
• Footstools or chairs to allow students to reach the ceiling

1. Attach each Styrofoam ball to the end of a length of string by using bits of tape.
2. Make several holes in the black construction paper. Place holes in random locations about the paper. Make them a variety of sizes by using the single-hole punch, the pen or pencil, and the pin.
3. Tack the black sheet to the front of the classroom to form a screen for the overhead projector.
4. Place the overhead projector in the back of the classroom to make its image as large as possible on the black screen.
5. Place a construction-paper star field on the projector so that the holes in the paper create points of light on the sheet.
6. Explain to students that we on Earth see the stars in the sky as if they were on a flat plane. However it only appears as though they are all the same distance away, since our eyes can't detect their relative distances because they are points of light.
7. Give each student a ball and string. Ask them to find a star beam by moving their hands in the area between the overhead projector and the sheet. When they find a star beam, have them tack their string to the ceiling in a manner that allows the star beam to strike directly on the sphere. Encourage students to place their spheres at varying distances from the overhead projector to increase the 3-D effect of the demonstration.
8. Gather students in the back of the classroom and explain that this arrangement is more representative of the arrangement of stars. They are at different distances from the Earth, but because they are ALL so far away from the Earth, we have a difficult time seeing their varying distances.
9. Optional: To emphasize the need for parallax to determine distances, have kids pair up and stand at arms' length facing their partner. One student should hold up a finger. The second should reach out and grab hold of it. Explain that they used both their eyes and the principle of parallax to determine the distance with which they needed to reach out and grab the other's finger. Now have students repeat the process with one eye closed. The difficulty results from the lack of parallax with only one eye.

Have advanced students use astronomy reference books to pick a constellation and identify by name the stars in the constellation. Students should look up the distance in light-years that those stars are from Earth. Have these students reproduce their constellation on their black construction paper and, using an appropriate scale, place their spheres at the appropriate distances for the stars they represent.