Today we started by asking, “How far away are the stars? Are they all on a sphere? Are they different distances away? How do we measure how far away they are?” I asked them to hold up a thumb, close one eye and hold their thumb so that it covered up one of the parents heads, who was standing in the far end of the room, … then switch eyes! The thumb is no longer covering the parent’s head – this is parallax. In the pictures below I held my iPhone camera by my left eye and then the right, I did NOT move my thumb. Each eye is in a slightly different location and my thumb appears to move with respect to the trees in the background. Stars do the same thing and we call it stellar parallax. The Earth is moving around the sun, giving us different views of the night sky from different positions (like our right and left eye). Stars that are closer to us will appear to shift, like our thumb, with respect to stars that are further away (like the trees in the photos).
In the top diagram, the blue star, which is closer to us, appears to the left of the more distant ‘pink’ stars when we’re viewing from Earth in the ‘now’ position. But, 6 months later when our position has moved to the other side of the sun, the blue star now appears to be to the right of the ‘pink’ stars. You can see how this effect depends on the distance of the blue star from the more distant reference stars by looking at the bottom diagram. The blue star is further from us, and the effect – its apparent change in position is not as dramatic as the first diagram.
To really understand this the kids made an angle measuring device and found the distance to a paper star that I put in my living room by measuring the parallax angle with respect to a more distant paper star. I used the “Measuring Parallax” activity from the Universe at Your Fingertips dvd that I’v mentioned before. All I had to do was print out the pages for the measuring device (basically a protractor with pointers for lining up the stars) on card stock and let the kids cut them out and put them together. They measured the angles between the reference star and the star of interest from two different locations and using the sheet included in the activity, could figure out the distance to the star of interest. Most got a fairly reasonable answer. This activity was nice for younger kids because it has a table to look up their parallax angle, so they didn’t have to do any geometry or math besides some subtraction and multiple by 1/2 (the distance between where they took their measurements was 1/2 meter).
The other thing we talked about today was the Doppler effect. I made a doppler ball based on the instructions I found on planetseed.com (the link doesn’t appear to work any more so I took it down). Its basically just a buzzer from Radio Shack and a 9V battery stuffed inside a tennis ball. I spun it around while the kids listened to the frequency of the buzzer get higher and lower as it went toward and away from them. I then had them draw the buzzing ball and draw (or paste the pictures I provided) the sound waves coming from the ball. Then we talked about how light waves do the same thing – bunch up as the sources moves toward you, or spread out as the source moves away from you. But we don’t hear light waves, we see them and that change in frequency is actually a change in color so stars moving away appear to be red shifted. For a bit of comic relief, I showed the clip of Sheldon from Big Bang Theory in his Doppler Effect halloween costume:
When everyone finished taping everything into their books we had cupcakes to celebrate the birthday of one of my students… any excuse for cupcakes! Next week we start Unit 3: Stars.