Twinkle, Twinkle Little Star?
By John Cowens

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Heat currents in the atmosphere cause the twinkling of starlight. Light rays from stars are refracted or bent as they pass through cells (masses) of warm, less dense air into cells of cooler, denser air. This causes the path of the light rays to bend slightly many times each second, producing the twinkling effect. Heat currents cause focusing problems for astronomical telescopes used for photography; the star images dance around on the film and create fuzzy disks. The use of image-sensing and computer processing negate the twinkling effect somewhat, but the best way to minimize the problem is to place telescopes at high altitudes, such as on mountain tops or in orbit above Earth's atmosphere.

Materials:

• food-warmer fuel such as Sterno® or an electric hot plate


• matches (if using fuel)


• slide projector


• "star" slide materials:


• 35 mm slide frame


• aluminum foil


• pin


• scissors


• projection screen


• dark room

How to make a "star" slide

1. Obtain a 35 mm slide mount from a camera store.
2. Cut a small square of aluminum foil to fit the slide frame.
3. Using the pin, make about 20 or 30 pinprick holes in the foil. The slide is ready to be used. (See picture below.)

Note: An overhead projector can also be used. Make the aluminum foil slide large enough to cover the projector stage.

How to make heat currents

1. Darken the room and turn on the projector with the star slide. Observe the appearance of the stars.
2. Light the food-warmer fuel can (or hot plate) and place it near the projector lens between the projector and the screen.

   CAUTION: The alcohol fuel in the warmer can is ideal for this activity because it produces heat with little light. Be careful when handling the can in the darkness.

3. Stand back and observe the optical effects on the screen.

Extension
Encourage your students to observe the effects of heat currents rising from asphalt on hot summer days or from hot radiators on winter days.

Reference
These lessons were adapted from NASA's Space-Based Astronomy – Teacher's Guide with Activities. This publication is in the public domain and is not protected by copyright. Permission is not required for duplication. EG-103 August 1994.

John Cowens teaches sixth grade at Fleming Middle School in Grants Pass, OR.

October 2006, Vol.37, No.2