Scintillating Little Star
The twinkling of stars is well known—the cause, less so. Explore this “scintillating” phenomenon by beaming a laser pointer over a hot plate.
- Hot plate
- Laser pointer
- A few stacked-up books or cups on which to steady the laser pointer so it’s slightly higher than the hot plate’s surface
- Distant wall or other vertical surface
- Image of an eye, printed from here
- Optional: large binder clip
- Place the hot plate, turned off and cool, on a flat surface, such as a table or desk.
- Near the hot plate, rest the laser on your stack of books or cups so the laser’s beam passes no more than 1 or 2 centimeters over the hot plate’s surface, as shown in the photo below.
- Turn on your laser pointer, using tape to keep it in the “on” position. (You can also use a large binder clip, if that works better.)
- Place the laser flat down on your stack of books or cups. Aim it parallel to and directly over the hot plate’s surface toward a distant wall (the farther, the better), as shown in the photo below. Note: Always use caution when working with lasers. Never point a laser into anyone’s eyes.
- Tape the printed image of the eye to the wall, with the laser beam aimed directly at the center of one of the eyes in the image (see photo below).
Closely observe the laser pointer's "spot" on the image of the eye. Note how steady the spot seems to be. Then turn on the hot plate. (Caution: it will get very hot!)
As the hot plate heats up (this might take a few minutes), watch to see what happens on the spot of light (see photo below). Is it still as steady?
As the hot plate warms up, you’ll see the laser spot start to wiggle and dance, steered this way and that by convection currents in the air.
When air near the hot plate heats up, it becomes less dense. This density change causes the air to rise and mix with the surrounding cooler air in a process called convection. The optical properties of hot air are different from those of cool air. Light passes through hot air more quickly than it does through cool air. This slight change in speed causes the light to bend, or refract, as it passes through air of varying temperatures.
If you were viewing the laser spot from the vantage of the “eye” of the printed image, the laser spot would veer in and out of view, or twinkle.
This same twinkling occurs when you view a distant star. Located trillions upon trillions of miles away, a star appears to us as a single point of light, like the light from your laser. Starlight travels through the vacuum of space unimpeded and in a straight line until it enters Earth’s atmosphere. As it passes through regions of gas of varying temperature, the light refracts, bending this way and that in response to puffs of hot or cold air. You perceive this thermally driven jiggling as a flickering of the light—or the twinkling of the star.
Astronomers call this twinkling effect scintillation, and generally seek to avoid it when looking through telescopes, since it blurs and warps images. Many modern large observatories use adaptive optics to mitigate scintillation, relying on fast computers and adjustable reflecting surfaces to compensate for distortions caused by scintillation. But the best way to avoid distortions caused by our atmosphere is to leave Earth altogether: Orbiting telescopes, such as the Hubble Space Telescope, offer the best possible observational clarity.
This activity is a great launching point for learning about many physics and astronomy concepts, including telescope optics, index of refraction, light rays and their paths of travel, convection, gases, and the atmosphere.
NASA has some great resources related to observing astronomical objects:
Hubble Space Telescope
This Science Snack is part of a collection that showcases female mathematicians and math educators whose work aids or expands our understanding of the phenomena explored in each Snack.
Source: Wikimedia Commons
Maria Mitchell (pictured above) was an astronomer and educator who spent decades predicting how stars would move through the night sky, and one of the first women to discover a comet. She was also the first woman elected to the American Association for the Advancement of Science (AAAS). As a professor at Vassar College, she pioneered what were then unconventional teaching practices: advocating for small classes, individualized attention, and incorporating technology and mathematics into her lessons. Maria believed in the importance of the study of astronomy. She said, “When we are chafed and fretted by small cares, a look at the stars will show us the littleness of our own interests.” Explore why stars twinkle with the Science Snack Scintillating Little Star.