Unfortunately, many (if not all) of your students are probably unfamiliar with the cinematic masterpiece that is The Black Hole. However, they probably do know, or have at least heard, of actual black holes—the massive, super dense, inescapable gravity pits that scientists have been theorizing, hypothesizing, and studying since the late 18th century.
With this simple Demo Science science demo, you can help your students better understand how black holes form* in the depths of space.
For this experiment, you’ll need two small, round balloons (if you can find black ones, that would be perfect), two large-mouthed glass jars, a marker, a refrigerator or deep freezer (talk to your school’s kitchenaires), and a copy of Superunknown.**
Gather your students ‘round your desk and ask for balloon-blowing volunteers. Give the two chosen students (do not pick Kevin) one balloon and one jar each with which to work. Have them hold the balloon so that its spout is above the edge of the jar, with the rest of it inside the jar. Then, have them blow up the balloons are large as they can get them inside the jar—once the balloons hit the sides of the jar, they should stop inflating. They’ll probably be sticking up above the top of the jar, which is good. Make sure the balloons get tied shut nice and tight.
Mark each balloon just above the rim of its respective jar. Then, put one jar in the freezer and leave the other one out in the regular classroom environment. If your Kevin is anything like the Kevin in my class, there’s likely to be some waiting-induced anxiety and antsiness, so you might have to set up one of those industrial safety gates to keep the kids away from the cooler. No peeking until the experiment is complete, you smelly little punks!
After 30 minutes or so, take your balloon jar out of the freezer. Have your students observe and compare the markings on the balloons.
Crossing the Event Horizon of Science
You’ll notice that the balloon that remained at ambient temperature is essentially unchanged. Meanwhile, the balloon you placed in the freezer has shrunk and sunk into its jar. This is because of a change to the pressure balance betwixt the elastic of the balloon (pushing in) and the gases inside the balloon (pushing out), due to the cold.
At lower temperatures, gases condense and produce less outward pressure. If the gas (air) inside the balloon continued to drop in pressure, the elasticity of the balloon would continue to cause it to shrink more and more. The balance of these pressures may explain how black holes form.
The nonstop nuclear reactions that occur at the center of a star—like our sun—produce outward pressure. This pressure remains constant throughout the star’s life cycle, and the star will remain the same size as long as the gravity pulling in on it (star’s generate their own gravity) remains constant, as well, much like your room-temperature balloon.
When those nuclear reactions decrease or stop as the star reaches the end of its life, the balance of gravity and outward pressure is knocked askew. The star’s gravity pulls its materials inward toward the center of its mass. Popular hypothesis suggests that this type of shrinking could continue until the star collapses in on itself, thus forming a black hole.
* Probably—no one really knows for sure. The good news is, you’ll never be proven wrong.
** This last piece is not 100% necessary, but would certainly add to the experience.