The Universe in the Classroom

Black Holes

Activity #1: Shrinking


To demonstrate how a black hole might be formed.


two balloons (small, round size) two large-mouthed glass jars refrigerator marking pen


  1. Prepare two separate jars, with balloons inflated inside each one.
  2. Hold each balloon so that its mouth is above the edge of the jar, and the remaining part of the balloon is inside the jar.
  3. Inflate the balloons inside the jars.
  4. Tie the openings of the balloons closed.
  5. Mark the balloons just above the top edge of the jars with the marking pen.
  6. Place one Jar in the freezer for 30 minutes, and place the second jar on a table so that it remains at room temperature.
  7. After 30 minutes, remove the jar from the freezer.
  8. Observe the position of the mark on both balloons.

The balloon at room temperature remains unchanged, but chilling the balloon in the freezer causes it to shrink and sink into the jar.
At room temperature, the mark on both balloons remains the same. But after one is refrigerated and shrinks, the marks are at different levels.



Gas inside the balloon pushes out, and outside air and the elastic surface of the balloon pushes in. The size of the balloon remains the same as long as the outward gas pressure and the inward elastic pressure are equal. This was the case with the balloon which remained at room temperature.

The balloon in the freezer shrank when the, inside gas pressure decreased, gas pressure is proportional to the gas temperature. If the inside gas pressure continued to decrease, the inward force of the elastic surface of the balloon would cause the balloon to become smaller and smaller. It is the balance between the outside air pressure and the elastic surface pushing in, and the gas pressure inside pushing out, which can demonstrate the formation of a black hole. The nuclear reactions at the center of a star produce an outward gas pressure. As long as the outward pressure balances the inward pull of gravity, the star remains stable in size. When the nuclear reactions stop, the balance is upset, and gravity pulls the star's materials toward its center. If the mass of the star, and therefore its gravity, is large enough, then nothing can prevent the shrinking from continuing until the star is so small that it was invisible. It becomes a black hole.

Balloon diagram 1
Balloon diagram 2
Balloon diagram 3
1) At room temperature, the pressure of gas inside the balloon pushing out balances the pressure of air outside the balloon pushing in. 2) As the balloon cools in the refrigerator, the pressure inside decreases. The pressure is determined, in part, by how fast the gas molecules move. As the temperature cools, the molecules slow down, resulting in a decreased pressure inside the balloon. Without enough pressure to resist the inward push of the outside air, the balloon shrinks. 3) The balloon stops shrinking when the pressure inside once again balances the outside pressure. Even though the gas molecules inside are moving slower (due to lower temperature), they are moving within a smaller volume, and so the inside pressure starts to increase as the balloon continues to shrink. Once the two pressures are equal, the shrinking stops.

"Shrinking'' from Astronomy for Every Kid, by Janice Van Cleave. Copyright 1991, John Wiley and Sons, Inc. Used with permission.

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