Introduction: Black holes form during the “death” of very massive stars (at least several times the mass of our
Sun). A star consists of a hot core surrounded by many layers of gas. In the core of the star, lighter elements such as hydrogen and helium join together by thermonuclear fusion to form heavier elements such as metals.
This process emits massive amounts of energy in the form of heat and light, which is why stars shine! The radiation emitted in this process exerts an outward pressure, which counteracts the force of gravity pulling the gas towards the center of the star and gives the star its large size. When these two factors balance each other, the star is at equilibrium. However, when the star runs out of fuel in its core, it is unable to support these heavy outer layers of gas. If the dying star is very massive, gravity will pull on the gas and cause the star to become smaller and smaller until its density reaches infinity at a single point, which is called a singularity, or a black hole.
Close to the singularity, the attraction of gravity is so strong that nothing can escape. The escape velocity would need to be higher than the speed of light – so not even light can escape, which is why the black hole is black. (It is not actually a hole, though: there is a LOT of mass, we just cannot see it!)
At a certain distance from the singularity, gravity is weak enough to allow light to escape, thus objects beyond this distance are visible. This boundary is called the event horizon. Objects outside the event horizon still feel the black hole’s gravity, and will be attracted towards it, but they can be seen and can potentially escape falling in. Once objects are within the event horizon, however, there is no return. They will be pulled into the black hole and never come out again.
After the black hole forms, it can grow by absorbing mass from its surroundings, such as other stars and other black holes. If a black hole absorbs enough material, it can become a supermassive black hole, which means it has a mass of more than one million solar masses – but it is still very tiny in size! It is believed that supermassive black holes exist in the centers of most, if not all, galaxies, including the Milky Way.
Usually, astronomers observe objects in space by looking at the light. This is how they study stars.
However, since black holes do not emit any light, they cannot be observed in the usual way. Instead, astronomers have to observe the interaction of the black hole with other objects. One way to do this is to look at the motions of stars around the black hole, since their orbits will be altered by its presence. Another way is to use instruments which can detect X-rays. As matter spirals around the black hole, much like water swirling down your bathtub drain, it becomes so hot that it emits radiation in the form of X-rays.
Materials (per person):
Balloon
Aluminum foil
Ruler
Pin
Balance

Procedure:
1.
Cut 3 pieces of aluminum foil, approximately 30 cm square. (It doesn’t need to be perfect!)
2.
Inflate the balloon so it is not too big for step 3 and tie it closed. (about the size of a grapefruit)
3.
Wrap the balloon in your three layers of the foil. This is your model star. a.
The layers of foil represent the different gas layers of the star. b.
The air in the balloon gives the star its shape because the air is pressing out against the balloon. This is similar to hot core of the star. Inside the star’s core, the energy released by thermonuclear fusion exerts a pressure on the gas layers of the star, which keeps them from collapsing.
4.
Use the balance to find the mass of your model star. Record this value in your Journal. (If you are doing this at home, your parents might have a kitchen scale you could use. If not, you can skip this.)
5.
Simulate the effect of gravity by trying to lightly compress the balloon. Can you do it? (NO!)
6.
When a star reaches the end of its life, it runs out of fuel in the core and is no longer able to hold up the gas layers. Pop the balloon with the pin to simulate this process.
7.
Once again, try to compress the balloon with your hands to mimic the effect of gravity. Now can you do it? (YES!)
8.
This small ball of foil is your model black hole. Find the mass and record it in your Journal. (If you are doing this at home, see note on step 4.)
9.
Answer the discussion questions in your Journal. (RATS!)
Discussion Questions
1.
Why were you not able to compress the balloon until after you popped it?
2.
If a real star were the size of the balloon, is the crumpled ball too large or too small to represent a real black hole? Explain.
3.
What would happen if you used more pieces of aluminum foil to make the gas layers in the star?
Would the star be more massive? What about the black hole?
4.
Compare the masses of your model star and your model black hole. Are they the same? (They should be!)
5.
Recall that density is mass per unit volume (D=m/V). Which has a higher density, the model star or the model black hole? How about a real star and the black hole it forms? Explain.
6.
We know our sun is not massive enough to become a black hole. However, imagine that it suddenly did become one. What would happen to Earth? Explain. (Re-read the introduction if necessary.)