Having difficulties explaining black holes to your students? Why not try these simple activities in the classroom?
Many young people have heard of black holes and understand that if something falls into one, it cannot get out again – even light cannot escape. That is how a black hole gets its name: it is a point in space that does not emit any light (figure 1). This is not an easy concept to explain. In this article, therefore, I briefly introduce black holes and then describe two simple activities to help school students to visualise what is happening. Each activity should take about an hour; both are suitable for pupils aged 10-14 (although note that the reviewer suggests using the activities with students aged 10-19).
collapsed star or singularity;
the event horizon, a region
around the singularity where
not even light can escape;
and the region outside the
event horizon, where objects
can feel the gravity of the
black hole without becoming
trapped. Click on image to
Image courtesy of Monica
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 gasw1. In the core of the star, lighter elements such as hydrogen and helium are joined together by thermonuclear fusion to form heavier elements such as metals. The heat created in this process exerts an outward pressure, which counteracts the force of gravity pulling the gas towards the centre of the star and gives the star its large size. When the star runs out of fuel in its core, however, 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 (figure 2).
Close to the singularity, 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 in there, although we 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 sucked inside the event horizon, however, there is no return.
After the black hole forms, it can grow by absorbing mass from its surroundings, such as other stars and other black holesw2. 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. It is believed that supermassive black holes exist in the centres of many galaxies, including the Milky Way.
Usually, astronomers observe objects in space by looking at the light; this, for instance, is how they study stars (for example, see Mignone & Barnes, 2011). 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 presencew3.
This activity will demonstrate to students how a black hole is formed through the collapse of a massive star, once the core of the star is unable to support the weight of the outer layers of gas surrounding it. The time needed should be about one hour.
Each working group will need:
Answer: The crumpled ball is much too large to represent a black hole. Even a real black hole, formed from a massive star, is smaller than the tip of a pencil.
Building the star with more layers of gas (represented by the foil) would make the star more massive. It would also result in the formation of a more massive black hole, since there would be more material with which to form the black hole.
Although they have a different size, the star and black hole have the same mass, since they are made from the exact same amount of material. However, since the black hole is smaller, it has more material contained in less volume, and therefore has a higher density.
Each working group will need (figure 3):
When the speed of the marble is high enough, the marble has enough energy to escape the gravity of the black hole. However, if the speed of the marble is too low, the force of gravity from the black hole is too strong and the marble will not be able to escape.
Because more massive objects create a stronger gravitational force, in both cases you will need to throw the marble harder for it to escape the gravity of the black hole.
If a black hole becomes massive enough, stars that pass nearby will become trapped in its gravitational field and begin to orbit the black hole, much as the planets in our Solar System orbit the Sun. By observing the motions of many stars, astronomers can look for stars that have orbits around the same central point. If they cannot see an object at this central point, this is evidence that a black hole could be present there.
Activity 1 was adapted from the ‘Journey to a Black Hole’ demonstration manual on the Inside Einstein’s Universe websitew4. That activity was in turn adapted from the ‘Aluminum Foil, Balloons, and Black Holes’ activity on NASA’s Imagine the Universe websitew1.
Activity 2 is adapted from a resource in the UNAWE database by Ricardo Moreno from Exploring the Universe, UNAWEw5 Espagna.
Click here to find the original of Activity 1.
Székely P, Benedekfi Ö (2007) Fusion in the Universe: when a giant star dies.... Science in School 6: 64-68.
Boffin H, Pierce-Price D (2007) Fusion in the Universe: we are all stardust. Science in School 4: 61-63.
Rebusco P, Boffin H & Pierce-Price D (2007) Fusion in the Universe: where your jewellery comes from. Science in School 5: 52-56.
Rosenberg M (2012) Creating eclipses in the classroom. Science in School 23: 20-24.
Jeanjacquot P, Lilensten J (2013) Casting light on solar wind: simulating aurorae at school. Science in School 26: 32-37.