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Model of a Black Hole

Model of a Black Hole

Brief Description

Many children have heard of black holes and some have the understanding that they are bottomless wells. If something falls into a black hole, it is impossible for it to escape — even light cannot escape and is swallowed. The lack of light is how black holes get their name. These objects are mysterious and interesting, but it is not easy to explain them simply, but this experience activity will help children to visualize the concept.

Materials

Light elastic bandage used for muscular injuries (i.e. Tubifix)

Small marble

Very heavy ball (such as those used in games of boules, bocce or pétanque)

Learning Objectives

Use an interactive, hands-on activity to introduce children to the important astronomical concepts of black holes, gravity and space-time,

Build a physical model of the space curvature around a massive object and observe the effect on a less massive object,

Demonstrate what happens to an object passing a gravity well, if its velocity is not high enough or if the gravity well is too deep.

Background Information

A black hole is a region in space where gravity is so strong that nothing that enters it can escape, not even light! Black holes form when a massive star runs out of fuel and becomes unable to support its heavy outer layers of gas. If the star is large enough—approximately 25 solar masses—then gravity pulls on the gas and causes the star to grow smaller and smaller until its density reaches infinity at a single point. This is called a 'singularity'.

After the black hole forms, it can continue to grow by absorbing mass from its surroundings, such as other stars and other black holes. If a black hole absorbs enough material, growing to over one million solar masses, it becomes a ‘supermassive black hole’. It is believed that supermassive black holes exist in the centres of most galaxies, including the Milky Way.

A black hole is made up of three parts: the singularity (the collapsed star), the ‘inner event horizon’ (the region around the singularity where nothing, not even light, can escape), and the ‘outer event horizon’ (where objects will still feel the gravity of the black hole but not become trapped).

Astronomers usually observe objects in space by looking at the light. However, since black holes don’t emit any light, they can’t be observed in the usual way. Instead, astronomers have to observe the interaction between the black hole and other objects. One way is to observe the motions of stars around the black hole, since their orbits will be altered by its presence.

In the following activity, students will build a model of a black hole, which is intended to help them visualize how exactly a black hole can “bend” space and time and affect nearby objects. The activity should take about one hour.

Step 1

Cut a ~40 × 40 cm piece of elastic bandage. If it is tubular, you will need to cut it through one side to make it flat. (Image 2)

Step 2

Ask several students to stretch the bandage horizontally until it becomes taut to represent two-dimensional ‘space’.

Step 3

Place the marble on the bandage and make it roll across the surface of the bandage. Its path should follow a straight line similar to that of a ray of light travelling through space.

Step 4

Place the heavy ball on the bandage and you will see how it deforms the fabric of ‘space’. ‘Space’ becomes curved around the heavy object. (Image 3)

Step 5

Make the same little marble roll close the heavy object. Its trajectory should now be altered by the deformation of the bandage. This is similar to what happens to light passing close to a massive object that deforms the space surrounding it. Try varying the speed of the marble to see how its path changes. (Image 4)

Step 6

The more concentrated the central mass (i.e. the heavier the large ball), the more curved the bandage will be. This increases the depth of the ‘gravitational well’ from which the marble will not be able to escape. (Image 5)

Step 7

As the marble passes close to the large ball, it starts to revolve around the ‘black hole’ and eventually falls in. Once it is there, you can see how things may easily fall into a black hole but cannot come back out. This is what happens with black holes: their gravity deforms space in such a way that light or other objects fall in and cannot escape.

Additional Information

Accessible information about black holes from the ‘Ask an Astromer’ site at Cornell University. It provides answers to many different questions and specifies difficulty level (beginner, intermediate, advanced):

http://curious.astro.cornell.edu/blackholes.php.

A video by the European Southern Observatory showing real data taken of stars orbiting around a black hole:

http://www.eso.org/public/videos/eso0846a/.

A great interactive site from the Space Telescope Science Institute with extensive information about black holes as well as online activities and experiments:

http://hubblesite.org/explore_astronomy/black_holes/.

Follow-up questions

What happens when you decrease the speed of the marble? Why?

What happens when you use a heavier ball? What about a heaver marble?

How would you be able to know if there is a black hole somewhere by observing the motions of the stars?

Keywords

Black holes Gravity Space Time Mass Interactive Model

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