ARCturus

The life phases of a star depend largely on its mass. The last phase of a heavy star can proceed in many ways. One result may be a black hole. There is not much information on this mysterious object of the universe and it is difficult to illustrate. The picture introduces a model of a black hole in The Finnish Science Centre Heureka. The light curves are reflections of light from metal balls rolling fast around the centre (picture IH, Finland).

 

 

Aim: Produce an impression on the substance and effects of the strongest known gravitation object.

Explorations: First, memorize and discuss the various phases and aging processes in the life of the stars. Recall the current phase of our own Sun. Ponder together the etymology and meaning of the concept ‘black hole’.

1)  At first, get to know the concepts ’gravity field’ and ‘event horizon’. Ponder and discuss what it would be like to experience the effect of these fields. Let the students express their own propositions without correcting or changing their views.

2)  Sketch a diagram of the surroundings of a black hole, draw an event horizon and describe for instance a source of light at various distances from the centre of the hole: far outside of the horizon, near the horizon, at the border of the horizon and inside the border. Ponder the differences between these cases.

3)  A black hole has a strong gravity field. Some test arrangements may help in illustrating its effects. The students need to be strongly emphasized that this topic is too unknown and rare to illustrate, let alone explore experimentally.

In science centres there is a large plastic table in which the centre surface is curved down. The diametre of the deepest part of this basin is no more than a few centimetres, whereas on the edges where the curving started it was more than one metre. Set a heavy ball in rolling motion along the edges of the table. When rolling on the curved surface it starts to sink downwards. The deeper the ball sinks the higher the speed increases. Finally the ball has fallen in the middle of the basin that indicates a black hole. The students can test the fall into the black hole by setting the ball in motion at various speeds and by comparing the results.

4)  * In addition to an excursion to a science centre or instead of it, prepare with the students a model of a curving universe which of course only is a modest copy of the prefabricated plastic model mentioned above. Obtain about a one square metre piece of synthetic rubber with excellent stretching and recovery properties. Stretch the rubber around a strong metal circle. Put a heavy metal ball in the middle of the rubber. The ball of course sinks deep and stretches the surface of the rubber to a curve. Now it is possible to do similar ball tests on this surface as in task 3.

A structural disadvantage of this sinking surface is that it is nearly straight, not so curving as in the prefabricated plastic table. But nevertheless, it produces the rolling and sinking effect although the sinking happens much faster.

Instead of a rubber plate, a strong and stretching net may also be used. However, the elasticity of the net is not as good as that of the rubber. A net is a very good instrument in illustrating the curving universe.

Methods: Get to know the characteristics of a black hole by using literature and the Internet. Watch computer-based video clips illustrating events near the black hole. Use two models to test the falling of an object in the gravity field.

Materials: Literary sources, prefabricated plastic model of a black hole, plate made of synthetic rubber, elastic net, balls in various sizes.

Pondering: How could a strong gravity field be illustrated? What happens to a human being when he falls into a black hole? What is the future of a black hole? A black hole in the Milky Way.

Evaluation of the results: Ponder the conclusions that can be drawn from the results. What would be a better way to illustrate the falling into a black hole? What is a right kind of a surface in sinking?

Hints: Test various weights in the middle of the plate or the net. Which one produces the best sinking motion and how does the speed alter?

Keywords: Black hole, gravity field, event horizon, gravitation.

 

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