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Black Holes



Suppose you go outside and throw an apple straight up into the air. The Earth's gravity slows the upward speed of the apple until it finally stops--and then starts falling back down. But suppose you could throw the apple so fast that Earth's gravity could never quite make it stop and fall back down. That speed is called escape velocity-- for Earth, the escape velocity is 25,000 miles per hour. On planets or stars with more gravity, the escape velocity would be even higher. Some planets and stars have the same mass but different sizes. In that case, the smallest one will have the greatest gravity at its surface. Stars can change size (and gravity) very dramatically. A star's gravity works not only on objects outside the star but also among the particles that make up the star. These particles attract one another and the star would collapse if it weren't for the nuclear energy pushing outward. When the nuclear fuel runs out, there is no more energy to keep the star from collapsing into a very small and dense sphere. The ultimate size of this sphere depends on the original mass of the star. Oddly, the largest stars result in the smallest spheres. If a star has a mass less then 1.4 times the sun's, gravity among its particles will not be able to overcome the tendency of the electrons, protons, and neutrons to remain separate. The forces will balance when the star has become a white dwarf the size of a small planet. If a star has between 1.4 and 3.6 solar masses, the gravitational force is so great that electrons combine with protons to form neutrons. The resulting neutron star will be only a few miles in radius. For stars with more than 3.6 solar masses, even the forces that hold elementary particles apart cannot overcome gravity and the entire star shrinks until its radius is essentially zero. In the region very near this pinpoint of matter, gravity becomes almost infinite. Even four or five miles from this singularity, gravity is so great that the escape velocity equals the speed of light. Einstein's general theory of relativity showed that light, though it does not react to gravity in the same way as ordinary matter, is nevertheless affected by strong gravitational fields. In fact, light itself cannot escape from inside this region. The imaginary surface at this radius is known as the "event horizon." From outside, the event horizon would appear perfectly dark (since no light can escape) and hence would appear to be a black hole in space. How do astronomers observe black holes? When charged particles accelerate, they emit electromagnetic radiation (like visible light or X rays). Astronomers look for regions in the sky with radiation consistent with charged particles accelerating toward a black hole. Researchers attempt to rule out other possible causes of this radiation to prove that black holes really do exist.


Much of science involves making inferences about things that you cannot see. If there is nothing falling into a black hole, there is no way to see the black hole. Could we still find it? In this activity you will simulate trying to find a black hole by shooting "light beams" through a region of space and inferring the position of the black hole from the deflections of the beams. Materials
  • graph paper
  • protractor
  • ruler
  • pencil
  1. Find a partner. One player operates the "black hole"; the other is an astronomer.
  2. Each player draws a large square 20 cm (about 8") on a side. The black hole player secretly marks the hole's position in her/his square.
  3. Using a ruler, the astronomer draws a light beam through her/his square.
  4. The black hole player uses the protractor to deflect each beam. Here's how:
    • Copy the beam from the astronomer's paper to your own.
    • Measure how close the beam comes to the black hole.
    • From the point of closest approach, bend the beam according to the following table: 0-1 cm The beam is absorbed by the black hole. It doesn't come out. 1-2 cm Deflect 60* 2-3 cm Deflect 30* 3-4 cm Deflect 15* 4-5 cm Deflect 5* >5 cm No deflection
    • On the astronomer's paper, mark where the beam comes out (not how close the beam came). Repeat steps 3 and 4.
  5. When the astronomer knows where the black hole is, that player marks the place (or the region) where it must be and gives it to the black hole player to check. Take turns hiding the black hole. Extensions
    1. Use coordinates to give your hypothesis and receive your result: "My beam goes in at (0, 7) and is heading for (18, 20)." "Well, it actually comes out at (11, 20)."
    2. A teacher can draw the "astronomer's map" on the blackboard for all to see. Questions
      1. What do you know after one move of this game?
      2. What strategies can you use to find the black hole?
      3. This activity is really "science fiction" since this method couldn't really be used to find black holes. An authentic search would require an array of astronomers to cover a large part of the galaxy (or many galaxies) on the other side of the black hole from the origin of the light beam. Second, given the finite speed of light, even if such an array could be put in place, the experiment would require hundreds, if not thousands of years to complete. Given this information, if we were really doing this in space, what would be needed? What light beams could be used?


  • Asimov, I. (1978) How did we find out about black holes? New York: Walker and Company.
  • Asimov, I. (1988) Quasars, pulsars, and black holes. New York: Gareth Stevens.
  • Boslough, J. (1989) Stephen Hawking's universe: An introduction to the most remarkable scientist of our time. New York: Avon Books.
  • Folger, T. (1993, Jan) In the black. Discover, pp. 29-30.
  • Powell, C. S., & Horgan, J. (1991, July). Live from off-center: Astronomers follow the energetic trail of the Great Annihilator. Scientific American, pp. 29-30.
  • KCET/LA videotape: "Searching for black holes." The Astronomers, part 2. PBS Video: (800) 328-7271.