There are plenty of stars that are more than three times the mass of the Sun.
Astronomers agree that many of these stars have long since blown up, gone through their white dwarf phases, and presumably also gone through the collapsing process. This suggests that black holes comprise a significant proportion of the matter in the Universe.
As long as we can’t see them, we do not have a good way to find out if black holes exist. Fortunately, however, real black holes are not as black as theory implies. According to the research of the well-known cosmologist Stephen Hawking, black holes gradually lose their mass by emitting ener- gy. This need not necessarily all be in the form of EM radiation; energy also can be lost as gravitational waves.
The idea of a gravitational wave was first made plausible when Einstein developed his general theory of relativity. Figure 14-8 is a simplified depic- tion of a gravitational wave as it leaves a black hole and travels through the space-time continuum. Just as a pebble, when dropped into a still pond, produces concentric, expanding, circular ripples in the two-dimensional surface of the water, so does the black hole produce concentric, expanding, spherical ripples in the three-dimensional continuum of space. Ripples are also produced in time—as hard as this might be to imagine!
Gravitational-wave detectors have been built in an effort to detect rip- ples in space and time as they pass. Because they involve the very fabric of the Cosmos, such waves can penetrate anything with no difficulty whatso- ever. A gravitational disturbance coming from the nadir (straight down as you see it while standing upright) would be every bit as detectable as one coming from overhead.
As of this writing, no conclusive evidence of gravitational waves has been found. However, astronomers are reasonably confident that they exist.
The challenge is nothing more (or less) than continuing to refine the detec- tion strategy until ripples in space and time are discovered and can be attributed to some real, however bizarre, celestial object.
When we expand our scope of observation to an intergalactic scale, there is other evidence for the existence of black holes. This will be dis- cussed in the next chapter.
Quiz
Refer to the text if necessary. A good score is 8 correct. Answers are in the back of the book.
1. Photons are (a) antiprotons.
(b) antielectrons.
Time
Schwarzchild radius Space-time
continuum
Waves
Figure 14-8. Some black holes should be expected to emit gravitational waves.
(c) antineutrons.
(d) energy packets.
2. As a black hole swallows up more and more matter, (a) its mass decreases.
(b) the gravitational radius decreases.
(c) its mass increases.
(d) None of the above
3. The waveforms of the pulses from a pulsar are (a) smooth.
(b) irregular.
(c) long.
(d) short.
4. If an object collapses so that its radius is less than the Schwarzchild radius, then
(a) things can come out but cannot go in.
(b) the escape velocity at the surface is greater than the speed of light.
(c) the escape velocity at the surface is less than the speed of light.
(d) the object is, by definition, a pulsar.
5. Radio waves travel through interstellar space at a slightly different speed than visible light because of
(a) magnetic fields.
(b) differences in photon energy.
(c) dispersion.
(d) No! Radio waves always travel through interstellar space at precisely the same speed as visible light.
6. Suppose that a neutron somehow forms, and it is floating all by itself in space.
How long can we expect it to last?
(a) Forever
(b) Until it collapses into a black hole (c) Until it becomes part of a neutron star (d) none of the above.
7. The EM energy from pulsars is believed to be a product of (a) neutrons decaying into energy.
(b) matter interacting with antimatter.
(c) electrons rising into higher orbits.
(d) none of the above.
8. As a rotating white dwarf dies out and collapses into a neutron star, (a) it rotates faster and faster.
(b) it rotates more and more slowly.
(c) its rotational speed does not change.
(d) rotation loses meaning because of the incalculable density of the object.
9. Suppose that a 1,000-kg spacecraft from Earth touches down on a planet and that planet turns out to be antimatter. What will happen?
(a) There will be an explosion of incalculable violence, and all the matter in both the planet and the spacecraft will be annihilated.
(b) There will be an explosion, and approximately 90 quintillion (9.0 ⫻1019) joules of energy will be liberated.
(c) There will be an explosion, and approximately 180 quintillion (1.80 ⫻1020) joules of energy will be liberated.
(d) The spacecraft will be quietly swallowed up by the planet and will disappear.
10. When an electron moves into a larger orbit within an atom, (a) the electron’s energy increases.
(b) the electron’s energy decreases.
(c) the electron’s charge increases.
(d) the electron’s charge decreases.
Galaxies and Quasars
When telescopes became powerful enough to resolve nebulae into definite shapes, one type of nebula presented a conundrum. Many of the spiral neb- ulae, which looked like whirlpools of glowing gas, had spectral lines whose wavelengths were much longer than they ought to be. This phenomenon, called red shift, suggests that an object is receding. Red shifts were seen commonly for spiral nebulae, but blue shifts—foreshortening of the waves when an object is approaching—were almost never seen, and when they were observed, they were minimal. Some astronomers thought that the spi- ral nebulae actually were huge congregations of stars at immense distances and that our own Milky Way was just one such congregation. Until indi- vidual stars could be resolved within the spiral nebulae, however, this idea remained an unproved hypothesis.
Types of Galaxies
Today we know that the spiral nebulae do consist of stars, and we call them spiral galaxies. Spirals are not the only type of galaxy. Other objects, pre- viously thought to be emission nebulae or globular star clusters within our Milky Way, turned out to be irregular galaxiesor elliptical galaxies. Some of these are billions (units of 1 billion or 109) of light-years away from us.
Many contain hundreds of billions of individual stars.