Black Dwarfs and Neutron Stars Help
After a star such as our Sun has gone through its red-giant phase, the outer materials will drift away into space, and a white dwarf will remain. It will be only about as big as the Earth and will be dim compared with the Sun as we see it today. A similar fate awaits many other stars in the Universe. Some stars have more interesting destinies, though. Some of these end states are such that the term star does not seem appropriate.
Like a glowing cinder in an abandoned fire, a white dwarf becomes dimmer and dimmer until it is a globe having mass comparable with that of a star, temperature near absolute zero, and diameter comparable with that of a terrestrial planet such as Earth, Venus, or Mars. Such a star is called a black dwarf .
If space travelers ever come across a black dwarf, it will look something like a planet. However, the gravitation in the vicinity of and on the surface of such an object will give it away. In fact, landing on a black dwarf would be a suicide mission. The force would be so great that the spacecraft would be pulled down into a violent crash, and the bodies of the astronauts, as well as their vessel, would ooze into the surface.
After the Sun becomes a black dwarf, those planets not vaporized during the red-giant phase will continue to faithfully orbit. Much of the Sun’s original mass will remain in the dark ball at the center of the Solar System.
In general, the larger a star is to begin with, the more dense is the final object, the “ghost of a star that once shone brightly.” A black dwarf with more than a certain mass finds itself under the influence of gravitation so strong that all the electrons in the atoms are driven into the nuclei. Atoms are mostly empty space; solids don’t pass through each other because of the repulsive electrical forces produced by their electron shells. This repulsive force, on an atomic scale, is powerful indeed. However, gravitation can overcome it if sufficient mass is put into a small enough physical volume.
When an electron combines with a proton because of gravitational forces, the result is a neutron. It is believed that some black dwarfs are balls of neutrons stuck together with little or no intervening space between them. This type of object is known as a neutron star .
The fate of a massive neutron star challenges even the most vivid imagination. If the original star had spin, the neutron star retains much of that rotational momentum. Because the neutron star is so much smaller than the original star, however, the neutron star spins faster (Fig. 14-3). Some neutron stars spin at dozens or even hundreds of revolutions per second. How can we know this when neutron stars are not visible directly through telescopes? The answer lies in the fact that some of them emit pulsating “signals” at radio, infrared, and optical wavelengths.
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