Black Holes: Gravity's Ultimate Victory

I mentioned that the maximum possible mass of a neutron star was about 3 . This is analogous to the Chandrasekhar limit, but remember that the Chandra limit applies to white dwarfs and is about 1.4 . What happens if a star is so massive that the supernova explosion is unable to blow enough matter away to keep the neutron star below this limit? Such a star might have an original mass of, say, 50 or more. It turns out that there is no known source of pressure that can support the core against gravitational collapse once it exceeds about 3 . When the core becomes so small that the escape velocity at its surface exceeds the speed of light, it becomes a black hole.

A black hole has no ``surface'' but does have a size scale associated with it, the Schwarzschild radius. Sometimes called the event horizon, this describes a sphere around the black hole separating objects which can (in theory) escape the black hole's gravity from those which cannot. At the event horizon, the escape velocty is the speed of light (c); inside the event horizon, not even light can escape the black hole. The Schwarzschild radius is given by:

where M is the mass of the black hole and G is Newton's gravitational constant. For the Sun, this radius is only 3 km. Bear in mind that well outside the event horizon, the black hole behaves just like any other object of the same mass. So if the Sun suddenly became a black hole, the Earth would continue orbiting it just as before.

As the formula above shows, the event horizon of a black hole grows larger as it accumulates more mass. In the centers of many galaxies, we believe there are black holes that have accumulated so much mass (from eating interstellar gas and stars) that they have masses of up to . These supermassive black holes may have quite different origins from the stellar black holes that result from individual massive stars. However, even for one of these ``monsters'' the corresponding Schwarzschild radius is only about 20 A.U.--much smaller than our solar system. So the universe is in no danger of being eaten up by black holes.

How do we observe black holes? Although the black hole itself emits no light, it can make itself known via its gravity. If we see a star orbiting an unseen companion that is well over 3 in mass, and if there is evidence that the companion is very small, we can be pretty sure it's a black hole. One indicator of small size is the emission of X-rays from hot gas falling in towards the companion. (X-rays have very high energies, but these energies would be possible just outside a black hole's event horizon because the orbital speeds there approach the speed of light.) Internal friction of the gas falling towards the black hole could then account for the X-ray emission. In our Galaxy, there are a handful of bright X-ray sources which are likely candidates for stellar black holes.

For a more in-depth understanding of black holes, you will need to familiarize yourself with some of the basic ideas of general relativity, including the curvature of space and the gravitational redshift. Many good, introductory physics and astronomy books can begin to help you with this. Best wishes for happy exploring!

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