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Astrophysics: Black holes

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A black hole is an object that is so massive that nothing, not even light, can escape the pull of its gravity. In 1916, Karl Schwarzchild was the first person to suggest the existence of black holes. He used his solutions to Einstein’s general-relativity equations to explain the properties of black holes. In 1967, the physicist John Wheeler coined the term “black hole” to describe these objects.

In order for an object to escape the gravitational pull of a planet, such as Earth, the object must be moving away from the planet faster than a certain threshold speed, which is called the escape velocity. The escape velocity at the surface of Earth is about 1.1 x 104 m/s, or about 25 000 mi/h.

The escape velocity for a black hole is greater than the speed of light. And, according to Einstein’s special theory of relativity, no object can move at a speed equal to or greater than the speed of light.

Thus, no object that is within a certain distance of a black hole can move fast enough to escape the gravitational pull of the black hole. The distance, called the Schwarzchild radius, defines the edge, or horizon, of a black hole.

Newton’s laws say that only objects with mass can be subject to forces. How can a black hole trap light if light has no mass? According to Einstein’s general theory of relativity, any object with mass bends the fabric of space and time itself. When an object that has mass or even when a ray of light passes near another object, the path of the moving object or ray curves because space-time itself is curved. The curvature is so great inside a black hole that the path of any light that might be emitted from the black hole bends back toward the black hole and remains trapped inside the horizon.

Because black holes trap light, they cannot be observed directly.

Instead, astronomers must look for indirect evidence of black holes. For example, astonomers have observed stars orbiting very rapidly around the centers of some galaxies. By measuring the speed of the orbits, astronomers can calculate the mass of the dark object--the black hole--that must be at the galaxy’s center. Black holes at the centers of galaxies typically have masses millions or billions of times the mass of the sun.

Material that orbits a black hole can move at such high speeds and have so much energy that the material emits X rays coming from such disks, scientists have discovered several black holes within our galaxy.

A black hole is an object whose escape velocity is greater than the speed of light. Escape velocity is independent of the mass of the escaping object. Understanding how a black hole can trap light, which has no mass, requires an appeal to Einstein’s general theory of relativity. You cannot construct a simple model to help student’s grasp the difficult concept of gravity as a curvature of space-time. Attach a sheet of flexible rubber to a hoop, and then place a heavy object in the center of the sheet. The sheet will bend downward into a cone. You can then role a coin in an orbit around the object in the center. Point out to students that this is only a rough three dimensional model, while the curvature of space-time is four dimensional.

The extreme bending of space-time that prevents light from escaping the Schwarzchild radius of a black hole occurs in a less extreme form around the stars that are less compact than black holes. During the solar eclipse in 1922, astronomers observed light, which came from a distant star, bend as the light passed near the sun. This observation was one of the first pieces of evidence to support Einstein’s general theory of relativity.

Nuclear Decay Modes

So far, we have considered what happens when nucleons are bound together to form stable nuclei. However, not all nuclei are stable. There are about 400 stable nuclei; hundreds of others are unstable and tend to break apart into other particles. This process is called nuclear decay.

The nuclear decay process can be a natural event or can be induced artificially. In either case, when a nucleus decays, radiation is emitted in the form of particles, photons, or both. The emission of particles and photons is called radiation, and the process is called radioactivity. For example, small amounts of radium salts with the nuclei within these salts decay, releasing light energy that causes an object to glow in the dark. The nucleus before decay is called the parent nucleus, and the nucleus remaining after decay is called the parent nucleus, and the nucleus remaining after decay is called the daughter nucleus. In all nuclear reactions, the energy released is found by the equation E= mc2.

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