Black Hole
Black holes are among the strangest and most mysterious objects in the cosmos. Simply put, a black hole is the remains of a massive star that has burned out its nuclear fuel and collapsed into a point of zero volume under tremendous gravitational force. This point is its "singularity," where the pressure and density are infinite. It cannot be directly detected by any means, yet black holes are not only believed to exist, they may represent 90 percent of the content of the universe.
The name "black hole" was first proposed by American physicist John Wheeler (1943-) in 1969. It had a novel sound to it, was highly publicized, and became a favorite with the general public. Russian scientists came up with the term collapsar, which, though more descriptive of the object, never caught on.
The final end of a star is directly related to its mass. Any star will collapse once its nuclear fuel is spent. Without the radiation from nuclear fusion pushing outward from the star 's core to balance its immense gravity, a star will begin to fall into itself. Average-sized stars, like the Sun, will end up as a white dwarf star about the size of the Earth. Astronomer Subrahmanyan Chandrasekhar postulated that a star with a mass greater than 1.4 solar masses would become very unstable once nuclear fusion at its core ceased, unless it could shed some material. Massive stars, at the end of their lives, shed mass by exploding as a supernova. A supernova might result in the creation of a neutron star with a radius of 6 miles (10 km).
But what about stars that are still more massive? Theory suggests that a star with more than roughly two or three times the mass of the Sun undergoes a catastrophic collapse when its nuclear fuel is spent. The mass becomes so concentrated as the star collapses that the force of gravity becomes completely overpowering. As the star becomes more and more dense, gravity increases until not even light has enough velocity to escape the collapsed star's surface. That surface is called the event horizon, and its radius is known as the Schwarzschild radius, after astronomer Karl Schwarzschild (1873-1916), who described the phenomenon in 1916. Anything crossing into the event horizon cannot escape. Light and electromagnetic radiation are forever trapped within its invisible confines. The only way a black hole can be detected is by seeing its effect on visible objects like adjoining stars, dust or gas.
In 1783, English astronomer John Michell first suggested that light would be affected by gravity in the same way as ordinary matter. He claimed light coming from an object like the Sun would have its velocity reduced as its distance increased. The concept was so outlandish to scientists of the day that it promptly fell into obscurity. It was not until more than a century later that Albert Einstein proposed a revolutionary concept of gravitation in his general theory of relativity. An essential component of his theory was that light was affected by gravity. A massive object, with its strong gravitational field, should be able to affect light passing nearby. This was proven during a total eclipse of the Sun in 1919. During the short period of totality, photographs of the sky near the covered Sun were made. Apparent shifts in the positions of the stars were noted, verifying Einstein's hypothesis.
Initially there was a great deal of excitement. Schwarzschild had determined his radius based on a non-rotating black hole, four years earlier, and J. Robert Oppenheimer and Hartland Snyder (1913-1962) made calculations of gravitational collapse in 1939. Interest in black holes waned, however, until the discovery of quasars in 1963.
Quasars are small and extremely distant objects which emit tremendous quantities of radiation including visible light and X-rays. Astronomers could not determine how energy is produced in these objects. In the mid-1960s, mathematician Roy Kerr concluded that black holes could be responsible for the observed quasars. As huge quantities of matter spiraled into a rotating black hole, tremendous emissions of radiation would occur until the matter reached the event horizon to vanish forever. Many astronomers now believe that quasars are indeed massive black holes at the cores of distant galaxies. Other abnormal galaxies are also posited to have black holes at their cores; a well-known example is the giant elliptical galaxy M87, in the Virgo galaxy cluster, which has an enormous jet of material being ejected from its core. A supermassive black hole is thought to be necessary to so forcibly eject such a large, energetic stream of material.
The first serious black hole candidate in our galaxy, Cygnus X-1, was discovered in 1972 using an X-ray telescope aboard the satellite Uhuru. A seemingly normal blue supergiant star emitting intense amounts of X-rays, this star is believed to be a binary star system in which one of the "stars" is a black hole which is now pulling in the stellar material of the blue star. Measurements indicate the unseen companion may have as much as ten times the mass of the Sun.
In 1997, a team of astronomers at Harvard announced the possible observation of a black hole's event horizon in a binary system thought to contain a black hole. Binaries with black holes should be observable by the X-rays emitted from the heated gas being drawn into the hole from the companion star, but since most of this gas simply dissapears through the event horizon, the X-radiation should be less intense than in a system containing a neutron star. The Harvard group's observations appear to corroborate this hypothesis.
Controversy about black holes still rages. As yet no definite proof exists that stellar cores can maintain the necessary critical mass to evolve into a black hole. A supernova explosion rips the star apart when it reaches the neutron star stage. If the critical mass is always blasted into space, the collapse cannot progress, and black holes are not formed. On the other hand, if black holes do exist, they might account for a great deal of invisible dark matter in the universe. The gravitational attraction of this matter could determine the ultimate fate of the universe: Will it eventually slow down and contract, or will it continue to expand forever?
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