Data Bases • Custom Term Papers • Free Term Papers • Free Research Papers • Free Essays • Free Book Reports • Plagiarism? Links • Top 100 Term Paper Sites • Top 25 Essay Sites • Top 50 Essay Sites • Search 97,000 Papers @ DirectEssays.com • Search 101,000 Papers @ ExampleEssays.com • Search 90,000 Papers @ MegaEssays.com Free Essays • Term Paper Sites • Chuck III's Free Essays • Free College Essays • TermPaperSites.com • My Term Papers • Get Free Essays • Essay World • Planet Papers	Search Lots of Essays Back to Subjects - Science Black Holes3 Black Holes3 A Black hole is a theorized celestial body whose surface gravity is so strong that nothing, including light, can escape from within it's surface. Gravity is the key to a black hole's immense power. The black hole's strong gravity keeps captured material from escaping. For example, if Earth were the same mass it is now but had only one-fourth its present radius, the escape velocity of someone standing on its surface would be twice what it is now. Black holes have a power far greater than our minds can imagine. This report will go into further discussion on these massive holes in space. Now, though, astronomers have uncovered a much better candidate for a black hole in our galaxy. It lies in the constellation Monoceros some three or four thousand light-years away. Monoceros was discovered in 1975, when it emitted a shower of light and x-rays. Observations soon revealed that Monoceros was a binary consisting of an orange dwarf star and a dark companion. Astronomers continue to observe the object and other black holes like it. Despite its name, A0620-00, it is a better candidate for a black hole than Cygnus X-1.(Croswell 30-37) As I said before, gravity is the key for a black hole’s immense power. The black holes strong gravity keeps captured material from escaping. Of course, every moon, planet, and star has gravity. Earth has enough gravity that you have to travel faster than 11 kilometers per second to overcome the force of gravity. This number is Earth’s escape velocity. The gravity of Jupiter is even stronger: it’s escape velocity is 60 kilometers per second. A black hole has so much gravity that to escape one you would have to be traveling faster than the speed of light, 300,000 kilometers per second. But traveling faster than the speed of light is impossible, so once you get into a black hole you can’t get out.(Levitt 83) Black holes may form during the course of stellar evolution. As nuclear fuels are exhausted in the core of a star, the pressure associated with their heat is no longer available to resist contraction of the core to ever higher densities. Two new types of pressure arise at densities a million and a million billion times that of water, respectively, and a compact white dwarf or a neutron star may form. If the core mass exceeds about 1.7 solar masses, however, neither electron nor neuron pressure is sufficient to prevent collapse to a black hole. Knowing more about galactic black holes will help astronomers learn more about the evolution of galaxies and the relationship between galaxies, black holes, and quasars. Astronomers Holland Ford, Richard Harms, and colleagues have used data from the Hubble Space Telescope to develop strong evidence that a black hole exists in the center of the elliptical galaxy M87 in Virgo. Ford and Harms have shown that a small mass, fast rotating disc lies at M87’s heart.(Powell 12) There is also a black holethat is eating the Milky Way. The core of the Milky Way galaxy, 180 quadrillion miles from Earth, exerts enough force to hold together a system of 100 billion stars. Astronomers trying to determine the origin of this force think they may have found the answer-a black hole at the center of the galaxy with the mass of three million suns, sucking in matter and emitting odd radio waves and other signals. Astronomers first noticed a glow of gamma radiation at the center of the galaxy in the 1970’s. But the most convincing evidence was a motionless source of radio waves that were detected at the center of galaxy. All of these factors indicate a supermassive center that is absorbing matter into it’s gravitational field at great speed.(Sawyer 38) Using a series of radiotelescopes stretching 5,000 miles across the United States from the Virgin Islands to Hawaii, an international team of astronomers have discovered strong evidence of an incredibly powerful black hole as massive as 40 million suns. The findings, presented to the American Astronomical Society in Tucson Wednesday and published today in the journal “Nature,” is considered the strongest evidence yet of the existence of black holes. It is difficult to prove the existence of black holes primarily because, by definition, they can never be seen. At best, scientists can only study the that would be expected in the surrounding territory if in fact a black hole is there. By several accounts, astronomers projects from the 1980’s have come closer than any of the others. Scientists lust after evidence of black holes because they are among the most mysterious objects in the universe, devouring everything within their gravitational reach. Their immense pull is believed responsible for swirling masses of stars that radiate brilliantly across the heavens. A fuller understanding of black holes is considered essential to comprehending the physics that drives celestial objects from quasars to Abraham Loeb believes that black holes fueling active galactic nuclei form when protons left over from the creation of the universe interact with electrons or dust in gas clouds to create a drag force. The force slows the clouds rotation enough to allow extreme gravitational collapse.(Cowen 86) Scientists believe that a black hole forms when a very massive star runs out of nuclear fuel and is crushed by its own gravitational force. While a star burns fuel, it produces an outward push that counters the inward pull of gravity. When no more fuel remains, the internal pressure drops, and the star can no longer support its enormous weight. It throws off its outer layers in a gigantic explosion. At the same time, it’s core collapses. Gravity can crush a core measuring 10,000 miles (16,000 kilometers) across to an object 10 miles (16 kilometers) across in about one Very few stars become black holes when they die. Most massive stars collapse to become neutron stars (dense stars made up almost entirely of neutrons). Scientists are sure that neutron stars form by collapse. They suspect that a black hole will form in the same way if a star’s burned out core contains at least three times the mass of the sun. The idea of a black hole is based on physicist Albert Einstein’s theory of gravitation, known as general relativity. Einstein’s theory predicts that a black hole formed by the collapse of a star about 100,000 times smaller than the sun and almost featureless. Therefore, it would make it extremely difficult to pinpoint a black hole.(World Book)*** Astronomers believed that black holes could only be an intermediate step toward the completion of spherical gravitational collapse or a disappearance in what the astronomers call a “singularity.” The singularity is a region of space-time where infinitely intense gravitational forces deform matter and photons beyond recognition and, as the radius of a sphere shrinks to a point of zero dimension and the volume goes to zero, matter and energy are squeezed out of existence. The singularity, to some scientists, is nature’s way of saying that the present physical laws we are using are not adequate to cope with the situation-perhaps we have missed the proper application of some existing laws or, in the extreme, because new laws are needed. Other scientists are just as certain that once we have a black hole, the singularity is ruled out; they indicate that as it takes an infinite time to reach the gravitational radius and as the universe spans a finite time, the black hole simply does not have enough time to go to a singularity. Perhaps an example will serve to illustrate what happens in space-time that could give rise to a singularity. Picture a thin sheet of rubber stretched over a large frame, and let us assume that this rubber represents a corner of the universe. If we take a ball and place it in the middle of the sheet, the ball will sink into or depress the sheet to deform it. If we replace the ball with a heavier one and place it on the sheet, the ball will stretch the rubber more and the deformation will be greater, with the ball A still heavier ball will deform the sheet more and the ball will sink still farther into the rubber. Finally, if the ball had almost infinite weight and we assumed the rubber sheet could not tear, the ball would drop to an almost infinite distance from the frame supporting the rubber. And if at that instant the rubber sheet did open up a tiny hole, the ball might pop through the tiny hole to escape the sheet. With the escape of the ball, the pressure on the rubber would be relaxed and it would spring back to its initial position as a flat sheet. The gravitational stress would have been removed from space-time, but the ball would have effectively left our universe. Where would the ball be now? This situation has been deeply explored by many astronomers that we know To return to our rubber sheet analogy, we can visualize a second rubber sheet directly under the first; as the black hole deforms the top sheet in some mysterious manner the bottom sheet is also deformed as a mirror image of the top. Or one can picture a softly inflated rubber balloon into which one is poking a finger. We will poke a finger in from the other side along a diameter. Now imagine a marble being pushed into the balloon by one of the fingers; the finger coming in from the other side of the balloon just touches it. Further imagine that the marble mysteriously passes through the two distended layers of the rubber. When the pressure of the fingers is removed, the marble ends up at the other side of the balloon, diametrically opposite to the point where it was introduced. If we imagine the marble to be a black hole in this fashion, we have transferred it to another part of the universe. One serious drawback must be mentioned. At this time one cannot visualize an astronomer being compressed to the densities found within a black hole. However, this should not be considered an impossibility, for this concept possesses many fascinating overtones. One must remember that if we move with the speed of light, time literally stops, dimensions in the direction of motion shrink to zero, and mass becomes infinite. One cannot help concluding that an astronaut traveling at the speed of light would have zero dimension with infinite mass. There is one difference --The singularity is a point while the astronaut becomes a line at the speed of light. Theory tells us that even though the astronaut was compressed to a line-this is what we on the outside world would see, if indeed it were possible to see him-the fast- moving astronaut would mot notice any difference in shape, motion, or time. This exposition gives rise to a most intriguing thought. Perhaps at some future date, by moving just within the gravitational radius, an astronaut may be able to move to Gravitational Collapse is the catastrophic fate that befalls a massive object when it's gravity completely overwhelms all other forces. During most of a star's lifetime, it's tendency to contract as a result of it's gravity is balanced by the outward pressure produced by the heat of it's nuclear reactions. Eventually, however, the nuclear fuel will be exhausted. If the star's mass is less than about 3 solar masses, it will eventually contract to a stable configuration as either a white dwarf (about the size of Earth but hundreds of thousands times denser) or a neutron star (a similar mass compressed into a sphere only a few miles across). More massive stars, however, will continue to shrink even further when their thermal and rotational energy is exhausted. Unless the star sheds its excess mass, gravity will overcome all conceivable forces and gravitational Once gravity exceeds the other forces, the star will fall in on itself in a few hours. When the size of the collapsing star falls below what is called the Scharzschild Radius, the escape velocity becomes equal to the light. When not even light can escape from the surface, the star is said to be inside a black hole. Theorems by Roger Penrose and Stephen Hawking show that, according to general relativity and similar theories of gravitation, a singularity or edge to the space-time continuum must occur. It is believed, but has not been proved, that everything inside a black hole will hit the singularity and be utterly destroyed within a few microseconds; however, some claim that matter and energy may reappear in another universe. The collapse of a star or a dense cluster of stars can release large amounts of energy, perhaps 10% of the total rest-mass energy of the system if the collapse is nonspherical. Most of the energy will probably be emitted as gravitational waves. Matter falling into a black hole already formed can also release electromagnetic energy. This is a possible source of X rays from Cygnus X-1 in our galaxy and of visible light radio waves from quasars and from certain other distant galaxies. The universe as a whole may also undergo gravitational collapse. The universe is presently expanding as distant galaxies move apart, but if they do no have escape velocity relative to each other they will eventually fall back together and bring the universe to an end. Whether this will happen depends on the density of matter in the universe, which is not precisely known.(Asimov 255) For the past couple of centuries astronomers have done many things to try to unlock the mystery of these natural wonders called black holes. The curiosity of many scientists has motivated them to use every means possible to do this. If there is one scientist who, in the last couple of decades, has contributed more towards this cause, it would probably be Ken Croswell. He has had something to do with most of the major advances in this field of study. However, even with people like Croswell, our little world is still far from unlocking the mind-boggling mystery of black holes. Bibliography: Works Cited Asimov, Isaac. “Black Holes.” Grolier Electronic Publishing. 1993 ed. 255 “Black Holes.” The World Book Encyclopaedia. 1994 ed. Cowen, Ron. “New Spin on Making a Black Hole.” Science News. 6 Feb. 1993: 86 Croswell, Ken. “The Best Black Hole in the Galaxy.” SIRS Researcher. Jan. 1992: 30-37 Dye, Lee. “Evidence of Massive Black Hole Discovered by Astronomers.” SIRS Researcher. 1995: A3+ Levitt, I. M. “Beyond the Known Universe.” New York: New York City, 1974 Powell, Corey. “A Black Hole is Identified in the Core of the Galaxy.” Scientific American. August 1991: 12 Sawyer, Kathy. “Is a Black Hole Eating the Milky Way?” SIRS Researcher. Mar. 1990: 38 Word Count: 2483
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