Black Hole

Group No. 6

A black hole is a theorized celestial body whose surface gravity is so strong that no light can escape from within it. It is one of the three postulated final stages of stellar evolution wherein a star’s core cools and contracts and begins to collapse under the enormous weight of the outer layers. No black hole has ever been identified but scientists have described certain parts of the universe where they might exist.

A black hole is a theorized celestial body whose surface gravity is so strong that no light can escape from within it. Although black holes have been of intense scientific interest only in the later 20th century, the concept goes back to the French mathematician Pierre Simon de Laplace. In a 1798 treatise Laplace agreed with Isaac Newton that light is composed of particles. He reasoned that if enough mass were added to a star like the sun, the gravitational force of the star would eventually become so great that its escape velocity would equal the velocity of light. At that point, light particles would not be able to leave the surface of the star, and it would blink out and become an invisible black star.

More than a century later, Albert Einstein, in his special theory of relativity, maintained that nothing can move faster than light. This means that Laplace’s black stars are also black holes, because if light cannot escape, all other matter must be trapped as well. The surface of the black hole thus acts like a one-way membrane: material may fall into a black hole, but no information or energy can come out of a black hole. The detailed properties of black holes are studied by using Einstein’s general theory of relativity and gravitation.

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In 1917 a German astrophysicist, Karl Schwarzchild, used Einstein’s theory to calculate that if a star of any given mass were compressed to a size smaller than a critical radius, now called the Schwarzchild radius in his honor, the density would become so high and the gravitational force so great that the star would become a black hole. The spherical surface about the star at the Schwarzchild radius is called the event horizon. It marks the outer surface of the black hole at which the escape velocity just equals the velocity of light. Further calculations revealed to Schwarzchild that the critical radius of a black hole is proportional to its mass. For the sun, this radius is about three km (about 2 miles). To find the Schwarzschild radius of any other object, one need to divide the mass of the object by the mass of the sun, and then multiply by 3 km.

If a star more massive than the sun undergoes gravitational collapse at the end of its evolution, it will form either a white dwarf, a neutron star, or according to theory a black hole, depending primarily on its mass. If its collapse process is nonspherical, perhaps because the star is flattened at the poles, then theoretical gravitational waves could be given off just before the black hole is formed. Attempts are being made to detect such waves.

The only other way to identify a black hole would be through its interactions with other matter. For example, if the black hole is formed in a Binary Star System, gas from the normal star may later flow toward the black hole. As the gas falls toward the hole, its molecules increase in speed and approach the speed of light. The molecules begin to bunch up and collide, heating them to temperatures at which X-rays are emitted. Such X-rays have been detected in eclipsing binary star systems in which the X-ray source is not visible. The binary star Cygnus X-1 may include such a companion; other candidates are an X-ray source in the nearby Large Magellanic Cloud galaxy, and X-ray novas in the constellations Monoceros and Vulpecula. No black holes has yet been positively identified.

Black holes, if they exist, could come in a great range of sizes. The English physicist Stephen Hawking has speculated that tiny black holes with masses no longer than that of a 1 large mountain are possible. Such black holes, in the size range of elementary particles, would have been formed only under extreme conditions that cosmology theories indicate existed in the very first moments of the universe. On the other hand, gigantic black holes may lie at the center of galaxies. Some astronomers suggest that such black holes may be linked to the differences that exist between galaxies, ranging from normal ones such as our own to the highly active galaxies called radio galaxies and quasars.

Various speculations have been made as to the permanence or instability of black holes themselves. Laws of physics suggest that black holes would emit particles and shrink with time. According to Hawking, their temperatures would rise as they shrank, and they might finally evaporate with an enormous burst of energy. Other theorists suggest that certain factors would halt this process along the way. Some theorists have been suggested the existence of so-called white holes in which matter would flow through a completely collapsed black hole – a singularity, or body of zero radius and infinite density – into another universe. This notion, however, is extremely speculative.

A popular misconception is that black holes act like cosmic vacuum cleaners, sucking up everything within reach. In fact the gravitational attraction of a black hole would be no stronger than that of a normal star of the same mass. For instance, if the sun could collapse to form a black hole (it cannot, however, and will instead end its life as a white dwarf), the earth would continue to orbit just as it does now.

Bibliography

“Black Hole.” The 1995 Grolier Multimedia Encyclopedia. USA: Grolier Electronic Publishing Inc., 1995.

“Black Hole.” Microsoft Encarta 97 Deluxe Edition. USA: Microsoft Corporation, 1996.

“Black Hole.” 1998 World Book Multimedia Encyclopedia. USA: World Book, Inc., 1997.

“Black Hole.” Compton’s New Century Encyclopedia and Reference Collection II. USA: Compton’s Learning Company, 1995.

Space Telescope Science Institute. “Hubble Provides Multiple Views of How to Feed a Black Hole” (PRC98-14). http://oposite.stsci.edu/pubinfo/pr/1998/14/.

Robert Nemiroff and Jerry Bonnell. National Aeronautics and Space Administration. “Astronomy Picture of the Day Index – Stars: Black Holes.” http://antwrp.gsfc.nasa.gov/apod/black_holes.html.

Steven Degennaro. The Search for Black Holes. http://www.owlnet.rice.edu/~spac250/steve/index.html.

Peer Reviews

Here's what a star student thought of this essay

pictureperfect

31st of Jul 2012

Quality of writing

This essay is reasonably well written and the candidate has used appropriate technical terms throughout their work which demonstrates a good understanding of this topic. However there are one or two grammatical errors and some issues with spelling in which the candidate has used the American version of some words.

Level of analysis

It is clear that the candidate has taken the time to research this topic, which is always a good idea when writing a scientific essay. This enables you to include additional information within your essay which demonstrates a broader knowledge and can make your essay more interesting to read. Furthermore, the candidate has sensibly included a bibliography stating the resources they have used, this is important as it shows the reader the steps you have taken to prepare for your essay and it can show that your work is not plagiarised. That said, the candidate has unfortunately not written a conclusion for their work, this is a mistake, by not concluding your work you are unable able to leave the reader with a good impression. A good conclusion should summarise key points from your essay with reference to why these are important as these helps to tie together any loose ends.

Response to question

The candidates overall response is good, they have mentioned scientific theories, they have discussed contributions from important scientists and they have used images to help with their discussion which can be useful when talking about such a difficult concept. That said, the candidates introduction is a little unclear and could be improved. When writing your introduction you need to state exactly what you are going to discuss within your essay, this is important as it makes the purpose of your essay clear. It can often be easier to add this into your introduction once you’ve finished as you will then know what you have discussed.

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