Our solar system consists of ten planets revolving around the Sun. The Sun serves as a magnet that uses its gravitational pull to hold the solar system together. If the Sun were to disappear, what would hold the planets together? The answer might be a black hole. A black hole is a theorized body whose gravity is so strong that even light can't escape from within it (Shipman 64). If light can't escape from a black hole, then it must be invisible – therefore how can we know that the black holes exist? How do they form and where can we find them? This paper will discuss the theory behind the black holes and physical evidence of their existence. In order to understand black hole's properties better, lets review basic principles of gravity.

Lets assume that a person standing on a planet's surface throws a rock in the air. The rock will rise up to a point until the gravity will pull it back, making the rock fall. If the person will throw rock hard enough, it will escape planet's gravity. The speed at which the rock will leave a gravitational pull of a planet is called the "escape velocity'. The escape velocity differs on the planet's mass; the more mass the planet has – the higher escape velocity will be.

A black hole has so much mass concentrated in a small radius that its escape velocity is greater than the velocity of light (Bunn). Since it is impossible for anything to travel faster than light, it means that nothing can escape a black hole (Gribbin and White 75). Black holes may form after a star is overwhelmed by its gravitational force, that it can't keep from collapsing. During their lifetime stars remain at a constant size, because they contain a balance of forces: heat generated by burning nuclear fuel expands the star outward while the force of gravity pulls it in. This is illustrated in Figure 1, taken from the book Black Holes, Quasars, and the Universe. Figure 1.

Excess pressure in the hot core (white arrow) counterbalances the weight of the envelope (solid arrow). The interior constantly loses energy to the envelope and ultimately to outer space because of the flow of radiation from the core, through the envelope to the photosphere, and to space as the sun shines (Shipman 26). When a star exhausts its nuclear fuel and collapses under its own weight, it begins to shrink in size. If the core of the star is massive enough the collapsing star will shrink to a point where the gravity will become strong enough to trap even light (Shipman 24).

Such strong gravity disturbs space and causes a black hole to have some certain properties like "event horizon'. Event horizon is " a spherical surface that marks boundary of the black hole'. As soon as matter passes through the horizon it cant get back out, it will move closer to black hole's center – approaching singularity (Bunn). In 1969, American relativist John Wheeler named these massive collapsed stars as "black holes' (Gribbin and White 74).

The idea that a star can shrink and result in a great concentration of mass goes back to the 18 th century. In the early 18 th century, Isaac Newton researched and experimented with light. From his experiments he concluded the corpuscular theory of light, which states that light consists of tiny particles that move in straight lines at great speeds (Compton's Multimedia Encyclopedia). The French mathematician Pierre Simon de Laplace, in 1796, reasoned that light particles could not escape from a massive body (Shipman 65). The scientists disregarded Laplace's theory, until Albert Einstein in 1916 came up with the theory of relativity (Shipman 65). In theory of relativity Einstein stated that "gravity is not a force but a curved field in the space-time continuum that is created by the presence of mass' (Compton's Multimedia Encyclopedia).

Not long after Einstein developed the theory of relativity, the German astronomer Karl Schwarzschild calculated how compressed an object with a given mass (in this case a star) should be in order to form a black hole (Shipman 65). His equation became known the Schwarzschild radius, which shows to what critical radius a given mass should be compressed to become a black hole (Gribbin and White 77). In 1939, the United States physicists J. Robert Oppenheimer, Hartland S. Snyder and Volk off showed that it is possible for massive stars to collapse and form black holes (Bunn and Shipman 65). In 1970's, the British scientist Steven Hawking developed a theory that black holes are not completely black (Bunn).

Hawking noticed that black holes comply with the second law of thermodynamics. The second law of thermodynamic says that "the entropy of an isolated system always increases, and that when two systems are joined together, the entropy of the combined system is greater than the sum of the entropies of the individual systems' (Ferris 229). It means that "the area of event horizon increases whenever matter fell into a black hole'. This was researched by student at Princeton named Jacob Bekenstein. Such a proposal was logical, but it had a flaw in complying with the second law of thermodynamics.

If a body has entropy it also must have a temperature, which means that black holes should emit radiation. But how can black holes emit anything when by the definition nothing can escape from their gravitational pull? When Hawking was visiting Moscow in 1973 he had a chance to discuss black holes with two leading Soviet scientists Yakov Zeldovich and Alexander Starobinsky. They convinced Hawking that "according to the quantum mechanical uncertainty principle, rotating black holes should create and emit particles' (Ferris 230). Hawking decided to calculate how much radiation is emitted from rotating black holes.

He found out from his calculations that even non – rotating black holes should emit radiation. However this radiation does not directly comes out of black hole itself. The answer lies in quantum mechanics theory, which tells that "the particles do not come from within the black hole, but from the empty "space' just outside the black hole's event horizon' (Ferris 231). Since black holes are invisible, astronomers have been trying to locate them by observing their effects.

Black holes have tremendous gravitational pull, matter and light particles around them are attracted towards the center. As matter and light particles approach a black hole they "form a swirling accretion disc, like water going down the plughole of a bath, with gas piling up and getting hot as gravitational energy is converted into energy of motion' (Gribbin and White 137). When light enters a violent spin under strong gravitational pull, it emits rapidly pulsating and detectable X-rays. In 1965 astronomers observed intense X-rays coming from the constellation Cygnus (nearly 10, 000 light years away). When satellite technology was born, in 1971, the world's first X-ray satellite pinpointed the exact location of these X-rays. It was found to be a massive but invisible object that astronomers have named Cygnus X-1.

Since Cygnus X-1 exhibit all the hypothetical properties of a black hole, there is a strong belief that Cygnus X-1 might be the first identified black hole (Gribbin and White 138). At present time, the Hubble Space Telescope provides us with images that prove the existence of black holes. The Figure 2 shows a recent picture taken by the Hubble Space Telescope of a nearby galaxy, which has a massive black hole. Figure 2. Astronomers have obtained an unprecedented look at the nearest example of galactic cannibalism – a massive black hole hidden at the center of a nearby giant galaxy that is feeding on a smaller galaxy in a spectacular collision. Such fireworks were common in the early universe, as galaxies formed and evolved, but are rare today (The Space Telescope Science Institute).

During the second half of the 20 th century, due to scientific effort and new satellite technology, research has contributed to find more about black holes. As a result, scientists have found many of the black holes' properties and physical evidence of their existence. However since we can't recreate a black hole, the best proof would be if we were able to go near a black hole and observe it at close range. Unfortunately such a mission is impossible right now because suspected nearest black hole is 10, 000 light years away.

For now we can only observe at great distance the effect that black holes impose on surrounding space. Bibliography Bunn, Ted. "Black Holes Frequently Asked Questions.' web (Sept 1995). Compton's Multimedia Encyclopedia.

Version 2. 0 P. CD-ROM. Compton's Learning Company, 1991. Eisen hamer, Jonathan and Levy, Zola. "Hubble Provides Multiple Views of How to Feed a Black Hole.

' web /pub info / pr /1998/14 (14 May 1998). Gribbin, John and White, Michael. Stephen Hawking – A Life in Science. London: Penguin Books, Ltd. , 1992. Ferris, Timothy.

Physics, Astronomy, and Mathematics. New York: Back Bay Books, 1991. Shipman, Harry. Black Holes, Quasars, and the Universe. Boston: Houghton Mifflin Company, 1976.