Black Holes

Everyday we look out upon the night sky, wondering and dreaming of what lies beyond our planet. The universe that we live in is so diverse and unique, and it interests us to learn about all the variance that lies beyond our grasp. Within this marvel of wonders our universe holds a mystery that is very difficult to understand because of the complications that arise when trying to examine and explore the principles of space. That mystery happens to be that of the ever clandestine, black hole.

Most black holes are formed from the gravitational collapse of a star, usually having a great, massive, core. A star is created when huge, gigantic, gas clouds bind together due to attractive forces and form a hot core, combined from all the energy of the two gas clouds. This energy produced is so great when it first collides, that a nuclear reaction occurs and the gases within the star start to burn continuously. The Hydrogen gas is usually the first type of gas consumed in a star and then other gas elements such as Carbon, Oxygen, and Helium are consumed. This chain reaction fuels the star for millions or billions of years depending upon the amount of gases there are. The star manages to avoid collapsing at this point because of the equilibrium achieved by itself. The gravitational pull from the core of the star is equal to the gravitational pull of the gases forming a type of orbit, however when this equality is broken the star can go into several different stages. Usually if the star is small in mass, most of the gases will be consumed while some of it escapes. This occurs because there is not a tremendous gravitational pull upon those gases and therefore the star weakens and becomes smaller. It is then referred to as a White Dwarf. If the star was to have a larger mass however, then it may possibly Supernova, meaning that the nuclear fusion within the star simply goes out of control causing the star to explode. After exploding a fraction of the star is usually left (if it has not turned into pure gas) and that fraction of the star is known as a neutron star(Oxlade2000).

A black hole is one of the last option that a star may take. If the core of the star is so massive (approximately 6-8 solar masses; one solar mass being equal to the sun's mass) then it is most likely that when the star's gases are almost consumed those gases will collapse inward, forced into the core by the gravitational force laid upon them. After a black hole is created, the gravitational force continues to pull in space debris and other type of matters to help add to the mass of the core, making the hole stronger and more powerful. Most black holes tend to be in a consistent spinning motion. This motion absorbs various matter and spins it within the ring (known as the Event Horizon) that is formed around the black hole. The matter keeps within the Event Horizon until it has spun into the centre where it is concentrated within the core adding to the mass. Such spinning black holes are known as Kerr Black Holes(Asimov1977).

In simplest terms a black hole is an area of space that has so much mass concentrated in it that there is no way for nearby objects to escape its gravitational pull. To understand the magnitude of this gravitational pull we need to explore our best theory of gravity, at the present moment, Einstein's general theory of relativity. His premise states that anything with mass will bend the fabric of space and time. The greater the mass the more distortion in the fabric of the universe. Einstein visualized the universe as a giant rubber sheet with objects, varying in weight, placed upon it, representing planets, asteroids, moons, etc.

Long before Einstein and his theories, in the late 1700's, an astronomer and mathematician named Pierre Laplace was one of the first people to theorize the existence of black holes. In his book Exposition of the System of the World, he theorized that if a star became so massive that it's escape velocity would become greater than the speed of light. Escape velocity is the speed you must achieve to get away from a planet or star and into space. The earth escape velocity is 25,000 mph, while the moon, with a smaller mass, is about 5,300 mph. When a star has such a large concentration of mass in such a small area that the escape velocity is faster than the speed of light. Then, since nothing travels faster than light, nothing would escape the star's gravitational field. Even a beam of light would be pulled back by the gravity and be unable to escape. Laplace theorized that this star would have to be 250 times the diameter of the sun. This phenomenon was called a dark star (Oxlade2000).

There are a lot of different types of black holes, but they all seem to be formed in pretty much the same manner. When a star has used up all of it fuel it can no longer stand the immense force of gravity that has been pulling on it for its entire life. The star expands quickly and then collapses in on its self. The tremendous weight of the outer layer cause the core to collapse and the star begins to form tremendous mass and dig deeper into the fabric of space and time. It continues to compress until it forms a singularity, which is an object with infinite density(Taylor1998)

Around this singularity forms the event horizon. Once you cross the event horizon you cannot escape the gravitational pull and will be drawn into the singularity. Outside the event horizon is the photon sphere where light particles orbit around the black. These particles cannot stay in orbit forever though, in fact most of the time it is a very short orbit. This is because of two main reasons. Firstly, the photon sphere is a tiny place, just one precise distance. A light ray would have just one chance to hit the right trajectory. Secondly, things aren't that precise out there in the universe. There are all these other light rays bouncing around, making things very unstable. A light ray may orbit the photon sphere for a long time but, eventually it will encounter other light rays and its perfect orbit will be disturbed(Oxalade2000).

Since a black hole is a mass in space where the gravitational pull is so strong that nothing, not even light can escape it is impossible to see with the naked eye or even the strongest of normal spectrum telescopes. In the strictest scientific terms, to prove something you must absolutely, positively show that what you've seen matches exactly the properties of your theory, and that there is no other possible explanation. So how can black holes be proven to exist if there is no visual way to see one. The first scientists to really take an in depth look at black holes and the collapsing of stars, were a professor, Robert Oppenheimer and his student Hartland Snyder, in the early nineteen hundreds. They concluded on the basis of Einstein's theory of relativity that if the speed