Free Form: Black Holes

Black hole in front of Large Magellanic Cloud
Image Credit: http://en.wikipedia.org/wiki/Black_hole#mediaviewer/File:BH_LMC.png

Black holes are objects in the universe that are so dense and so massive that their gravitational pull absorbs all matter and light that falls within a certain radius away from them.  They can be created when massive stars that are at the end of their life cycles no longer have enough pressure to support the gravitational force attracting the mass to the center, causing the star to collapse. This collapse is so fast and energetic that it releases a supernova explosion, and what remains can become a black hole. Black holes can also theoretically be created from high energy collisions.

A common misconceptions of black holes is the idea that they ‘suck’ everything around them. Black holes, in essence, are just stars that are very dense. For example, if the Sun were replaced with a black hole of equal mass, there would be no effect on the orbits of the planets since the it would have the same gravitational attraction as the Sun. How black holes really work depends on the escape velocity. The escape velocity is the velocity required for an object of certain mass to have enough kinetic energy to overcome the gravitational pull of a massive object. In other words, the kinetic energy must equal the potential energy. An equation for the escape velocity can be written as the following:

\(v_{escape}=\sqrt{\dfrac{2GM}{R}}\)

Where G is the gravitational constant, M is the mass of the black hole, and R is the the distance from the center of the black hole. The event horizon is the distance away from the center of the black hole at which the escape velocity would equal the speed of light, meaning that any light within the event horizon will never escape the gravitational pull. This is why black holes can't be seen- none of the light gets out for us to see it.

Another important feature of black holes is the Schwarzschild radius. The Schwarzschild radius is the radius within which an object of certain mass must be compressed in order to be a black hole. In other words, an object that has a radius smaller than the Schwarzschild radius will have an escape velocity that exceeds the speed of light, making it a black hole. The equation for the Schwarzschild radius, \(R_s\), is just a rearrangement of the equation for the escape velocity:

\(R_s=\dfrac{2GM}{c^2}\)

Where c is the speed of light. To give you an idea of how dense black holes are, from the equation for the Schwarzschild radius, we can find that if the Sun were to be a black hole of the same mass, it would have to be condensed into a radius of 3 km, and the Earth would have to be condensed into a radius of 8.7 mm.

Through gaining a better understanding black holes, we can better understand galaxies, and perhaps the evolution of the universe. Astronomers believe that black holes play an important role in the formation of galaxies and there is extensive research studying the effects of black holes on their host galaxies.

Citations:

http://en.wikipedia.org/wiki/Stellar_evolution

http://en.wikipedia.org/wiki/Black_hole

http://hyperphysics.phy-astr.gsu.edu/hbase/vesc.html

http://en.wikipedia.org/wiki/Schwarzschild_radius