Black HolesBlack holes are points in space where there are extreme gravitational pulls that prevent anything, including light, from escaping. The reason for such a strong gravitational attraction is due to the fact that large quantities of matter are contained in a small amount of space. Stellar black holes form from stars with a mass greater than 20 solar masses and can be the result of gravitational collapse. Gravitational collapse is the result of the star's internal pressure being unable to resist the star's own gravity. When the star is depleted of its nuclear fuel to the point that it cannot maintain a high enough temperature, it will begin to collapse under its own weight (Seidel 2011). The collapse of the star causes a supernova that causes the outer layers of the star to explode into space while the core collapses completely under its own weight. If the remaining core left behind exceeds 3 solar masses there are no known forces that can prevent the core from completely collapsing into a black hole (p. 568 Bennett et al. 2013) Since black holes do not emit light and completely absorb nearby light to them it would seem impossible to detect. Although black holes do not emit light, their effects are detectable. Due to the strong gravitational pull of a black hole, any matter drawn into the black hole accelerates and heats up. This causes the atoms to become ionized and when they reach high enough temperatures they begin to emit X-rays that can be detected and observed from Earth (Netting 2014). Studying X-ray binaries is an excellent way to detect stellar black holes since binary systems provide enough matter to provide the black hole's X-ray emissions. Cygnus X-1 is an example of a black hole detected through the observation of an When a small to medium-sized main sequence star (less than 10 solar masses) begins to run out of fuel in its core, the core will begin to collapse where the hydrogen on the edges of the collapsed core can be compressed and heated (Chandra 2012) . The nuclear fusion of this new hydrogen will create a new wave of energy that will cause the outer layers of the star to expand; this is known as the red giant phase. In the red giant phase, over millions of years, all of the stars' energy reserves are used up, leaving behind a hot core that is still surrounded by the expanded outer layers. The outer layers are eventually expelled by stellar winds which end up creating a planetary nebula and the remaining hot core forms a white dwarf star where the pull of gravity is supported by degeneracy pressure (p. 538 Bennett en al. 2013).