Black Holes Michael Beattie

Life Cycle of a Star Formed when a large amount of gas (mostly hydrogen) starts to collapse in on itself due to is gravitational attraction Eventually, the gas will be so hot that hydrogen atoms will coalesce to form helium Eventually the star will run out of its nuclear fuels

When a Star Runs Out of “Fuel” In 1928 Indian graduate student Subrahmanyan Chandrasekhar worked out how big a star could be and still support itself against its own gravity The idea: a star can maintain itself at a constant radius by a balance between the attraction of gravity and the repulsion from the Pauli exclusion principle

The Chandrasekhar Limit Mass of more than one and a half times the mass of the sun White dwarf: radius of about 5000 kilometers, density of 1 ton per cubic centimeter, supported by exclusion principle repulsion between electrons in its matter Neutron star: radius of about 10 kilometers, density of 100 million tons per cubic centimeter, supported by exclusion principle repulsion between neutrons and protons in its matter

Beyond the Limit Chandrasekhar showed that the exclusion principle could not halt the collapse of a star more massive than the Chandrasekhar limit Robert Oppenheimer solved the main problem of understanding what would happen to such a star and his work was later extended by a number of people As the star contracts more and more, the gravitational field becomes increasingly stronger, to the point where light can no longer escape

The Event Horizon Imagine an object with such an enormous concentration of mass in such a small radius that its escape velocity was greater than the velocity of light. Then, since nothing can go faster than light, nothing can escape the object's gravitational field Think of the event horizon as the place where the escape velocity equals the velocity of light, a one-way membrane around the black hole where anything can fall in, but nothing can come out

Size of a Black Hole There is no limit in principle to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a black hole if you compress it to a high enough density. The Schwarzschild radius (the radius of the horizon) and the mass are directly proportional to one another: if one black hole weighs ten times as much as another, its radius is ten times as large.

Evidence of Black Holes John Mitchell pointed out that a black hole still exerts a gravitational force on nearby objects Astronomers have observed systems in which one visible star orbits around an unseen companion, attracted to it by gravity Some of these systems, such as one called Cygnus X-1, are also strong sources of X rays