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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 oneway 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