Black holes, 18 November 2013!Black holes occur when a type 2 supernova has enough mass to implode into 1.a singularity. !Stars with less than 8 times the mass of the sun (usually around 2 times the A.mass of the sun) turn into a neutron star when they implode. !The exact boundary is still a measure of debate. !a.When a star with over 8 times the mass of the sun implodes, it overcomes B.the Pauli exclusion principle – the force that keeps neutrons apart from one another. !When the Pauli exclusion principle is overcome, the matter is scrunched C.into a singularity (single geometric point that has a size of 0). !The mass of the star is still in the black hole, so it continues to exert gravity. 2.Like its mass, the gravity of a black hole is extremely concentrated.!When gravity is condensed, it operates more strongly over a shorter A.distance. !The sun can attract objects from far away, but its gravity is distributed a.over its large volume. You would be vaporized before you were pulled apart by its gravity. If the sun was condensed into a neutron star the size of San Francisco, it would exert the same gravity only five kilometers around itself. You could be pulled apart by this concentrated gravity.!When gravity exerts unequal force on an object, thus pulling it apart, 1.it is said to have a "tidal" effect.!Condensing the mass of a star to zero size leads to such concentrated b.gravity that even light can't escape. !The escape velocity from the gravity of a black hole is faster than the 1.speed of light. !Nothing goes faster than the speed of light, so nothing can escape a A.black hole. !This was explained by Einstein in the beginning of the 1900s, but a.it was known 10 years before Einstein's work. !The zone around a singularity where gravity is so concentrated that light 3.cannot escape is called a "black hole."!If a star the size of the sun was scrunched into a singularity, the black hole A.around it would have a radius of 3 kilometers.!Light could escape farther than 3 kilometers from the singularity, but not a.particles which cannot go as fast.!The farther a particle is from the black hole, the more likely it escape b.the black hole. ! Astronomers cannot find out what goes on immediately around a B.singularity because they get their information from photons of light, which cannot escape it. !In the early 1970s, astronomers started searching for black holes by looking 4.for their effects on the visible stars around them. !The idea of a black hole was first proposed in the 1700s. It was mostly a A.topic for philosophers, practitioners of physics, and mathematicians until Penrose and other scientists used computations to propose they were real the 1960s. !Astronomers find black holes by searching for the effect of something B.massive but unseen on visible stars. !Black holes can only be detected when they are near a visible object.!a. A large number of "candidate" black holes were found in the seventies and C.eighties, which have been sustained to this day. !Despite rigorous scientific review for over 30 years, none of them have a.been disproven. !Every critic who attacks the idea of a black hole ends up sustaining it 1.by the time they finish their research, even if it is contrary to their wishes and personal gain. !There are few skeptics any more. !A.Black holes could still be disproven with better information, but it 2.would be quite remarkable. !This phenomenon happens so frequently in science that it is called A."a beautiful theory slain by an ugly fact."!This class will just refer to these objects as black holes rather than b.candidate black holes.!The best candidate black hole is the unseen companion to a blue giant star D.in Cygnus. !The star has no other distinction.!a.There are many candidate black holes being discovered to this day. !E.The structure of a black hole: !5.There is a singularity in the center of a black hole with many times the A.mass of the sun. !The area of gravity surrounding the singularity is so intense that no light or B.particles can get out. This is the so called black hole. !Surrounding the black hole is a disc of hot gas that swirls around the black C.hole very fast. !As the gas goes spiraling into the black hole, its density and pressure rise. a.This heats the gas to over a million degrees kelvins, causing it to emit x rays. !Because of the density and pressure needed to create them, x rays 1.are telltale signs of the gravitational fields of black holes.!There is a stream of gas that feeds the disc of a black hole that comes from D.a nearby giant star. !The black hole is massive enough to pull gas from the outer shell around a.the giant star to itself. !Any red, orange, yellow, white, blue giant can feed a black hole. !1.This stream of gas falls into the black hole in clumps, causing it to emit b.random amounts of light like an irregular variable star. !The variation is detectable in two hours.!1.The star looks and behaves differently when near a black hole than if it c.was isolated.!If the star had no neighbors, it would be still and spherical. !1.A star near a black hole orbits a center of gravity between itself and 2.something many times the mass of the sun that we cannot see (a black hole). !You can tell the mass of what the star is orbiting by how fast the A.star moves. The larger the objects, the faster the star has to move to avoid being pulled in. !Astronomers can also see that the star is dumping gas into a massive 3.unseen object. !There is so much energy from the disk of in falling hot gas around a 4.black hole, that it often makes a hot spot on a giant star. !Because the black hole shines so intensely on them, these are A.almost always variable stars. !There are hundreds of these known.!a.Sometimes these stars are eclipsing binaries: they eclipse the black 5.hole periodically, blocking x ray emissions from the disc of gas around it.!Though the equations of Astronomers say that everything should be pulled 6.into a black hole, there are hundreds of proven cases of bipolar outflow from black holes. !This bipolar outflow comes out of the black hole in two opposite A.directions at the poles of the disk of in falling material. !There are intense magnetic fields in both jets of gas. !B.Bipolar outflow has so much energy, speed, and temperature that it can be C.seen across the entire detectable spectrum.!This is a lot of energy– the ratio from the shortest wavelength and the a.longest wavelength in the spectrum is about 1043. !It can