In 1915, while serving on the front with the German army during the First World War, astronomer Karl Schwarzschild got his hands on a copy of Einstein's theory of general relativity. Oppenheimer just had to find out how to get there. This answer had already been delivered by a German physicist in 1916. But Oppenheimer and his students wanted to know where this gravitational collapse path leads and, thus, what is the final state of the universe's biggest stars. The most massive stars undergo a series of these collapses and bouts of nuclear fusion. That will be the fate of our star, the sun, after it exhausts the hydrogen at its core in around 5 billion years.įor stellar cores at least 1.4 times more massive than the sun, there is enough pressure, and thus heat, generated during gravitational collapse that further bouts of nuclear fusion can be triggered, with the helium created by the fusion of hydrogen itself forging heavier elements like nitrogen, oxygen and carbon. This would come to be known as the Chandrasekhar limit, and any star below it - unless it has a stellar companion feeding it material - is doomed to end its existence as a smoldering stellar remnant called a white dwarf. Indian-American physicist Subrahmanyan Chandrasekhar realized that, for stellar cores with a mass less than 1.4 times that of the sun, gravitational collapse would halt due to quantum effects that prevent particles from "squashing" too close together. The nature of the remnant depends on the mass of the stellar core. While the star's outer layers are shed, its core rapidly contracts, leaving an exotic stellar remnant. When this fuel is exhausted, a star can no longer support itself against gravitational collapse. Eight years before Oppenheimer's theory of star collapse and black hole birth, another theoretical physicist was thinking about what happens when stars run out of fuel for nuclear fusion.
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