Squeezing the Life Out of a Star

1939: J. Robert Oppenheimer

The scientific papers that describe startling insights into the workings of nature seldom appear beneath headlines like, "The Most Amazing Thing Ever!" Instead, the titles are a bit more subtle, along the lines of, "On Continued Gravitational Contraction." Yet their conclusions are no less impressive.

The paper on gravitational attraction, for example, for the first time spelled out the inevitable fate of a massive star:

When all thermonuclear sources of energy are exhausted a sufficiently heavy star will collapse. Unless [something can] reduce the star's mass to the order of that of the sun, this contraction will continue indefinitely. — Physical Review, 1939

In other words, the star will form a black hole.

The paper was written by J. Robert Oppenheimer, a physicist at the University of California at Berkeley, and a graduate student, Hartland Snyder.

At the time, physicists were just beginning to understand the process that powers stars, known as nuclear fusion, which combines lighter and simpler chemical elements to make heavier and more complex ones. (The most basic reaction, which powers stars like the Sun, fuses hydrogen to make helium.) Fusion releases enormous amounts of energy. It also produces an outward pressure that counteracts the pull of gravity, preventing a star from collapsing.

By combining this model of a star's interior with Albert Einstein's theory of gravity and the physics of quantum mechanics, Oppenheimer and Snyder expanded on earlier studies of the fates of stars.

Those studies had found that stars like the Sun will end their lives as white dwarfs, which are small and dense, while heavier stars would end as neutron stars. Oppenheimer and another graduate student, in fact, had calculated that the maximum mass of a neutron star should be about 3.2 times the mass of the Sun. None of the studies, however, suggested what might happen to the heaviest stars of all.

Oppenheimer and Snyder determined that once fusion shut down in a star of a critical mass, gravity would overwhelm all other known forces of nature and cause star to collapse to an infinitely dense point. "The star thus tends to close itself off from any communication with a distant observer," they wrote, and "only its gravitational field persists."

Neither Oppenheimer and Snyder nor any of their colleagues could follow up on their startling assertion, though, because World War II intervened. Oppenheimer joined the war effort, and soon became scientific leader of the Manhattan Project, which developed the first atomic bombs.

Resources

On Continued Gravitational Attraction

Landmarks: Forgotten Black Hole Birth

Timeline