Black Holes
A star exists in a delicate balance between the crushing force of gravity, on the one hand, and the push of incredibly hot gases on the other. This balance exists as long as there is fuel for the fusion process that powers the star.
What happens when the star runs out of fuel? Then, gravity collapses the star. The more massive the star, the more drastic the collapse and the more condensed the remaining object.
Cas A is the remnant of a star that exploded about 300 years ago. The X-ray image shows an expanding shell of hot gas produced by the explosion. Courtesy of NASA/CXC/SAO
A star about the size of the sun:
The collapse produces enough pressure to squeeze atoms right out of existence, leaving electrons and nuclei packed tightly together in an object about the size of Earth. The result is called a white dwarf.
A star six to eight times larger than the sun:
Upon exhausting its nuclear fuel, a star this large undergoes a catastrophic explosion as a supernova. (See Neutrino Astrophysics) The force of gravity in the leftover object is so strong that the electrons are jammed into the nuclei to make a neutron star, with a diameter of only about 16 km.
A star ten or more times larger than the sun:
At this size, the force of gravity collapses the neutron core right out of existence to form a black hole. Gravity near the black hole is so strong that nothing can escape, not even light. More precisely, any matter or radiation inside a sphere called the event horizon falls inward and cannot escape. (See drawing.) For a black hole with ten times the mass of the sun, the radius of the event horizon is about 30 km.
History of Black Holes
The existence of black holes was predicted well before the 20th century. About a hundred years after Newton worked out his theory of gravitation, the English astronomer John Michell recognized in 1784 the possibility that the gravity of a very large star might be so great that nothing, not even light, could escape it.
In fact, a correct description of a black hole requires Einstein's general theory of relativity. Armed with this theory, Karl Schwartzschild in 1916 found a solution of Einstein's equations corresponding to the simplest kind of black hole. In the 1930s, other physicists, including Robert Oppenheimer, who later became the director of the Manhattan Project, produced more detailed calculations. John Wheeler coined the name “black hole.”
After a Star's Gravitational Collapse
