Formation of Stellar Black Holes
A star is created when gases are pulled together by gravity. As hydrogen atoms combine with other hydrogen atoms to release incredible amounts of nuclear energy in a process known as fusion, the star threatens to explode and rip apart. But gravity, the same force that created the star, now threatens to crush it.
Think of it as a tug of war: Gravity (red) is pulling toward the center with inward force, as radiation pressure (blue) is trying to escape with outward force. The result is a balance or equilibrium between gravity and heat pressure. Yet radiation pressure from the fusion reactions, unlike gravity, is lost with time. Over billions of years, the star fuses hydrogen into helium and on so on until iron, when the fusion process requires more energy than it releases. At that point, gravity wins. When a star has exhausted its fuel supply, gravitational forces crush the star to one of three possible outcomes:
Concerning which outcome a star would take, the deciding factor is mass. More mass equals more gravitational force. To put it simply, "the bigger the star, the harder the fall." If the star is over 8 solar masses, the energy from the collapses causes a violent explosion of the star's top layers known as a supernova. What is left of the star after the collapse is known as a collapsed star or stellar remnant. Generally, a star about the size of our sun would become a white dwarf. Our sun would not collapse into a black hole because of the Pauli exclusion principle. The exclusion principle states that two electrons cannot occupy the same energy state at the same time in an atom. As gravity crunches the star's atoms together, the electrons of the atoms repel and push away from each other. The closer electrons are crunched, the faster they bounce back and forth to repel other electrons. This electron degeneracy pressure gives the star an outward repulsion force to counteract the inward attraction of gravity. However, the speed that the electrons can move back and forth is limited by the speed of light. When electrons need to move faster than light (which they cannot) to counteract the attraction forces, gravity dominates the exclusion principle for electrons. The Chandrasekhar limit (about 1.4 solar masses) is the mass limit at which electron degeneracy pressure can prevent the gravitational collapse of the star. A collapsed star with mass greater than the Chandrasekhar limit will either become a neutron star or a black hole. When the exclusion principle for electrons is dominated, the electrons are crushed into the atoms' nuclei. The electrons and protons combine and neutralize to form neutrons, the star becomes a neutron star and gravity is now balanced by the exclusion principle for neutrons, which is stronger than the exclusion principle for electrons. But the exclusion principle for neutrons, like the exclusion principle for electrons, has its limits. If the collapsed star's mass is over 3 solar masses, gravity will dominate both exclusion principles and become a stellar black hole. Continue to Middleweight Black Holes. Back to the top of the page or the Formation outline. |
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