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Life of a Star

Before looking at how a star collapses to become a black hole, let’s look at its life. Stars are formed inside vast clouds of gas and dust to drift together to form clumps called protostars. Each protostar shrinks until its center becomes so dense that nuclear reactions begin inside it, and it starts to shine. The Orion nebula, a huge cloud of gas and dust is lit by the light of nearby stars. Stars come in different sizes. The Sun is a pretty average star, which glows yellow. Larger stars glow blue or white because they are hotter, but they don’t shine for as long. Smaller stars glow Orange or red. They are cooler and last longer.
After thousands of millions of years, the nuclear reactions in the sun will stop. Gravity will then squeeze the core, creating heat that will make the outer layers swell, swallowing Earth. The outer layers will drift into space, leaving a planet-sized star called a White dwarf.

Formation of Black Holes

Karl Schwarzschild predicted that a Black hole would be formed when a massive star stopped shining and collapsed in on itself. The gravity would be so large that the material would become more and more dense, making the gravity still stronger. Gravity would eventually become so great that the star would keep collapsing past its gravitational field! In 1939, American astrophysicists Robert Oppenheimer and Hartland Snyder used mathematics to show that this collapse would happen.

Gravitational Radius

The French Mathematician Pierre Laplace used Newton’s laws to calculate the size of a body that would stop light from escaping. Karl Schwarzschild calculated at what distance from the center of a body the escape velocity would be the speed of light. He used Einstein’s relativity theory. Remember that the force of gravity between two objects gets greater as the objects get close together. At a certain distance from a body, the gravity becomes so great that the escape velocity becomes greater than the Speed of light.
Schwarzschild calculated the relationship between this distance from a body’s center and the mass of the body. This distance is known as the Schwarzschild radius. For bodies such as planets and stars, Schwarzschild’s radius is mush smaller than the body. For example for Earth it is less than one half of an inch, and it is about one and a half miles for the sun.
Schwarzschild’s theory was that a black hole was formed if the gravitational radius of a body was larger than its actual radius. This means a body would have to be squeezed into an extremely tiny space. For example, Earth would have to be squashed to the size of a pea for it to become a Black hole.