By Sarah 2019 batch A black hole is defined by the escape velocity that would have to be attained to escape from the gravitational pull exerted upon an object. A black hole itself is invisible because no light can escape from it. When black holes were first hypothesized they were called “invisible stars”. A common type of black hole is the type produced by some dying stars. A star with a mass greater than 20 times the mass of our Sun may produce a black hole at the end of its life. In the normal life of a star there is a constant tug of war between gravity pulling in and pressure pushing out. Nuclear reactions in the core of the star produce enough energy to push outward. For most of a star’s life, gravity and pressure balance each other exactly, and so the star is stable. Howerver, when a star runs out of nuclear fuel, gravity gets the upper hand and material in the core is compressed even futher. The more massive the core of the star, the greater the forc...
