What are black holes in sciencia and astronomy?

Answer 1

Stars begin as interstellar clouds of gas that gradually organize into clumps. Eventually these growing clumps become so massive that they begin to exert their own uniform gravitational force. This force causes the gas clumps to ball up and become dense. As the gravitational field becomes stronger, the nascent star begins to collapse inward under its own gravitational force. As this happens, the temperature and pressure at the very interior gets so high that nuclear fusion begins to occur in which Hydrogen atoms combine to form Helium, and emit energy. This nuclear reaction expels such a strong outward force that it counteracts the inward gravitational force - resulting in a spherical star with a relatively fixed size.

Eventually, all of the Hydrogen and "fuel" that drives this nuclear fusion is used up, and thus the outward force begins to die off. When this occurs, the inward gravitational collapse goes unhindered, and star begins to become very tiny and enormously dense. As the density grows and grows, the gravitational force gets higher and higher.

The higher a gravitational force gets, the harder it becomes for anything to overcome the gravitational force. For example, if you are on the moon (low gravity) and shoot a rocket upward, the gravitational force of the moon is too weak to stop its ascent. It will therefore leave the moon and fly indefinitely outward into space. If you were hypothetically able to try this on Jupiter (very high gravity), the rocket you launch may not even get a few hundred feet in the air before it is slowed down, then slammed back into Jupiter's surface.

So on the dying star with a high gravitational force, nothing can escape its gravity - not even a beam of light. Because of this, an observer would look in the direction of the dying star and see nothing but black.