A black hole is a field in space that exhibits very strong gravitational pull that nothing not even electromagnetic particles or light can pass from it. Strong gravitational pull occurs as a result of matter that has been pressed into a tiny space. Typically, an ordinary black hole is formed by specific stars that are dying. A star that is about 20 times massive than the sun may produce a black hole at the end of its life, essentially, black holes are not invisible because light cannot  penetrate from them.

In essence, black holes are invisible because of strong gravitational pull that is pulling all the light into the center of the black hole. In most cases scientists can utilize space telescopes to examine the effects of strong gravity on the stars and surrounding gases. Generally, black holes come in a spectrum of sizes that is mainly determined by black holes’ size and mass.  The black hole that is small is the size of an atom, with mass equivalent to that of a large mountain and is known as primordial black holes. The medium-sized black hole, which is also the commonest, is known as stellar black hole. The mass of this type of black hole can be twenty times greater the mass of the sun. Supermassive black holes are the largest; they have masses greater than one million suns compounded together. Studies suggest that supermassive black holes lie at the center of every big galaxy. For instance, the Sagittarius A is a supermassive black hole found in the heart of the Milky Way galaxy.

In the lifetime of star, there is always a constant tug-of-war between pressure pushing out and gravitational pull. Nuclear reaction at the heart of the star generally produces adequate pressure and energy to push outwards. For the most part, a star is able to moderate pressure and gravity to remain relatively stable in its lifetime. However, in case a star is to run out of nuclear energy then gravity shall have an upper lead, compressing materials even further in the center. The massive the stars core the dominant the gravitational force that compresses the material. Ordinarily, for small stars, when nuclear energy is used up and there are no underlying nuclear reactions that can combat gravity, repulsive force within the stars’ electrons develop sufficient pressure to stop further gravitational collapse after which the star cools off and dies.

On the other hand, if a supermassive star uses up its nuclear energy, then, it explodes as a supernova. The stars’ outer parts are violently expelled into space while the core collapses. In case the core remains after explosion of the supernova, there is no repulsive force that is able to withstand the force to prevent gravity from collapsing the core into a black hole. Drawing on the collapsing star, the core is indurated, virtually with null volume, into infinite density. Following this, it requires velocities higher than the speed of light to escape from the gravity of the objects.

In retrospect, since no object can move at the speed of light, regardless of the radiation, nothing can escape from the boundary of the black hole. As such anything and everything that passes within the black holes boundary including light is trapped forever. Nonetheless, a black hole is unlikely to destroy the earth. Most black holes do not wander around the earth, like other objects in space they stick to gravitational laws. Therefore, for a black hole to be able to affect the earth it has to be very close to the solar system which is an unlikely scenario.