A black hole is a place in space where gravity is so strong that even light cannot escape. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.  Since no light can escape from a black hole, they are invisible. Black holes are detected as surrounding material (like gas) is funneled by the force of gravity into a disk around the black hole. The gas molecules in the disk swirl around the black hole so fast that they heat up and emit X-rays. These X-rays can be detected from Earth.  Also, we can detect black holes by observing the objects around them, like in the animation below.

This is why we think there’s a Black Hole in middle of our galaxy: these stars seem to be orbiting around nothing.

Black holes vary in size – the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or “stuff,” in an object.

Another kind of black hole is called “stellar.” Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar mass black holes in Earth’s galaxy. Earth’s galaxy is called the Milky Way.

The largest black holes are called “supermassive.” These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.

Here’s how super massive black holes work.

Source:  www.phdcomics.com