So what exactly are black holes? Lets put it this way. A black hole is just another thing a star can turn into when it runs out of fuel, in this case it is simply a massive ex-star that has collapsed. Black holes are very interesting and the properties that define them are how they are in mass, size, and what makes them black, how they are created, how they are observed. The creation of black holes is very interesting in my opinion. Basically you have a star with “critical mass” that has collapsed under its own weight until it became a black hole. If the star is heavy enough, it can develop into a black hole. Due to its density the star makes a dent in space-time curvature that is infinitely long. Because of this, it is very possible that they play an important role in galaxy formation and evolution according to scientists.
But why is a black hole black? The reason for this is because the escape velocity of a black hole, which is the speed needed to leave the earth, is greater than the speed of light therefore the photons cannot leave. The explanation is fairly easy, light does not have a high enough speed to leave a black hole so it cannot escape. Think about shining a flashlight on a black hole. This would be useless because when flashlight photons hit an object, we only see the because they bounce off the object . In this case though, the photons of the light will just fall into the black hole and never come back out. The same with stars because we cannot see light coming out from the star so it appears black. This is the reason why a black hole is black. The first evidence provided for the existence of black holes is called the event horizon. The event horizon for a black hole is defined as the specific distance from its center where the velocity needed to escape is exactly the speed of light. Anything that gets closer to this cannot leave and it will never be seen again by anyone outside the black hole. The event horizon can help us study black holes by simply watching things approach them. What happens to things as they fall in is that they go through spaghettification, which is the process of an object stretching out and then being ripped apart as a result of the gravitational pull. Another example is the fact that scientist have found instances where it seems that stars are orbiting around nothing. We know that this nothing is a black hole though. As this happens the atoms are forced closer together which produce light (mostly x-rays) that can be seen here on earth. As a result scientists can also measure the mass of a black hole by studying the orbit of a nearby star. Because of this we can determine in fact that a star is orbiting a black hole. These two terms plus the fact that stars orbit around a black hole provide evidence about black holes which helps scientists identify them. In order to understand black holes we first have to know how they are created and what makes a black hole a black hole. After understanding this everything else falls together such as explanation of why a black hole is black and why light cannot escape from it. Upon understanding these two things, then we can better understand the evidence of black holes such as event horizon and escape velocity, which helps scientists identify a black hole.