what is event horizon?

Answer 1

The event horizon is a fictious border. Anything inside it cannot escape the black hole, even light, as it is pulled in faster than it could travel in the opposite direction (lightspeed, maximum possible speed).

Because even light cannot escape from within this fictious border, we cannot see anything inside the border. Any light that is emited or reflected inside the border, cannot make it to our eyes.

The most weird part: if you are watching someone being sucked into a black hole, you do not see him suddenly vanish. Instead, this person seems to slow down further and further, until he appears to be motionless. Red shift will then occur, and this person will cease to be visible for the human eye.

Answer 2

Regular black holes are thought to form from heavy stars (perhaps those which start off with masses more than 20 or 25 times that of the Sun, but this is still an area of active research). When these stars end their lives in a supernova explosion, their cores collapse and gravity wins out over any other force that might be able to hold the star up.

Eventually, the star collapses so much that it is contained within its Schwarzschild radius, or event horizon, the boundary within which light cannot escape. At this point, the black hole is extremely tiny; a black hole with the mass of the Sun would fit in a small town, while one with the mass of the Earth would fit in the palm of your hand! The material inside the Schwarzschild radius will continue to collapse indefinitely, reaching the point where our understanding of the laws of physics breaks down. But no information from inside the Schwarzschild radius can escape to the outside world.

Supermassive black holes, meanwhile, form differently - perhaps from the merger of many smaller black holes early in the universe's history - and grow over the years as they suck in gas from their surroundings. The formation of these objects and their relationship to the galaxy that harbors them is still an area of active research.

Answer 3

The event horizon is the gravity field of a black hole where the space-time is so bent that light cannot escape it. The event horizon creates a region in space where nothing can escape, if nothing can go beyond the speed of light. Thus when something enters the event horizon, it will vanish without a trace. Should the object be emitting something, after it is enveloped by the event horizon, not even the emissions that traced its existence will escape the black hole.

Here is a picture I use when teaching this at our observatory

http://math.ucr.edu/home/baez/PUB/CygnusXR1plunge.jpg

Answer 4

An event horizon is any barrier that allows only one way penetration.

Answer 5

it is the edge of a black hole beyond which nothing - not even light - can escape the gravitational pull of said black hole.