I'm sure most of you might have heard the word "Black Hole" some time in your life, either from a documentary or a Sci-fi TV series etc. i thought of writing this overview for those people that are fascinated about Black Holes but don't really understand what they are, but first before getting in to a black hole there are some basic things that you have to be familiar with lol.
1. What is Gravity? - Gravity is a very weak force that accompanies any thing that have a Mass, everything that has a mass are attracted to each other because of gravity, gravity gives us weight by pulling us constantly towards the earth, remember gravity is one of the weakest forces in the universe, take earth for example it is quite huge, and has quite a bit of mass but still we can walk and jump around on it.
2. what is Escape Velocity? - Escape Velocity is the minimum speed that is needed to escape from a gravitational field. for example if you throw a rock in the air no matter how hard you throw, it just fall back on to earth, but if you are somehow able to throw the rock at the escape velocity of the earth it will break free from earths gravity and venture in to outer space.
3. Speed of light? - Speed of light is according to General Theory of relativity is the absolute maximum speed attainable. in simple terms nothing can go faster than the speed of light, it equals to 300,000 Kmps
4. What is a Black Hole? - Black Hole is a point in space that have a Gravitational feild so massive, nothing can escape a black hole not even light, so we call it a Black Hole....
Ok well thats a very draft Answer to that question, now lets get in to more details shall we
Formation of a Black Hole
Black Holes are formed in two ways as far as we know and this is the most common one
Gravitational Colapse of a Star -
Take our good old sun, you know that its extremely large, hence it has a massive gravitational feild enough to sustain 8 planets and billions of smaller debree and some minor planets, and you know that sun is a star, Stars dont have a solid surface of sorts, they are just massive balls of gas held togeather by their own massive gravitational feild, stars reach gravitational colapse when it no longer has enough fuel to keep it from shrinking because of its own Gravity, when a star cools down, all the matter in the star starts moving toward the center of it and the star starts to shrink, in the case of our sun this shrinking will stop at certain point and will produce a dwarf star, because suns mass is not enough to produce a black hole, say we are talking about a star 20 times the size of our sun, the gravity of this star would be so massive, when it colaps all the matter in it will shrink in to a point in space called a singularity and will form a black hole. but remember that the overal gravity of the black hole is still equal to the Gravity of the colapsed star, but since now all the matter is in the state of singularity, gravity is oncentrated in a much smaller space. all Black holes have a hypothetical distance from the center of the singularity where the gravity will cause escape velosites exeeding the speed of light, this hypothetical barrier is called the Event Horizon, anything that falls in to the Event Horizon will get stuck there with no hope of escaping, even light. Beyond event horizon our laws of physics seize to exisist.
Some cool things that can happen arround a black hole
In General Theory of relativity Time is affected by gravity, and this is scientificaly proven, Time usualy bends near objects with a lots of mass, and also it relatively slows down, say if ur a observer in a earth orbit where earths gravity is much less, in your point of veiw a clock on the surface of the earth runs slower than the clock on your wrist, but this amount is extremely small, we can never detet the difference. But its different in the case of black holes, say ur some distance away from e event horizen of a black hole and you send a ship towards the event horizon in your point of veiw you will notice that the ship slows down as it gets near and near to the black hole and finally you will see the ship stop to a halt as it enters Event horizon, in your perspective the ship will stay in a complete stop forever near the Event Horizon, but if you were on the ship you will be having a very painfull journy towards the singularity lol. this effect is called the Time dialation. so you can stop time near a black hole isnt that cool lolz
Space is also effected by gravity, and space can bend near a object with a extremely large Gravitational field, and thus creating a worm hole, a worm hole connect two dsitant spots in space time, acting as sort of a bridge, remember they are strictly hypothetical and not scientifically observed, it is belived that even if worm holes exist they will not be traversable, because of their fragile nature and the amount of radiation, time dialation involved, but its cool to know such thing can Theoritically happen. this image shows how a worm hole connects two points in space time.
I hope this gave you some idea of what a black hole is, just browse arround the net and you will find tons of information regarding all the stuff that are discussed here.