12/04/2019
Tha Basic Idea Of Black hole....
The simplest definition of a black hole is an object that is so dense that not even light can escape its surface. But how does that happen?
The concept of a black hole can be understood by thinking about how fast something needs to move to escape the gravity of another object – this is called the escape velocity. Formally, escape velocity is the speed an object must attain to "break free" of the gravitational attraction of another body.
There are two things that affect the escape velocity – the mass of object and the distance to the center of that object. For example, a rocket must accelerate to 11.2 km/s in order to escape Earth's gravity. If, instead, that rocket was on a planet with the same mass as Earth but half the diameter, the escape velocity would be 15.8 km/s. Even though the mass is the same, the escape velocity is greater, because the object is smaller (and more dense).
What if we made the size of the object even smaller? If we squished the Earth's mass into a sphere with a radius of 9 mm, the escape velocity would be the speed of light. Just a wee-bit smaller, and the escape velocity is greater than the speed of light. But the speed of light is the cosmic speed limit, so it would be impossible to escape that tiny sphere, if you got close enough.
The radius at which a mass has an escape velocity equal to the speed of light is called the Schwarzschild radius. Any object that is smaller than its Schwarzschild radius is a black hole – in other words, anything with an escape velocity greater than the speed of light is a black hole. For something the mass of our sun would need to be squeezed into a volume with a radius of about 3 km.
Seeing the unseen
If light can't escape a black hole, how can we see black holes?
Astronomers don't exactly see black holes directly. Instead, astronomers observe the presence of a black hole by its effect on its surroundings. A black hole, by itself out in the middle of our galaxy would be very difficult to detect.
Imagine you arrive home one night to find the kitchen a mess. You know that it was clean when you left, but now there are dirty dishes in the sink and crumbs strewn about the counter. From the evidence, you know someone used the kitchen while you were out – in fact, you can even say that they made a sandwich and chips because of the types of crumbs you see on the counter. You might even be able to identify who in your household was in the kitchen based on what kind of chips they had or what they put on their sandwich. You never saw that person in the kitchen, but their effect on the kitchen was evident.
Studying black holes relies heavily on indirect detection. Astronomers cannot observe black holes directly, but see behaviors in other objects that can only be explained by the presence of a very large and dense object nearby. The effects can include materials getting pulled into the black hole, accretion disks forming around the black hole, or stars orbiting a massive but unseen object.
