How big is black hole?

Answer 1

Before you can answer 'how big is a black hole?', you have to understand exactly what a black hole is. A black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull.

That definition leads you to wonder slightly about gravity. If, for some reason, you throw a rock straight up into the air it will rise for a while, but eventually the acceleration due to the planet's gravity will make it start to fall down again. If the acceleration is enough, you could make it escape the planet's gravity entirely. It would keep on rising forever. The speed with which you need to throw the rock in order that it just barely escapes the planet's gravity is called the "escape velocity." As you would expect, the escape velocity depends on the mass of the planet: if the planet is extremely massive, then its gravity is very strong, and the escape velocity is high. A lighter planet would have a smaller escape velocity. The escape velocity also depends on how far you are from the planet's center: the closer you are, the higher the escape velocity. The Earth's escape velocity is 11.2 kilometers per second and the Moon's is only 2.4 kilometers per second. After taking those two facts into consideration, look at a black hole. It is so massive that light does not travel fast enough to escape its gravity. Since nothing known travels faster than the speed of light, nothing can escape a black hole.

Now, back to the original question 'how big is a black hole?'. There are many different ways to describe how big something is. Let's just look at much mass it has and how much space it takes up. There is no limit(in principle) to how much or how little mass a black hole can have. In theory, any amount of mass at all can be made to form a black hole if you compress it to the right density. Most of the black holes were produced by the deaths of massive stars, so scientists believe those black holes weigh about as much as a massive star. A typical mass for such a black hole would be about 10 times the mass of the Sun, or about 1031 kilograms. Astronomers also suspect that many galaxies harbor extremely massive black holes at their centers. These are thought to weigh about a million solar masses.

the more massive a black hole is, the more space it takes up. In fact, the Schwarzschild radius(radius of the event horizon) and the mass are directly proportional to one another: if one black hole weighs ten times as much as another, its radius is ten times as large. A black hole with a mass equal to that of the Sun would have a radius of 3 kilometers. So a typical 10-solar-mass black hole would have a radius of 30 kilometers, and a million-solar-mass black hole at the center of a galaxy would have a radius of 3 million kilometers. Whether you measure by mass or space taken up, black holes can be some of the largest objects in the universe.