Black Holes

Black holes are the endpoint of the life of super-massive stars.

If a star ten or more times the size of our Sun undergoes a supernova explosion, it can leave behind a burned out stellar remnant (matter left over from the original star). With no force but its own gravity acting on it, the remnants will collapse in on itself. The remnants eventually collapse to the point where it has zero volume and infinite density creating "singularity ". As the density increases, light rays emitted from the now collapsed star remnants are bent and wrapped around a center point. When this happens, the gravitational force creates an intense gravitational field. A black hole has been created.

The huge gravitational pull of a black hole pulls in anything that crosses inside its Schwarzschild Radius. Nothing can escape- not even light itself. At the Schwarzschild Radius, the escape speed is equal to the speed of light. The Radius traps everything from stars to single protons.

* The Schwarzschild radius can be calculated using the following equation for escape speed: Vesc = (2GM/R)1/2 For photons, or objects with no mass, we can substitute c (the speed of light) for Vesc and find the Schwarzschild Radius, R, to be: R = 2GM/c2

Let’s say the Sun is replaced with a black hole with the same mass. The Schwarzschild Radius would be 3 km, compared to the Sun's radius of nearly 700,000 km. The Earth would have to get very close to be sucked into the black hole.

Identifying Black Holes

Since black holes can be rather small, and the light they have can’t escape, a black hole alone would be hard to see. We can see black holes, though, when they pass through a cloud of interstellar matter or get close to a star becuase matter from the star is sucked in. As matter is sucked into the black hole it gains kinetic energy and heats up. When the matter reach a few million Kelvin it emits X-rays. The rays escape the pull before the matter reaches the Schwarzschild radius, and we in turn can see it.

Another sign of a black hole is a random variation of emitted X-rays. Matter that emits X-rays does not fall into the black hole at a steady rate. Instead, it releases the rays sporadically, which causes a variation in the intensity of the X-rays.

~Sophia Thomson