Black Hole

All astronomical objects have an escape velocity, the speed required for an object to escape the gravitational influence from another. For Earth, the escape velocity is ~11.2 km/s, and that means you need a rocket that flies faster than 11.2km/s to leave the Earth. The higher the mass of an object, the greater its escape velocity. But what happens when you have an object that is so massive that light—which already travels at the cosmic speed limit—cannot escape the object? What you have is a black hole, an object with an escape velocity greater than the speed of light.

Despite its name, black holes are not perfectly black. They do emit radiation through a mechanism known as Hawking’s radiation. We can also observe black holes, albeit indirectly, through from their interaction with their surroundings. Active black holes at the center of galaxies accelerate matter near its event horizon and spew them out as relativistic jets occasionally. Black holes also heat the material around them, providing themselves with a ring of luminous gas as we see in the picture of the black hole in M87 on the right, taken by the Event Horizon Telescope.