Black holes are so named because A. they emit a perfect blackbody spectrum. B. no light or any other electromagnetic radiation can escape from inside them. C. all their electromagnetic radiation is gravitationally redshifted to the infrared, which leaves no light in the optical region. D. they emit no visible light, their only spectral lines are in the radio and infrared.

B. no light or any other electromagnetic radiation can escape from inside them.

Explanation:

Black holes are the result of the gravitational collapse of a very high-mass star.

High-mass stars, when they run out of hydrogen for the nuclear fusion inside their cores, start burning heavier and heavier elements, until only carbon is left; Nuclear fusion of carbon does not occur (because it is not energetically convenient), so the nuclear fusion in the core of the star stops, gravity prevails and the star collapses on itself. As a result, the material "bounces back" out into the space in a huge explosion, called supernova.

If the mass of the star was high enough, the remnant left after the supernova explosion is a black hole, which consists of a region of space where the density of matter is so high that the the gravitational attraction is so strong that even electromagnetic radiation cannot  escape it.

This means that light and any other electromagnetic radiation cannot escape from the gravitational field of a black hole.

The radius of a black hole, called Schwarzschild radius, is given by:

where

G is the gravitational constant

M is the mass of the black hole

c is the speed of light

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