Read this excerpt from A Black Hole Is NOT a Hole. One of the best black-hole clues is X-ray energy. As matter approaches an event horizon, its atoms "shine" with invisible X-rays. Although our eyes can't see the X-rays, we can make detectors that respond to their tremendous energy.

X-ray-catching telescopes make pictures of otherwise invisible X-rays. The pictures reveal that certain areas in outer space are emitting intense X-rays. Each bright location points out where something extraordinary is going on. It is a hint that a black hole might be there.

Why does the author include this description?

to explain why humans cannot see X-rays
to explain how X-rays help scientists study space
to explain that black holes are extraordinary
to explain where black holes are located