A black hole is a "place in space where gravity pulls so much that even light cannot get out." Black holes are often created by the death of a star, when an immense amount of matter is squeezed into an impossibly smaller space, drastically strengthening gravity's pull. Some black holes are as tiny as a single atom — others have a mass equivalent to millions of stars.
Since no light can get out, black holes are invisible. Scientists can only hope to catch a glimpse of a black hole's "shadow." "As dust and gas swirls around the black hole before it is drawn inside, a kind of cosmic traffic jam ensues," — like water draining in a bathtub. That "shadow" is known as a black hole's "event horizon" — and it can be photographed.
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