Why are black holes so difficult to study?
By definition, they cannot produce light, so they're elusive from that point of view. They are also the most compact objects that can be produced - if you could compress the Sun to a radius of about three kilometres, then you would produce a black hole. Because they are intrinsically compact and normally at very large distances, their projected size on the sky is extremely small, which is why it's so hard to see them.
So how can we 'see' black holes?
We can see the light produced by material falling onto a black hole. As it falls, the material becomes denser because it will be confined to a smaller and smaller region. It becomes hotter and more energetic and will eventually start shining. So the light we see is from outside the black hole. Some of that is very close to the black hole, but manages to be emitted before entering.
How did the Event Horizon Telescope (EHT) help image black holes?
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