Hints: quasar paradoxically Nukers Alan Dressler the Palomar Telescope in California

In reality, although a quasar burns brightly, it is actually impossible to see if there's a black hole in the middle. Paradoxically, the black hole is made invisible by the fact that it swallows light. So for years, no one could be certain if supermassive black holes really did exist at the heart of these strange active galaxies. The Nukers have spent the last two decades hunting for these elusive monsters. The first problem they faced was to prove that supermassive black holes existed at all. What they were to discover would be stranger than most people could have imagined. One of the first of the Nukers to try to find one was Alan Dressler. In 1983 he came to the Palomar Telescope in California convinced that he'd found a way to prove that supermassive black holes exist.
You can't see a black hole directly. That's what makes it a black hole. So what you are looking for is evidence of its gravity. You are ever looking at how it pulls on the stars that are coming nearby.
Dressler knew that although a black hole is invisible, its immense gravity would hurl stars around it at over 500,000 kilometers an hour.

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