How did scientists know about the existance of black holes, how they behave etc… long before getting the very first image of one
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The first X-ray telescopes in the 1960s and 1970s showed a few hundred stars glowed very brightly in X-rays. Some of the spots where X-rays were found were where there were binary star systems. We couldn’t tell the two stars apart by sight but by how their motion shifted their colors. In some cases only one star could be detected moving about an unseen companion. Some of those star systems contained neutron stars. Neutron stars have a maximum mass of something like 2 or 3 times the Sun’s mass, according to our theories.
But some star systems like Cygnus X-1 showed bright X-rays and that the visible star was being pulled on by something MUCH more massive than is possible for a neutron star (the mass of the unseen object can be known from how fast it makes the visible star orbit around.) Stephen Hawking actually bet AGAINST black holes being real because he figured he’d win either way: either he’d win the bet, or black holes would be real and his life work would be meaningful! He eventually had to admit that the evidence that star systems like Cygnus X-1 were black holes was so convincing that there was no doubt he lost the bet. This was long before we were able to make images showing the event horizon.
Also there are some galaxies that are REALLY bright in their centers. These are known as quasars. How could so much energy come from such a small space? The only way anyone could figure out to make so much energy in such a small space was if gas was falling onto a really massive black hole. Starting with the Hubble Space Telescope we were also able to measure how fast gas was spinning around galaxy centers, and from that figure how massive those galaxy centers were. In some cases, there were billions of times the Sun’s mass coming from areas comparable to our solar system. The only way that could happen would be if there were a black hole there.
And in the center of the Milky Way, Nobel Prize Winner Andrea Ghez and her team were able to make movies of stars orbiting an unseen object over decades, and from the speeds of those orbits it was clear there was an unseen object millions of times more massive than our Sun in the center of the Milky Way.
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