A super-massive black-hole can emit enough gamma rays to wipe out the entirety of life on earth from about 1,000LY away.So can supernovas. The radiation destroys the outer most layers of our atmosphere which causes solar radiation to drastically affect our entire ecosystem (by killing almost all forms of plants and algae, which kills all the herbivores, which kills the carnivores).
Black holes are not visible but their accretion disks are, if they have them. Accretion disks is debris that have yet to fall into the event horizon, which is what fuels the radiation that it emits. Radiation is not affected by gravity which is why it can escape black holes.
And quasars aren't black holes per say, it's a name given to a phenomenon, when a black hole becomes so supermassive that it affects an entire galaxy. A quasar is an accretion disk made from an entire galaxy.
Sorry for the lengthy response time. I was doing my own researching. I was trying to look up an explanation why electromagnetic radiation is capable of defying the gravitational pool of radiation. See, according to the latest theories, Black holes evaporate as time goes on. This happens when the astronomical pressures that forms inside a singularity create temperatures so awesome that it causes the black hole to radiate these electromagnetic waves. This means that the mass of the black hole is transforming into energy, slowly depleting the black hole of its mass. Eventually it will disappear and all that will be left is background radiation. I failed to find an explanation on how radiation manages to escape the gravitational pool of the black hole. It was an interesting read though.
