The roughly 100 million lights from cars, houses and streets in the city correspond to the stars in a galaxy. How bright is a quasar? Imagine hovering over a large city like Los Angeles at night. ![]() And if they are in their active quasar phase, you’ll be blasted by high-energy radiation. If you get too close, the enormous gravity will suck you in. Massive black holes are dangerous in two ways. These black holes are dark most of the time, but when their gravity pulls in nearby stars and gas, they flare into intense activity and pump out a huge amount of radiation. It’s over a thousand times bigger than the black hole in our galaxy, whose discoverers snagged this year’s Nobel Prize. Just last year, astronomers published the first-ever picture of a black hole and its event horizon, a 7-billion-solar-mass beast at the center of the M87 elliptical galaxy. That’s like the difference between an apple and the Great Pyramid of Giza. Nature knows how to make black holes over a staggering range of masses, from star corpses a few times the mass of the Sun to monsters tens of billions of times more massive. Over the past 30 years, observations with the Hubble Space Telescope have shown that all galaxies have black holes at their centers. They are the nearest examples of about 10 million that are expected to be scattered through the Milky Way. Since then, about 50 black holes have been discovered in systems where a normal star orbits a black hole. The first black hole to be confirmed was Cygnus X-1, the brightest X-ray source in the Cygnus constellation. The properties of the normal star allow astronomers to infer the properties of its dark companion, a black hole. Since black holes are dark, they are found when they orbit a normal star. That’s so dense that protons, neutrons and electrons are no longer discrete particles. After the star’s nuclear fuel is exhausted, its core collapses to the densest state of matter imaginable, a hundred times denser than an atomic nucleus. Death by black holeīlack holes are expected to form when a massive star dies. ![]() With all we’ve learned about black holes over the past few decades, there are still many mysteries to solve. Galaxies where the black holes are active are called quasars. Most of the time they are inactive, but when they are active and eat stars and gas, the region close to the black hole can outshine the entire galaxy that hosts them. In particular, I’ve focused on the supermassive black holes that lurk at the center of galaxies. I’ve been studying black holes for over 30 years.
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