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Black holes: nothing can escape these regions of space

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Black holes: nothing can escape these regions of space

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Black holes – regions of space where gravity is so strong that nothing can escape – are a hot topic these days. Half of the 2020 Nobel Prize in Physics went to Roger Penrose for his mathematical work showing that black holes are an inescapable consequence of Einstein’s theory of gravity. Andrea Ghez and Reinhard Genzel shared the other half to show that a massive black hole sits at the center of our galaxy.

Black holes are scary for three reasons. If you were to fall into a black hole left over when a star died, you would be shredded. In addition, the massive black holes observed in the center of all galaxies have insatiable appetites. And black holes are places where the laws of physics are obliterated.

Black holes are expected to form when a massive star dies. Once the star’s nuclear fuel is used up, its nucleus collapses into the densest state of matter imaginable, a hundred times denser than an atomic nucleus. It’s so dense that protons, neutrons and electrons are no longer discrete particles. Because black holes are dark, they are found when they orbit a normal star. The properties of the normal star allow astronomers to infer the properties of its dark companion, a black hole.
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The first black hole to be confirmed was Cygnus X-1, the brightest x-ray source in the constellation Cygnus. Since then, around 50 black holes have been discovered in systems where a normal star orbits a black hole. These are the closest examples of around 10 million that are expected to be scattered across the Milky Way.
Black holes are tombs of matter; nothing can escape them, not even the light. The fate of anyone falling into a black hole would be a painful “spaghettification”, an idea popularized by Stephen Hawking in his book “A Brief History of Time”. In spaghettification, the intense gravity of the black hole would pull you apart, separating your bones, muscles, tendons, and even your molecules. As the poet Dante described the words on the gates of hell in his poem “The Divine Comedy”: Give up hope, all of you who come in here.
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A hungry beast in every galaxy

Over the past 30 years, observations with the Hubble Space Telescope have shown that all galaxies have black holes at their centers. Large galaxies have bigger black holes.

Nature knows how to make black holes on a staggering range of masses, from the corpses of stars a few times the mass of the sun to monsters tens of billions of times the mass. It’s like the difference between an apple and the Great Pyramid of Giza.

Last year, astronomers released the very first image of a black hole and its event horizon, a 7 billion solar mass beast at the center of the elliptical galaxy M87.

It is over a thousand times the size of our galaxy’s black hole, and its discoverers won this year’s Nobel Prize. These black holes are dark most of the time, but when their gravity attracts stars and gas nearby, they turn into intense activity and pump out an enormous amount of radiation. Massive black holes are dangerous in two ways. If you get too close, the enormous gravity will suck you in. And if they are in their active quasar phase, you will be blown up by high energy radiation.

How bright is a quasar? Imagine flying over a big city like Los Angeles at night. The roughly 100 million lights emitted by cars, houses and city streets correspond to stars in a galaxy. In this analogy, the black hole in its active state is like a 1 inch diameter light source in downtown LA that outshines the city by a factor of hundreds or thousands. Quasars are the brightest objects in the universe.

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Supermassive black holes are weird

The largest black hole discovered to date weighs 40 billion times the mass of the Sun, or 20 times the size of the solar system. While the outer planets of our solar system orbit once every 250 years, this much more massive object rotates once every three months. Its outer edge moves at half the speed of light. Like all black holes, huge ones are shielded from view by an event horizon. In their centers is a singularity, a point in space where the density is infinite. We cannot understand the inside of a black hole because the laws of physics are breaking down. Time freezes on the event horizon and gravity becomes infinite at the singularity.
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The good news about massive black holes is that you might survive by falling into them. Although their gravity is stronger, the stretch force is weaker than it would be with a small black hole and it wouldn’t kill you. The bad news is that the event horizon marks the brink of the abyss. Nothing can escape from within the event horizon, so you couldn’t escape or report your experience.

According to Stephen Hawking, black holes are slowly evaporating. In the distant future of the universe, long after all the stars are dead and the galaxies have been plucked from view by the acceleration of cosmic expansion, black holes will be the last surviving objects.
The most massive black holes will take an unimaginable number of years to evaporate, estimated at 10 to the power of 100, or 10 with 100 zeros after. The scariest objects in the universe are almost forever.

Chris Impey is Professor Emeritus of Astronomy at the University of Arizona.

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