by Chris Impey, University of Arizona
Halloween is a time to be haunted by ghosts, goblins and ghouls, but nothing in the universe is more frightening than a black hole.
Black holes – areas of space where gravity is so strong that nothing can escape – are a hot topic in the news these days. Half of the Nobel Prize in Physics in 2020 was awarded to Roger Penrose for his mathematical work showing that black holes are an inevitable consequence of Einstein’s theory of gravity. Andrea Ghez and Reinhard Genzel split the other half to show that a massive black hole sits in the center of our galaxy.
Black holes are scary for three reasons. If you fell into a black hole that was left when a star died, you would be shredded. Also, the massive black holes seen in the center of all galaxies have insatiable appetites. And black holes are places where the laws of physics have been obliterated.
I have been studying black holes for over 30 years. In particular, I have focused on the supermassive black holes lurking in the center of galaxies. 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. Galaxies where the black holes are active are called quasars. With everything we’ve learned about black holes over the last few decades, there are still many mysteries to solve.
Death of black hole
Black holes are expected to form when a massive star dies. After the star’s nuclear fuel is depleted, its nucleus collapses to the densest state of matter imaginable, a hundred times closer than an atomic nucleus. It is so dense that protons, neutrons and electrons are no longer discrete particles. Since black holes are dark, they are found when they orbit a normal star. The properties of the normal star allow astronomers to derive the properties of its dark companion, a black hole.
The first black hole to be confirmed was Cygnus X-1, the brightest X-ray source in the Cygnus constellation. Since then, about 50 black holes have been discovered in systems where a normal star orbits a black hole. They are the closest examples of about 10 million that are expected to be spread through the Milky Way.
Black holes are graves of cloth; nothing can escape them, not even light. The fate of anyone who falls into a black hole would be a painful “spaghettification,” an idea popularized by Stephen Hawking in his book “A Brief History of Time.” In spaghettiification, the intense gravity of the black hole would pull you apart and separate your bones, muscles, tendons and even molecules. As the poet Dante described the words over the gates of hell in his poem Divine Comedy: Give up hope, all of you who enter here.
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 in their centers. Larger galaxies have larger black holes.
Nature knows how to make black holes over a dizzying array of masses, from stellar that are few times the mass of the Sun, to monsters that are tens of thousands of times more massive. It’s like the difference between an apple and the Great Pyramid of Giza.
Just last year, astronomers published the first ever image of a black hole and its event horizon, a 7-billion solar mass animal in the center of the elliptical M87 galaxy.
It is over a thousand times larger than the black hole in our galaxy, whose discoverers received this year’s Nobel Prize. These black holes are dark most of the time, but when their gravity pulls in nearby stars and gas, they flare up into intense activity and pump out a huge amount of radiation. Massive black holes are dangerous in two ways. If you get too close, the enormous force of gravity will suck you in. And if they are in their active quasar phase, you will be blown away by high energy radiation.
How light is a quasar? Imagine hovering over a big city like Los Angeles at night. The approximately 100 million lights from cars, houses and streets in the city correspond to the stars of 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.
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, the huge ones are shielded by an event horizon. In their centers is a singularity, a point in space where the density is infinite. We cannot understand the interior of a black hole because the laws of physics break down. Time freezes at the event horizon, and gravity becomes infinite at the singularity.
The good news about massive black holes is that you can survive falling into one. Although their gravity is stronger, the tensile force is weaker than it would be with a small black hole and it would not kill you. The bad news is that the event horizon marks the edge of the abyss. Nothing can escape from within the event horizon, so you could not escape or report on your experience.
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According to Stephen Hawking, black holes are slowly evaporating. In the distant future of the universe, long after all stars have died and galaxies have been torn out of sight by the accelerating 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 100th power, or 10 with 100 zeros left behind. The most sinister objects in the universe are almost eternal.
Chris Impey, Professor of Astronomy at the University, University of Arizona
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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