Mentioning “black holes” always seems to pique people’s interest. Everyone has heard of them and knows they are some of the strangest and most extreme objects in the universe. They have been the subject of numerous works of fiction in literature, television and film – “Interstellar” being the most recent.
Black holes are aptly named because anything that falls into them is lost forever. Only the most massive stars in the universe – those that are more than about 20 times the mass of our sun – will become black holes. These stars quickly burn through their nuclear fuel and explode spectacularly, briefly outshining the billions of other stars found in a galaxy. What is left behind from the explosion is a small, extremely dense remnant whose surface gravity is so strong that nothing, not even light, can escape it. In our own galaxy, the Milky Way, we expect there to be about one of these supernova explosions every century, each of which could be the birth of a new black hole.
If astronauts were to approach one of these stellar-mass black holes, they would begin to experience the unusual effects of these extreme objects. Albert Einstein’s general theory of relativity tells us that the increase in gravity as the astronaut approached would cause time to slow down compared to some distant observer. A mere moment for the astronauts could be a lifetime for someone watching on Earth. The astronauts would eventually cross a region where the pull of gravity is so strong that light is forced into a circular orbit. When the astronauts enter this region, it would be possible for them to see the back of their own heads since the light would travel around the black hole to the eyes of the astronauts. Eventually the black hole would be the astronauts’ demise. They would reach a point where the difference in gravitational pull between their feet and head would be so extreme that they would be literally ripped apart.
These stellar-mass black holes are generally tens of miles across, but they are tiny compared to those found in the centers of galaxies, known as super-massive black holes. Our own Milky Way has a black hole that is 3.7 million times the mass of the sun and about 13.6 million miles across (approximately half the distance between the sun and Mercury). The largest black hole found so far is about 4,600 times larger and more massive than this!
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Ultimately, the greatest property of black holes is that even as we learn more about these unusual objects, they continue to fuel the public’s fascination.