Black holes are perhaps the most nightmarishly fascinating features of our universe. Like long dark tunnels to nowhere (or giant garbage disposals), these mysterious fixtures in space exert a gravitational pull so gripping that nothing close by — not even light — can escape from being swallowed. What goes in, (mostly) never comes out. (More on that later.)
For this reason black holes are invisible to the eye, as lightless as the empty, dark space surrounding them. Scientists know they exist not because they can see an actual hole, but because a black hole's tremendous gravitational clench affects the orbits of nearby stars and gas. Another clue is the detectable radiation emitted as gas that's being sucked in is superheated. In fact, these strong X-ray emissions led to the discovery of the first black hole, Cygnus X-1 in the constellation Cygnus, in 1964.
If all this sounds like science fiction, read on. It's only the tip of the cosmic iceberg. As scientists are discovering, black holes are even stranger than science fiction. Here are seven mysteries to ponder.
1. Black holes distort time and space around them.
If you happened to fly near a black hole, its extreme gravitational pull would increasingly slow down time and warp space. You'd be tugged ever closer, gradually joining an accretion disk of orbiting space material (stars, gases, dust, planets) spiraling inward toward the event horizon or "point of no return." Once you crossed this boundary, gravity would overcome all chances of escape and you'd be super-stretched, or "spaghettified" as you plunged toward the singularity at the black hole's center — an inconceivably small point with a monstrous mass where gravity and density theoretically approach infinity and space-time curves infinitely. In other words, you'd be gobbled up and annihilated in a place that utterly defies the laws of physics as we understand them.
Take a simulated journey here:
2. Black holes come in miniature, middling and mammoth sizes.
Middling-sized stellar-mass black holes are the most common type. They form when a massive dying star, or supernova, explodes and the remaining core collapses from the weight of its own gravity. Eventually, it compresses into a tiny, infinitely dense singularity that forms the center. In truth, then, black holes aren't really holes, but points of highly compacted matter with outsized gravitational footprints. Stellar-mass black holes typically weigh about 10 times more than our sun, though scientists have discovered a few that are significantly larger.
Supermassive black holes are the biggest in the universe, some with masses billions of times that of our sun. Scientists don't fully understand how they form, but these enormous celestial mind-bogglers may have appeared shortly after the Big Bang and are believed to exist at the center of every galaxy, even the tiniest ones. Our own Milky Way galaxy spirals around Sagittarius A* (or Sgr A*), which contains the mass of about 4 million suns.
Researchers have also recently discovered stealth black holes that appear to devour material and gases at a slower rate, meaning fewer X-rays are emitted so they're harder to detect. Astronomers also believe there are tiny primordial black holes formed in the seconds after the Big Bang. These mini-mysteries have yet to be observed, but the smallest may be more minuscule than an atom (but with the mass of an asteroid), and the universe may be swarming with them.
Supermassive black hole Sagittarius A* (center) lies at the heart of our Milky Way galaxy. This image, taken with NASA's Chandra X-Ray Observatory space telescope, shows light echoes (circled) from a recent X-ray outburst. (Photo: NASA/Wikimedia Commons)
3. There are too many black holes to count.
The Milky Way galaxy alone is thought to harbor some 100 million stellar-mass black holes, plus supermassive Sgr A* at its heart. With 100 billion galaxies out there, each with 100 million stellar-mass black holes and a core supermassive monster (not to mention other types being discovered), it's like trying to count grains of sand.
4. Black holes devour things — and regularly spit them out.
Rest-assured, black holes don't roam the universe like hungry predators, stalking planets and other space prey for dinner. Rather, these heavenly beasts feast on material that orbits too close, like this unfortunate star that scientists have watched being swallowed for the last decade (the longest black-hole meal ever recorded). The good news is that Earth isn't on a collision course with any known black holes.
But just because we're unlikely to be slurped down, doesn't mean we shouldn't worry. That's because Sgr A* (and presumably other supermassive behemoths) occasionally fling out planet-sized "spitballs" that could someday do us in.
How do spitballs escape a black hole's clutches? They're actually made of matter that slips from the accretion disk before passing the point of no return and coalesces into chunks. In the case of Sgr A*, these hefty pieces are spewed into our galaxy at up to 20 million miles per hour. Here's hoping one never zooms too close to our solar system.
5. Supermassive black holes also give birth to stars and determine how many stars a galaxy gets.
In the same way that planet-sized fragments are expelled from the accretion disk, a recent discovery shows that behemoth black holes occasionally unloose enough material to form whole new stars. Even more remarkable, some even land in deep space, well beyond their galaxy of origin.
And a new study in the journal Nature, released in January, suggests that supermassive black holes not only create new stars, they control how many stars a galaxy gets by directly impacting how quickly the process of star formation turns off. Star formation, perhaps strangely, stops more quickly in galaxies with smaller — in a manner of speaking — black holes at the center.
Learn more about black-hole star formation here:
6. It's possible to stare into the abyss.
The new Event Horizon Telescope — powered by nine of the world's highest-resolution telescopes — recently took first-time photos of the event horizons surrounding two black holes. One is our own Sgr A* and the other is a supermassive black hole at the center of the galaxy Messier 87, 53 million light-years away. The image of the latter, now dubbed Powehi, astonished astronomers in April 2019, but the photo session also raised new interest in ongoing questions about what black holes look like and the mind-warping laws of physics that drive them.
7. Yet another black-hole head-scratcher…
Astronomers in South Africa recently stumbled upon a region of distant space where supermassive black holes in several galaxies are aligned in the same direction. That is, their gas emissions all jet out as though they were synchronized by design. Current theories can't explain how black holes up to 300 million light-years apart appear to be acting in concert. In fact, the only way it's possible, say researchers, is if these black holes are spinning in the same direction — something that may have occurred during galaxy formation in the early universe.
Editor's note: This story was originally written in April 2017 and has been updated with new information.