Black holes have dominated astronomy headlines in recent years. In 2015, we detected gravitational waves—ripples in the fabric of spacetime—from two black holes, each about 30 times the mass of our Sun, spiraling into each other.
This discovery sent shockwaves through the scientific community. Just two years later, the Nobel Prize in Physics went to three pioneers of gravitational wave research. And there was more to come.
In 2019, astronomers captured the first-ever image of a black hole's silhouette. It looks like a glowing donut. [Note: As this article goes to press, the 2020 Nobel Prize in Physics has just been awarded to researchers studying black holes—a fitting coincidence!]
Children of the Stars
Black holes sound exotic, but they're essentially the skeletons of dead stars. The 2019 image may look like a donut, but the process that creates black holes is more like baking a soufflé.
To understand black holes, you need to understand stars.
The early universe was simple: mostly hydrogen and helium. Gravity slowly pulled these gases together into clouds. As the clouds compressed, they grew denser and hotter. Eventually, the cores became hot enough to ignite nuclear fusion—and stars were born.
Nuclear fusion is the process of squeezing small atoms together to make larger ones. Think of it like kneading multiple balls of dough into one big loaf. During this process, a small amount of mass converts to energy, following Einstein's famous equation E=mc². This energy is why stars shine.
That energy also pushes outward, balancing against gravity's inward pull. A star is like a soufflé in the oven—the heat makes it puff up and hold its shape.
But fuel doesn't last forever. When a star exhausts its nuclear fuel, it loses the energy that was holding it up. Gravity wins. The star collapses.
What happens next depends on mass. Smaller stars settle into a stable state called a white dwarf—like embers glowing faintly after a fire burns out. Eventually, even that glow fades.
Massive stars face a more dramatic end. When fusion stops, they collapse so violently that they explode outward in a supernova. Think of squeezing a chocolate soufflé until it bursts—the filling sprays everywhere. These explosions scatter heavy elements across space: the oxygen we breathe, the silicon and magnesium that make up Earth's crust.
Meanwhile, the star's core keeps collapsing. If it's massive enough, nothing can stop it. It becomes a black hole.
A stellar death produces two opposite outcomes: an abyss at the center, and a richer universe around it. The elements forged in dying stars became the building blocks of planets—and of us.
You and I, along with black holes, are all children of the stars. By studying black holes, we're studying our own origins. Where did we come from? Where are we going?
The Hunt for Hidden Black Holes
If black holes are stellar corpses, there should be billions of them in the Milky Way. So where are they?
This is one of astrophysics' great challenges. By definition, black holes don't emit light—nothing can escape their gravity, not even photons. You can't see them directly.
But we can detect them indirectly. When gas from a nearby star falls toward a black hole, it heats up to extreme temperatures and emits X-rays. By detecting these X-rays, we've found over twenty black holes—even before gravitational wave astronomy came along.
Here's a distinction worth knowing. Everything I've described so far involves "stellar-mass" black holes—the remnants of individual stars. But there's another category: supermassive black holes, millions or billions of times the Sun's mass, lurking at the centers of galaxies. Our own Milky Way has one that's four million solar masses.
The discovery of the supermassive black hole at our galaxy's center earned two researchers part of this year's Nobel Prize.
Think of stellar-mass black holes as individual skeletons. Supermassive black holes are more like mass graves at the heart of every galaxy. (Their exact formation is still hotly debated.)
The donut image from 2019? That's a supermassive black hole, billions of times the Sun's mass. The gravitational waves from 2015? Those came from stellar-mass black holes, only about 30 solar masses each.
The 2017 Nobel went to the smaller variety. The 2020 Nobel honored the giants.
Supermassive black holes are easier to spot because they're, well, supermassive. Research on stellar-mass black holes is just getting started. But with gravitational wave detectors online and the Gaia satellite tracking billions of stars, we're finally equipped to find them.
Black holes don't emit light, but they often have companion stars that do. Imagine watching a tango in a dark room: if one dancer wears a glow-in-the-dark shirt, you can infer the other's presence from how they move together.
Expect stellar-mass black holes to make headlines for years to come.
A Closing Thought
People often ask me: what's the point of astrophysics? Honestly, I'm not sure. But I've come to think that much of life is "pointless" in the narrow sense.
The universe began with nearly nothing. It will end with nearly nothing—just black holes and white dwarfs cooling in the dark.
What counts as useful then? Maybe the only thing we can do is follow the advice from Liu Cixin's The Three-Body Problem: "Make time for civilization, for civilization won't make time."