Over a billion years ago, two black holes in a distant galaxy spiralled together, rippling the very fabric of space. In December, those ripples reached the Laser Interferometer Gravitational‑Wave Observatory (LIGO) in the US, marking the second gravitational wave detection in history. The first occurred just three months before.

Black holes are real, and they’re speaking to us directly from across the cosmos.

Credit: Jeffrey Phillips

In popular culture, a black hole is the ultimate metaphor for an invisible destroyer: a dark, inescapable force swallowing everything in its path. Not even light can escape its gravitational pull. Yet black holes are some of the most essential, ubiquitous and brightest objects in the universe.

Black holes were first proposed as a natural result of the death of a star. Now there are hints that black holes might also be an essential part of the formation and evolution of galaxies.

Our own galaxy is teeming with black holes – hundreds of millions according to one estimate. The simplest way to make a black hole is to take a star many times as massive as the Sun and wait several million years for it to run out of fuel. When that happens, it will collapse in on itself.

To a physicist, a black hole is fairly simple: a pure spacetime object defined only by its mass, spin (leftover from the spin of the progenitor star), and perhaps an electric charge. According to a yet‑to‑be‑proven theorem, black holes cannot have any other properties. From an astronomer’s point of view, however, black holes are anything but simple.

Cygnus X-1 secured a place in the history of astronomy when a combination of X-ray and optical observations led to the conclusion that it was a black hole, the first such identification. Credit: NASA / CXC

The first stellar-remnant black hole was discovered in 1964, when astronomers zeroed in on a source of strong X-rays and radio waves. It was named Cygnus X-1 after the constellation where it was spotted.

Cygnus X-1 is producing light, rather than devouring it. That’s because it’s not alone: Cygnus X-1 is in a binary orbit with a hot blue star that it is slowly cannibalising.

As the stellar material nears the black hole, it condenses into a disc of hot material, making the black hole visible in the same way a whirlpool is made visible by colliding material swept up in the vortex.

Systems like this are called X-ray binaries and they’re easy to spot with X-ray telescopes. But more massive black holes can outshine entire galaxies.

Supermassive black holes, weighing in at millions to billions of solar masses, can be some of the brightest objects in the universe. As far as we can tell, every decent-sized galaxy has a supermassive black hole in the centre of it.

Such a monster might have formed through the merging of smaller black holes or through voracious consumption of nearby matter; most black holes probably grow through a mix of these two processes. When actively consuming gas and dust, supermassive black holes light up like the X-ray binaries, but on a much larger scale.

These powerful objects, called active galactic nuclei, serve as the central engines for violent outflows of matter and radiation. While matter falls into the black hole in a hot, swirling disc, the tangled magnetic fields produced by the interaction of the disc and the central black hole can also drive matter and radiation outward in powerful jets extending thousands of light years.

This violent upheaval helps make black holes, which are hard to see, at least easier to trace. The four million solar mass black hole in the centre of our own galaxy is detected through its consumption of small amounts of interstellar gas and dust, and by watching stars zip around it in close orbits.

Astronomers are still working to understand exactly how supermassive black holes in the centres of galaxies grow to such enormous sizes, but it seems clear that the growth of black holes and the growth of galaxies are inextricably linked.

Even though a supermassive black hole constitutes a tiny fraction of a galaxy’s total mass, the two appear to build up in lockstep with each other. How one manages to influence the other is still a mystery.

Black holes are far more than just a cool abstract concept about the stretching of time and the “spaghettification” of things that fall in. They are central to the evolution and structure of the cosmos. And we’re just starting to explore how.