How we see something black

You can't see a black hole. You can see what it does to everything around it. Astronomers have found four ways to catch one.

Stars on impossible orbits

Starting in the 1990s, two teams spent decades tracking stars near the center of our galaxy, whipping around something invisible. One star, S2, completes a lap every 16 years or so. To explain those orbits you need about 4 million Suns' worth of mass packed into a region not much bigger than our solar system. Nothing but a black hole fits. Andrea Ghez and Reinhard Genzel each won a share of the 2020 Nobel Prize in Physics for that work.

Gas that glows before it vanishes

Gas falling toward a black hole doesn't drop straight in. It swirls into a flat disk, rubbing against itself and heating to millions of degrees, hot enough to shine in X-rays. Cygnus X-1, an X-ray source spotted in 1964, became the first widely accepted black hole in the early 1970s. Stephen Hawking famously bet that it wasn't one, then conceded in 1990.

The supermassive ones can do this on a grand scale. A quasar is a giant black hole feeding so fast that its disk outshines the hundreds of billions of stars in its galaxy. Something that gives off no light of its own ends up among the brightest things in the universe.

Drag sideways to tilt this schematic disk from face-on toward edge-on. Real images also bend light around the hole, so their bright rings are not simple views of this disk.

Ripples in space

On September 14, 2015, two detectors called LIGO, one in Washington State and one in Louisiana, felt a faint wobble. Each detector has two arms 4 kilometers long, and their lengths changed by a tiny fraction of the width of a proton. The signal lasted under a second.

It came from two black holes, roughly 36 and 29 times the Sun's mass, spiraling together and merging about 1.3 billion light-years away. In that last instant they turned about three Suns' worth of mass into ripples in space itself, called gravitational waves. It was the first direct proof that black holes come in pairs and merge. Three of its leaders won the 2017 Nobel Prize, and detectors have since recorded hundreds of mergers.

The first pictures

In April 2019, the Event Horizon Telescope released the first image of a black hole. It shows a glowing ring around a dark center, at the heart of the galaxy M87, about 55 million light-years away. The team linked radio telescopes across the world to work like one telescope the size of Earth, and separate teams checked the processing.

The dark center is called the shadow. It isn't the horizon itself. Light bends around the black hole, so the shadow looks about two and a half times wider than the horizon.

In May 2022 they released a picture of Sagittarius A*. That one was harder. It's far smaller than the one in M87, so gas circles it in minutes instead of days, and the view kept changing while they recorded it. Both images come from observations made in 2017.

What this does and doesn't prove

Everything here is observed, and it all matches Einstein's description of black holes. None of it shows us the inside. The horizon itself is inferred from how everything around it behaves. That's a strong inference, but still one step removed.