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Black Holes: Seeing the Invisible!

Black holes are some of the most bizarre and fascinating objects in the cosmos. Astronomers want to study lots of them, but there’s one big problem – black holes are invisible! Since they don’t emit any light, it’s pretty tough to find them lurking in the inky void of space. Fortunately, there are a few different ways we can “see” black holes indirectly by watching how they affect their surroundings.

This animated gif of supercomputer data takes you to the inner zone of the accretion disk of a stellar-mass black hole. Purple and blue-colored swirls spin in toward a central black circle.
This animation of supercomputer data takes you to the inner zone of the accretion disk of a stellar-mass black hole. The event horizon is the boundary where all trajectories, including those of light, must go inward.
NASA's Goddard Space Flight Center/J. Schnittman, J. Krolik (JHU) and S. Noble (RIT)

Speedy stars

If you’ve spent some time stargazing, you know what a calm, peaceful place our universe can be. But did you know that a monster is hiding right in the heart of our Milky Way galaxy? Astronomers noticed stars zipping superfast around something we can’t see at the center of the galaxy, about 10 million miles per hour! The stars must be circling a supermassive black hole. No other object would have strong enough gravity to keep them from flying off into space.

A simulation of stars orbiting Sagittarius A*, the black hole at the center of our Milky Way. Several stars and their orbits appear over time, with a counter counting up from 1995 to 2018. As the stars orbit, their orbits are shown as lines, showing that they are clearly orbiting something at the center of our galaxy.
This animation shows orbits of stars near the Milky Way’s central black hole based on images taken between 1995 and 2018. Using these stellar orbits, we can estimate the mass of the supermassive black hole to be 4 million solar masses.
Keck/UCLA Galactic Center Group

Two astrophysicists won half of the Nobel Prize in Physics in 2020 for revealing this dark secret. The black hole is truly monstrous, weighing about four million times as much as our Sun! And it seems our home galaxy is no exception – our Hubble Space Telescope has revealed that the hubs of most galaxies contain supermassive black holes.

Shadowy silhouettes

Technology has advanced enough that we’ve been able to spot one of these supermassive black holes in a nearby galaxy. In 2019, astronomers took the first-ever picture of a black hole in a galaxy called M87, which is about 55 million light-years away. They used an international network of radio telescopes called the Event Horizon Telescope. And in 2022, they used the same network of telescopes to reveal the black hole at the center of the Milky Way.

sing the Event Horizon Telescope, scientists obtained an image of the shadow of the black hole at the center of galaxy M87. This donut-shaped image glows in orange against a black background. The bottom portion of the “dount” is brighter, appearing almost white in places. The image shows emission from hot gas swirling around the black hole under the influence of strong gravity near its event horizon.
Using the Event Horizon Telescope, scientists obtained an image of the black hole at the center of galaxy M87 outlined by hot gas swirling around it under the influence of strong gravity near its event horizon.
Event Horizon Telescope collaboration et al.

In the image, we can see some light from hot gas surrounding a dark shape. While we still can’t see the black hole itself, we can see the “shadow” it casts on the bright backdrop.

Shattered stars

Black holes can come in a smaller variety, too. When a massive star runs out of the fuel it uses to shine, it collapses in on itself. These lightweight or “stellar-mass” black holes are a few to hundreds of times as massive as the Sun. They’re scattered throughout the galaxy in the same places where we find stars, since that’s how they began their lives. Some of them started out with a companion star, and so far that’s been our best clue to find them.

A large pancake-shaped disk of material swirls in toward a black hole at the center of the disk. An orange star enters the image on the right and behind the disk, but a stream of gas connects to the disk in a long, thin arc. A blue flame-shaped region rises above the disk, from near the black hole. A similar region is just visible protruding below the disk as well.
In this illustration of MAXI J1820+070, a black hole pulls material off a neighboring star and into an accretion disk. Above the disk is a region called the corona.
Aurore Simonnet and NASA’s Goddard Space Flight Center

Some black holes steal material from their companion star. As the material falls onto the black hole, it gets superhot and lights up in X-rays. The first confirmed black hole astronomers discovered, called Cygnus X-1, was found this way.

Watch how monster black hole rip apart a star in this animation.
NASA’s Goddard Space Flight Center

If a star comes too close to a supermassive black hole, the effect is even more dramatic! Instead of just siphoning material from the star like a smaller black hole would do, a supermassive black hole will completely tear the star apart into a stream of gas. This is called a tidal disruption event.

Making waves

But what if two companion stars both turn into black holes? They may eventually collide with each other to form a larger black hole, sending ripples through space-time – the fabric of the cosmos!

In this simulation, a pansy-shaped structure emits vertical and horizontal purple ripples. Yellow structures, making up the pansy’s center, surround each black hole and illustrate the strong curvature of space-time. The purple of the petals and the purple waves emanating outward represent gravitational waves.
This visualization shows gravitational waves emitted by two black holes of nearly equal mass as they spiral together and merge.
NASA/Bernard J. Kelly (Goddard and Univ. of Maryland Baltimore County), Chris Henze (Ames) and Tim Sandstrom (CSC Government Solutions LLC)

These ripples, called gravitational waves, travel across space at the speed of light. The waves that reach us are extremely weak because space-time is really stiff.

Three scientists received the 2017 Nobel Prize in Physics for using LIGO to observe gravitational waves that were sent out from colliding stellar-mass black holes. Though gravitational waves are hard to detect, they offer a way to find black holes without having to see any light.

We’re teaming up with the European Space Agency for a mission called LISA, which stands for Laser Interferometer Space Antenna. When it launches in the 2030s, it will detect gravitational waves from merging supermassive black holes – a likely sign of colliding galaxies!

Simulation of two colliding galaxies. Two galaxies, seen edge on, are in the last stages of colliding. The two, coin-shaped galaxies have bright centers that approach each other, and then quickly orbit each other a few times as they get closer and closer until the merge together. Streams of gas, dust and stars stream behind the two galaxies, ultimately tangling together into a single giant ellipse around the merged galactic centers.
Two galaxies collide and merge in this simulation of the formation of the galaxy known as "The Mice."
Josh Barnes (University of Hawaii) and John Hibbard (National Radio Astronomy Observatory)

Rogue black holes

So we have a few ways to find black holes by seeing stuff that’s close to them. But astronomers think there could be 100 million black holes roaming the galaxy solo. Fortunately, our Nancy Grace Roman Space Telescope will provide a way to “see” these isolated black holes, too.

Animation showing the concept of gravitational microlensing. On the left is a camera, on the right the apparent position of a background star it is imaging. Between the camera and the star, a black hole passes through. The light from the background star bends around the black hole, making the star appear as though it is in a different position. The process also acts as a virtual magnifying glass, amplifying the brightness of the distant background star.
When one star in the sky appears to pass nearly in front of another, the light rays of the background source star become bent due to the warped space-time around the foreground star. This star is a virtual magnifying glass, amplifying the brightness of the background source star.
NASA's Goddard Space Flight Center/CI Lab

Roman will find solitary black holes when they pass in front of more distant stars from our vantage point. The black hole’s gravity will warp the starlight in ways that reveal its presence. In some cases, we can figure out a black hole’s mass and distance this way, and even estimate how fast it’s moving through the galaxy.