Black Holes: Seeing the Invisible! – NASA Universe Exploration

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 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. Credit: 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.
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. Credit: Keck/UCLA Galactic Center Group
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. Credit: 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.
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. Credit: 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.
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!
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!
Two galaxies collide and merge in this simulation of the formation of the galaxy known as "The Mice." Credit: Josh Barnes (University of Hawaii) and John Hibbard (National Radio Astronomy Observatory)
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. Credit: 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.