Say cheese, black hole! – you’re on camera!
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For the first time ever, astronomers have taken a photo of a black hole.
Black holes are regions in space where gravity is extreme. Everything that comes too close is sucked in. And nothing can ever get out again. Even light, traveling at 300,000 kilometers per second, cannot escape the gravitational grip of a black hole.
But how can you take a photo of something that doesn’t emit any light or any other form of radiation? Well, in fact, the picture doesn’t show the black hole itself, but its dark shadow. Hot gas close to the black hole emits radio waves. Some radio waves are sucked into the black hole. But other radio waves are merely bent by the intense gravity of the black hole. Eventually, they can escape on their way towards Earth.
Astronomers had calculated the expected effect: a fiery ring of bent radiation, surrounding a dark ‘shadow.’ But they also calculated that the ring would only be visible if the black hole were heavy enough. That’s why they focused on two supermassive black holes in our cosmic neighborhood. The first one is known as Sagittarius A*. It is located in the core of our own Milky Way galaxy, at a distance of some 26,000 light-years. The second is in the center of the galaxy M87, at a distance of 55 million light-years.
Even then, it wasn’t easy to capture an image of the black hole shadow. You would need a radio telescope as large as the Earth to be able to see it at all! That’s not possible, of course. Instead, the astronomers used eight radio observatories around the globe, including ALMA in Chile. They all looked at the black hole at the same time (in April 2017). The observations were then combined by a huge supercomputer. The result was an image just as sharp and detailed as what an Earth-sized radio telescope would have seen.
The image of the supermassive black hole in M87 is a milestone in the history of astronomy. In the future, it may also be possible to produce images of Sagittarius A*. And if more radio telescopes can be added to the network (maybe even in space!), the black hole photos may become even sharper. In the end, astronomers hope to learn much more about black holes and about gravity in general.
M87 is a giant elliptical galaxy in the Virgo cluster of galaxies, at a distance of 55 million light-years. Like many galaxies, it has a supermassive black hole in its core. The black hole produces a narrow jet of particles and energy that was first seen in 1918. And because of the black hole’s intense gravity, the stars in the core of M87 move at very high speeds. But the black hole itself had never been seen. The new photo is the very first image of the greedy monster. From the image of the shadow and the light ring, astronomers have calculated that the black hole in M87 is 6.5 billion times more massive than our own Sun.
More than two hundred scientists worked together to produce the image of the black hole. The project (called the Event Horizon Telescope project) is led by Shep Doeleman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts (United States), and Heino Falcke of the Radboud University in Nijmegen (the Netherlands). Apart from ALMA, the team used radio telescopes in the United States, in Hawaii, in Mexico, in Spain, in Chile, and at the South Pole. The new results have been published in six papers in a special issue of The Astrophysical Journal Letters.