Black holes keep their secrets. They lock up forever whatever happens. Light itself cannot escape the hungry pull of a black hole.
So it seems that a Black hole should be invisible– and impossible to photograph. Such great fanfare accompanied the release of the first image of a black hole in 2019. Then, in the spring of 2022, astronomers unveiled another photo of a black hole – this time of the one at the center of our own Milky Way.
The image shows an orange donut-shaped blob that looks remarkably similar to the earlier image of the black hole at the center of the galaxy Messier 87. But the Milky Way’s black hole, Sagittarius A*, is actually much smaller than the first, and was also more difficult to see, requiring peering through our galaxy’s foggy disk. So even though the observations of our own black hole were made at the same time as those of M87, it took three more years to create the image. This required international collaboration of hundreds of astronomers, engineers and computer scientists, and the development of sophisticated computer algorithms to assemble the image from the raw data.
Of course, these “photos” don’t directly show a black hole, defined as the region of space within a point-of-no-return barrier known as the event horizon. They are actually picking up bits of the flat pancake of hot plasma swirling around the black hole at high speed in what is known as the accretion disk. The plasma consists of high-energy charged particles. As the plasma spirals around the black hole, its accelerating particles emit radio waves. The fuzzy orange ring seen in the images is an elaborate reconstruction of these radio waves, captured by eight telescopes scattered around the Earth, collectively known as the Event Horizon Telescope (EHT).
The latest image tells the story of the epic journey of radio waves from the center of the Milky Way and provides unprecedented details about Sagittarius A*. The image also represents “one of the most important visual pieces of evidence for general relativity,” according to our current best theory of gravity Sera MarkoffAstrophysicist at the University of Amsterdam and member of the EHT collaboration.
Studying supermassive black holes like Sagittarius A* will help scientists learn more about them how galaxies evolve over time and how they accumulate in huge clusters across the universe.
From the galactic core
Sagittarius A* is 1,600 times smaller than Messier 87’s black hole imaged in 2019 and is also about 2,100 times closer to Earth. This means that the two black holes appear roughly the same size in the sky. Geoffrey Boweran EHT project scientist at the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan, says the resolution required to see Sagittarius A* from Earth is the same as would be required to see an orange on the lunar surface taking photos.
The center of our galaxy is 26,000 light-years from us, so the radio waves collected to create this image were emitted around the time one of the earliest known permanent human settlements was established. The journey of radio waves began when they were first emitted by particles in the black hole’s accretion disk. With a wavelength of about 1 mm, the radiation traveled towards Earth relatively undisturbed by the intervening galactic gas and dust. If the wavelength was much shorter, like visible light, the radio waves would have been scattered by the dust. If the wavelength were much longer, the waves would have been bent by charged plasma clouds, distorting the image.
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