Recently, astronomers produced a much-refined image of the black hole at the center of galaxy M87 in the constellation Virgo.
You can see the original 2019 image and the improved version at https://www.reuters.com/lifestyle/science/scientists-unveil-new-improved-skinny-donut-black-hole-image-2023-04-13/.
The original image didn’t come quickly. Over two years, astronomers gathered five petabytes (five million billion bytes) of data with a worldwide array of telescopes to produce a fuzzy donut.
Over the next four years, astronomers subjected the original image to computer models of every conceivable way they thought that black holes function. The result was 30,000 separate images.
Then they used the computers to look for similarities among the images. The computers learned to make better comparisons as they went, yet another application of the “machine learning” at the heart of artificial intelligence.
The result is the image you see on the right. A much thinner ring surrounds the black area that defines a black hole.
Astronomers use computer models to predict what happens when a star dies.
When a star much more massive than our sun ends its life as a gigantic thermonuclear bomb, the powerful gravitational effect of its massiveness causes it to collapse into an extremely dense sphere. Depending on the total mass, its volume might reduce to the size of a basketball or an orange.
Truly massive stars reduce themselves to volumes of zero size, a spot in spacetime astronomers call a singularity. The volume of all that stellar material has essentially disappeared from the universe. Yet, its collective gravity, an inconceivably concentrated gravitational force, remains as pure gravity — enormous mass distilled to its gravitational essence.
The black hole’s gravitational field is so great that even light cannot escape, effectively sealing it off from the rest of the universe.
We are thus left with two distinct black-hole realms: the gravity-bound region close to the black hole where even light cannot escape and the area just outside where material is streaming into the black hole.
Those two realms are divided by an almost magical boundary called the event horizon. It resembles our earthly horizon in at least one respect. You can see the things near it, but you can’t see past it.
As the material spirals toward the event horizon, it flattens into a swirling donut shape called an accretion disk.
The enormous gravity of the black hole tears atoms and molecules into their subatomic components.
Those subatomic particles slam into each other with velocities approaching the speed of light. The result is a prodigious number of high-energy particles, many of which will be light particles, collectively known as photons.
Since astronomers cannot take an image of a black hole, they must look instead for the effects that a black hole should have on the surrounding universe. They must look just outside the event horizon, where the black hole’s gravity cannot suck light in.
The ‘Donut of Doom’ conclusively fills that bill.
As it turns out, a second type of black hole exists in our universe. They reside at the centers of galaxies and make stellar black holes look like specks.
Most galaxies, including our own Milky Way, appear to have black holes at their center. They formed out of galactic gas and dust that suffered a cataclysmic gravitational collapse early in the galaxy’s formation. That material never got a chance to develop into stars.
So why didn’t the M87 team image the Milky Way black hole?
In fact, they did. Our black hole was not very photogenic at only (ha!) two million solar masses.
M87’s central black hole weighs in at 6.5 billion suns.
The refined image of M87’s black-hole accretion disk is resolved down to the “last photon orbit” before material falls into the black hole. In the middle is darkness as black as the total absence of light can make it.
By looking at that “last photon orbit” in 2023’s better-resolved image, astronomers get a better glimpse at the event horizon, the place of “zero width” where the laws of the universe cease to function.
They may thus examine the crack between the worlds — the mind-bending transition where our universe, governed by natural law, ends and the mysterious realm of the black-hole universe begins.
Tom Burns is the former director of the Perkins Observatory in Delaware.