In 2019, astronomers captured a hidden part of the universe for us to download onto our computer screens. It was the first ever image of a black hole – and it revealed the violence of the space beast. This chaotic void, dubbed M87*, spits out a jet of light and passes directly through the galaxy in which it lives. But to the untrained earthly eye, it looks a bit like a fruit loop.
On May 12, the same team of wide-eyed scientists managed to outdo themselves by piecing together another mind-bending image of a cosmic abyss. Although this time it was a “silent, inactive” black hole in our own Milky Way galaxy called Sagittarius A*, or Sgr A*. Nevertheless, it also looks like a Fruit Loop, but perhaps one that gets soggy.
Both images are remarkable achievements for the field of astronomy. These are arguably the most breathtaking images mankind has seen. And they look like fuzzy orange neck pillows — or, as University of Arizona astrophysicist and Event Horizon Telescope Collaboration member Feryal Özel puts it, “black holes look like donuts.”
So what are we looking at, exactly? “When we look at the core of each black hole, we find a bright ring surrounding the shadow of the black hole,” Özel said.
Before going into detail, however, it’s important to note that the two images of black holes we see are not the kind of photographs we are used to on a daily basis. They come from observations of radio waves, which somehow work by detecting the intensity of light particles, or photons, in space and then translating those signals into visible patterns. Super intense photons are “brighter”, for example.
Black holes aren’t really black holes
A black hole is not exactly black, nor exactly a hole.
Rather, it’s a complex entity with multiple moving parts, similar to humans having a bunch of biological body systems. But to understand the recent Sgr A* picture of the EHT, you need to know three main aspects of black hole anatomy.
First there is the singularity.
Black holes typically form when gigantic stars collapse and all of the old stellar matter morphs into a single point. This point is called the “singularity” and has such immense mass – from the dead star up – that its gravity overcomes anything and everything with the misfortune of walking too close.
This includes gas, dust and even light. In fact, the gravitational pull of this entity is so strong that it literally warps the fabric of space and time. But, in images M87* and Sgr A*, this point is invisible to us. We have to imagine it, right in the center.
Second, there is the event horizon.
The event horizon is basically the boundary between our universe and the elusive interiors of the void. It is located at a certain very specific distance from the point of singularity called the Schwarzschild radius. Every black hole has one, and that’s probably what gives black holes their “black” reputation.
Anything that falls beyond the event horizon is caught in a hidden realm that appears to us as darkness because even light is trapped there. Content beyond the horizon can never come back. We don’t know what happens to them.
In EHT images, this alternate reality spherical space between the singularity and the event horizon is signified by the black circles. More specifically, however, the dark central parts are the shadows of the event horizon.
“The shadow is the image of the event horizon, it’s our line of sight into the black hole,” said Michael Johnson, EHT fellow and astrophysicist at the Center for Astrophysics, Harvard Smithsonian.
But we will come back to it.
Third, and especially for flaming donuts, is the photon sphere.
All around the singularity and the event horizon, shrouds of hot gas and dust are trapped in an eternal orbit around these deafening chasms in what are called accretion disks. If anything from this disk falls within the Schwarzschild radius, that is, beyond the event horizon, it is lost to the black hole universe. But the light is doing something tricky here. And that gives us our picture of the black hole.
Unlike gas or dust, light can gently tiptoe into the Schwarzschild radius without flying off into a vacuum. And if these photons travel in only in a good way, “light escaping from the hot gas swirling around the black hole appears to us as the bright ring,” Özel said. “Light that is close enough to be swallowed by her eventually breaks through her horizon and leaves behind only a dark void in the center.”
This is why the EHT collaboration refers to its images as “hearts” of black holes. Images are magnified on the Photonic Sphere, which technically comes even closer to the mind-boggling, warping singularity of space-time than even the event horizon. If these voids were people, we look at their beating hearts.
In the night sky, for context, the shadow inside the ring is about 52 micro-arcseconds long, the team said, which is about the size of a donut on the Moon as seen from Earth. The video below (adorably) illustrates this point.
“We found that we could measure the diameter of the ring with an accuracy of about 5%,” Johnson said. “Most of the uncertainty here is actually because we don’t know whether the black hole is rotating or not, and the rotation has a small effect on the diameter of the shadow.”
But everything is distorted
Space and time, or spacetime, around black holes are totally distorted.
As light particles, or photons, escape from the swirling accretion disk of gas and test the boundaries of the event horizon, they follow this warped spacetime path. Therefore, the orange light you see on the upper part of the EHT black hole image is not really “on top” of the black hole. It is actually associated with the end of the event horizon and part of a Saturn-like ring around the entire object. The fact is that the distortion of spacetime forces these distant photons to “bend” towards us.
The following video clarifies this.
Although this is a simulation of a binary black hole system, notice how when the blue black hole is behind the orange black hole, you can see all of the blue at the top and bottom of the hole. ‘orange. This is pretty much what happens to solo black holes as well, except for light orbiting its singularity. In fact, it happens to every black hole in the video.
Likewise, the event horizon itself follows a kind of distortion. We can basically see the far end of the event horizon, and basically every angle of the horizon there too. Everything is “folded” towards us. Alas, the dark central portions of these images are best considered “shadows” of the event horizon. Just think of them as the point of no return for photons – a point that is viewable because we see the lucky particles of light that weren’t trapped there, shuffling around the barrier between the observable universe and…everything in the darkness of the black hole.
In a sense, not only are we looking at images of black holes, but we are looking direct evidence of distorted spacetime. Simply put, we’re looking at direct evidence for Einstein’s theory of general relativity, the genius’s earth-shattering take on gravity.
And, on a general relativity note, the reason some parts of the light ring are brighter than others is due to a phenomenon called gravitational lensing. Essentially gravitational lens.
Sgr A Specifications*
Now that we know what we’re looking at, here are some of the specs of the newly imaged black hole.
Sgr A* is 26,000 light-years from Earth and has a mass equivalent to about 4 million times that of our sun. M87*, on the other hand, is about 54 million light-years away from us and 1,000 times more massive than SgrA*. Moreover, Sgr A* is much less violent — or “hungry” as astronomers sometimes call it. It does not consume as much gas from its surroundings as the M87*.
“We see that only a trickle of material actually makes it to the black hole,” Johnson said. “If Sgr A* were a person, he would consume a single grain of rice every million years.”
So, he says, the black hole is inefficient. “It only produces a few hundred times more energy than the sun, despite being 4 million times more massive – the only reason we can study it is because it’s in our own galaxy. “