In 2019, astronomers captured a hidden slice of the universe for us to download onto our computer screens. It was the very first image of a black hole — and it revealed the beast’s violence in space. Called M87*, this chaotic void spews out a beam of light and penetrates the galaxy in which it lives. But to the untrained Earth eye, it looks a bit like a Fruit Loop.
On May 12, the same group of wide-eyed scientists managed to outdo themselves by putting together yet another stunning image of a cosmic rift. But this time it was a “silent, silent” black hole in our own Milky Way galaxy called Sagittarius A*, or Sgr A*. Nevertheless, it also looks like a Fruit Loop, but maybe one that gets soggy.
Both images are remarkable achievements for astronomy. They are arguably the most breathtaking photos mankind has seen. And they look like faint orange neck pillows — or, as Feryal Özel, a University of Arizona astrophysicist and part of the Event Horizon Telescope Collaboration puts it, “it looks like black holes are like donuts.”
So, what exactly are we looking at? “If we look at the heart of each black hole, we find a bright ring around the black hole’s shadow,” Özel said.
Before we get into the details, though, it’s important to note that both of the black hole images we see aren’t the kind of pictures we’re used to on a day-to-day basis. They come from radio wave observations, which work by detecting the intensity of light particles, or photons, in space and then translating those signals into visible patterns. For example, super intense photons are “brighter”.
Black holes are not really black holes
A black hole isn’t exactly black, nor is it exactly a hole.
Rather, it is a complex entity with several moving parts, similar to humans with a lot of biological body systems. But to understand the recent view of Sgr A* by the EHT, you need to know three main aspects of black hole anatomy.
First, there is the singularity.
Black holes are usually formed when giant stars collapse, turning all former star matter into a single point. Called a “singularity,” this point has such massive mass — coming from the dead star — that its gravitational pull overcomes anything and everything with the misfortune of getting too close.
That includes gas, dust and even light. In fact, the pull of this entity is so strong that it literally distorts the fabric of space and time. But in both the M87* and Sgr A* images, 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, in fact, the boundary between our universe and the elusive interior of the void. It is located at a certain, very specific distance from the singularity point called the Schwarzschild radius. Every black hole has one, and this is the bit that probably gives black holes their reputation as “black”.
Anything beyond the event horizon is caught in a hidden realm that seems like darkness to us because even light is trapped in it. Content beyond the horizon can never come back. We don’t know what happens to them.
In the EHT images, this alternate reality-like, spherical space between the singularity and the event horizon is indicated by the black circles. More specifically, however, the dark central areas are 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 member and astrophysicist at the Harvard Smithsonian Center for Astrophysics.
But we’ll come back to that.
Third, and especially for the burning donuts, there is the photon sphere.
All around the singularity and event horizon, veils of hot gas and dust are trapped in an eternal orbit around these deafening chasms in so-called accretion disks. If anything of that disk falls within the Schwarzschild radius, that is, beyond the event horizon, it is lost to the black hole universe. But light does something tricky here. And this gives us our image of a black hole.
Unlike gas or dust, light within the Schwarzschild ray can sort of subtly tiptoe without spiraling into the void. And if those photons travel inward? only appropriately, “light escaping from the hot gas swirling around the black hole appears to us as the bright ring,” Özel said. “Light close enough to be swallowed up eventually crosses the horizon, leaving only a dark void in the center.”
That’s why the EHT collaboration calls their images the “hearts” of black holes. The images are zoomed in on the photon sphere, which is technically even closer to the mind-boggling, spacetime-distorting singularity than even the event horizon. If these voids were people, we look at their beating hearts.
In the night sky, for context, the shadow within the ring is about 52 micro-arcseconds, the team said, which is about the size of a donut on the moon when viewed from Earth. The video below illustrates that point (cute).
“We’ve found that we can measure the ring diameter with an accuracy of about 5%,” Johnson says. “Most of the uncertainty here is actually because we don’t know if the black hole is spinning and the spin has a small effect on the shadow diameter.”
But it’s all crooked
Space and time, or spacetime, around black holes is completely distorted.
As light particles, or photons, escape the swirling accretion disk and test the event horizon’s limits, they follow this warped spacetime path. Therefore, the orange light you see on the top part of the black hole of the EHT is not really on the “top” of the black hole. It is actually associated with the tip of the event horizon and part of a Saturn-like ring around the entire object. The point is that the curvature of spacetime is forcing those photons on the other side to “fold” toward us.
The following video will help clarify this.
Although it is a simulation of a binary black hole system, notice that when the blue black hole is behind the orange black hole, you can see all the blue at the top and bottom of the orange. This is also roughly what happens with solo black holes, except with regard to light orbiting its singularity. In fact, it happens to every black hole in the video.
Likewise, the event horizon itself follows a warp of sorts. We can basically see the far end of the event horizon, and essentially all the corners of the horizon there too† It’s all “folded” toward us. Unfortunately, the dark central areas of these images are better thought of as “shadows” of the event horizon. Just think of them as the point of no return for photons – one that’s visible because we see the lucky light particles that didn’t get stuck there, hanging out the barrier between the observable universe and… anything in the darkness of the black hole.
In a sense, we’re not just looking at images of black holes, but we’re also looking at direct evidence of warped spacetime† That is, we’re looking at direct evidence of Einstein’s general theory of relativity, the mind-boggling genius’ take on gravity.
And, in light of general relativity, the reason some parts of the light ring are brighter than others is because of a phenomenon called gravitational lensing. Gravity lens actually†
Sgr A* Specifications
Now that we know what we’re looking at, here are some specs of the newly imaged black hole.
Sgr A* is located 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 and 1000 times more massive than SgrA*. Also, Sgr A* is much less violent — or “hungry,” as astronomers sometimes say. It doesn’t consume as much gas from its environment as the M87*.
“We see that only a trickle of material actually reaches 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 radiates a few hundred times as much energy as the sun, despite being 4 million times as massive — the only reason we can study it at all is because it’s in our own galaxy.”