First Image of the Black Hole at the Heart of Our Galaxy

SGR A* first image

I attended the ESO-hosted EHT 2022 event for you !

You might remember the Event Horizon Telescope (EHT) collaboration for their remarkable achievement two years ago: the first image of a black hole, in galaxy M87. Today, during multiple simultaneous press conferences around the world, they unveiled an even more incredible image: that of the black hole at the centre of our own galaxy the Milky Way.

ESO captions: "Beginning with a broad view of the Milky Way, we dive into the dense clouds of gas and dust at our galactic centre. The stars here have been observed with ESO’s Very Large Telescope and ESO’s Very Large Telescope Interferometer for decades, the black hole’s immense gravitational pull distorting the orbits of the stars closest to it. Finally, we arrive at Sgr A*, the first image of which has been captured by the EHT collaboration. The black hole is shown by a dark central region called a shadow, surrounded by a ring of luminous gas and dust. The various observations used here were taken at different times, by different teams and with different facilities, and put together for the purpose of the zoom effect. The images go from visible wavelengths at the beginning to infrared, with the very final image being taken at radio wavelengths."
The world’s largest database of astrophotography data is waiting for you! Start now your free trial.
Try it free

From their observations of the stars at the centre of the Milky Way and their orbits, astronomers have been confident for decades that there must be a very compact object there, baptised Sagittarius A* (Sgr A*) and with a mass roughly 4 million times that of the Sun. Exotic theories like boson stars have been proposed to explain the phenomenon, however with this new, direct EHT image of the centre of our galaxy, we have a very strong piece of evidence for this compact object being a supermassive black hole. Furthermore, this provides validation for Einstein’s theory of General Relativity which predicts the existence of these black holes.

Access anywhere, at any time, the world’s largest database of astrophotography data ready to be downloaded and post-processed.
Try it free

Combining 8 radio telescopes around the world into one via Very-Long Baseline Interferometry, the EHT team effectively created a virtual “super-telescope” the size of the Earth. It took the researchers 25 years to develop this technique, which provides an instrument 3 million times more sensitive than the human eye, and with a resolution such that it could image a donut on the surface of the Moon !

What does it take to image a black hole ?

This is necessary if we want to detect Sgr A*, which despite being sizeable (it would just about fit into Mercury’s orbit) is also 27 000 light-years away from Earth, hidden behind clouds of gas and dust that obscure our view of the galactic centre. After hours of observations in 2017, it was time for billions of hours of data analysis conducted with supercomputers (or “correlators”) like Raven at the Max Planck Institute for Radio Astronomy and Frontera at the Texas Advanced Computing Centre, which enabled the combining of the thousands of images into the one we are presented with today.

Is the world plotting against your observations? Start now your 1-week free trial and immediately access tons of top-notch quality data.
Try it free
The making of the image: the images can be clustered into four groups based on similar features. An averaged, representative image for each of the four clusters is shown in the bottom row. The first three show a ring structure but with differently distributed brightness around the ring. The fourth cluster contains images that also fit the data but do not appear ring-like.

How does this image compare with M87’s black hole, then ? Well, firstly, all the members of EHT present at the conference and the subsequent Q&A highlight that we are looking at our own, home galaxy, which gives us a sense of awe in and of itself. Beyond that, there is the awe at the technological achievement: Sgr A* is 1000 times less massive than M87; to go back to donut imagery, if Sgr A is out donut, M87* would be the size of the Allianz Arena in Munich !

Anatomy of a black hole

Extensive modelling of black holes and the gas and magnetic fields surrounding them was also a key part of the process, as this enabled the researchers to comment on properties like Sgr A’s rotation rate and the angle at which we see it. The most likely it that we are seeing it with its spin axis pointing towards us. Note that the orientation of the supermassive black hole doesn’t have much of an influence on the current rotation rate of the galaxy as a whole and direction, however it’s extremely significant to understand its past evolution – a collision with another galaxy, for instance, might have shifted the axis. What this could mean, though, is that if Sgr A emits jets as can be observed with other supermassive black holes, it would be even trickier to spot them due to our position. There is evidence for past jets in what are known as Fermi bubbles, but direct detection of jets is one of the issues that the EHT collaboration hopes to tackle next.

Start to explore space from the comfort of your home!
Try it free
Sgr A* and M87*

In terms of similarities, though, considering that M87* evolves in a giant elliptical galaxy and has a much higher accretion rate, i.e., is located in a very different environment than Sgr A, it is incredible to see how both are so consistent with the theory of General Relativity. More images, notably of Active Galactic Nuclei, are yet to be revealed by the EHT collaboration, and with the addition of 3 new telescopes to the network since the Sgr A images were taken, even higher resolutions are to be expected. The field of black hole astronomy is just getting started …

All image and video credit to EHT/ESO

The world’s largest database of astrophotography data is waiting for you! Start now your free trial
Try it free