A blurry image of Sagittarius A*. Image: ESO/GRAVITY collaboration/L. Calçada, N. Risinger (skysurvey.org)
- A global team of researchers called the Event Horizon Telescope collaboration has released an image of our neighbourhood black hole, named Sagittarius A*.
- The feat is fascinating given that the black hole’s size in the sky is comparable to a donut on the surface of the Moon, seen from Earth.
- In 2019, the same collaboration produced the first direct image of a black hole, at the centre of the M87 galaxy, which is 55 million light years away from us.
- Thanks to the EHT’s image, we know for the first time that the axis of rotation of Sagittarius A* is pointed roughly towards Earth – which is surprising.
On May 12, a global team of researchers called the Event Horizon Telescope (EHT) collaboration released an image of our neighbourhood black hole, named Sagittarius, or ‘Sgr’, A*. Sgr A* is the supermassive black hole residing at the centre of the Milky Way galaxy.
This image is only the second direct picture of a black hole ever produced. The image shows a bright ring enclosing a dark area. The dark centre represents the shadow of the black hole from which no light escapes. The bright ring is an area called the accretion disc – it consists of superhot matter rapidly orbiting the central black hole.
The scale of achievement can be understood from a comparison made in one of the papers that presented these results: “Because the black hole is 27,000 light years away from Earth, it appears to us to have the same size in the sky as a donut on the Moon.”
Zooming in to this level would require a telescope the size of Earth. Obviously building such a thing is impossible, so instead, the EHT uses data collected from an array of radio telescopes in eight locations around the globe to achieve the same effect. The final image that we see is the combination of thousands of images produced from this data.
In 2019, the same collaboration produced the first direct image of a black hole, at the centre of the M87 galaxy, which is 55 million light years away from us. The data for both these images were collected at the same time. The Sgr A* image just took longer to process.
That said, it is curious why a black hole in a galaxy far, far away was imaged before Sgr A* itself. There are a few reasons. First, there is a lot of galactic matter – including rocky debris, gas and dust – between Sgr A* and Earth. This scatters the radio signals coming from Sgr A* and distorts the image. So the image needs to be reconstructed to account for this distortion.
Second, at around 4 million times the mass of the sun, Sgr A* is a supermassive black hole but it is relatively tiny compared to the M87 black hole. As a result, the matter in the accretion disk of Sgr A* takes less time to go around. This presents a challenge because capturing a long exposure shot of a faster-moving object is harder.
Despite being black holes of different sizes and types, Sgr A* and M87* look remarkably similar in their images. Sera Markoff, a theoretical astrophysicist and co-chair of the EHT science council, tweeted on May 12:
“This tells us that General Relativity governs these objects up close, and any differences we see further away must be due to differences in the material that surrounds the black holes.”
— ESO (@ESO) May 12, 2022
Jordy Davelaar, a member of the EHT collaboration and a black-hole astronomer, compared the ring size of Sgr A* from the EHT’s image and the theoretical prediction – and concluded that it was yet another proof of Albert Einstein’s general theory of relativity, published 107 years ago.
If we then compare our results to what we expect from GR, based on the ring size SgrA* should have, the models are within 10% of the Kerr prediction. pic.twitter.com/WYymEhrNiM
— dr. Jordy Davelaar (@jordydavelaar) May 12, 2022
Another interesting observation from this image is of the orientation of the black hole as it spins. Some black holes, including M87*, emit jets of material from their poles perpendicular to the accretion disc. This allows us to know which way they are ‘tilted’ in space.
Scientists have observed no such jets from Sgr A*. But thanks to the EHT’s image, we now know that its axis of rotation is pointed roughly towards Earth. This means the black hole’s accretion disc is surprisingly not aligned with the rest of the galaxy. Theoretical physicist Matthew Strassler wrote on his blog:
Nobody yet understands how supermassive black holes form and grow, so there isn’t a strong prejudice one way or another [about the black hole’s tilt], as far as I can tell from talking to experts. Even if it started out aligned with the axis of rotation, it may get kicked around as it merges with black holes that were in the centres of galaxies the Milky Way merged with. [This is] Definitely an area of ongoing research.
To be able to look into space and capture the image of a black hole is a tremendous feat of science and technology. And now that we have images of two different black holes of different sizes, we can look forward to a deeper understanding of gravity and its many mysteries.
Swetha P. is a science communicator and an aspiring physicist.