On Thursday May 12th, 2022, the consortium of global observatories that calls itself the Event Horizon Telescope (EHT) announced it had successfully imaged the super massive black hole (SMBH) residing at the centre of our galaxy. It’s not the first time such a SMBH has been imaged – EHT captured the first direct look at one back in 2019, when it observed the black hole at the centre of the supergiant elliptical galaxy Messier 87 (M87*, pronounced “M87-Star”) 55 million light years away, but is still a remarkable feat.
Sitting at the centre of our galaxy and a “mere” 27,000 light years from Earth, Sagittarius A* (pronounced “Sagittarius A Star” or Sgr A*, and so-called because it lies within the constellation of Sagittarius close to the boundary with neighbouring of Scorpius when viewed from Earth) is some 51.8 million km in diameter and has an estimated mass equivalent to 4.154 million Suns.
Because of its distance and size (in terms of SMBHs, it is actually fairly middling (M87*, by comparison has a mass somewhere between 3.5 and 6.6 billion Suns) and factors such as the volume of natural light and interstellar dust between Earth and Sqr A*, we cannot see it in the visible light spectrum.
However, we can detect the infra-red radiation from the space around it. This is important because black holes are surrounded by an accretion disk – material attracted by the gravity well of the black hole and which fall into an orbit around it just beyond the event horizon. This material is travelling as such massive speed, it creates high-energy radiation that can be detected.
Even so, gathering the necessary data to image an SMBH, even one as relatively close to Earth as Sgr A* or as incredibly huge as M87* (which is thousands of times bigger than Sgr A*) requires an extraordinary observation system. Enter the Event Horizon Telescope (EHT).
This is actually a network of (currently) eleven independent radio telescopes around the world. It extends from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, and the Very Long Baseline Array (VLBA) in New Mexico, USA, down to the South Pole Telescope (SPT) located at the Amundsen–Scott South Pole Station, Antarctica; and from the James Clerk Maxwell Telescope and the Submillimeter Array, Hawaii to the Northern Extended Millimeter Array on the Plateau de Bure in the French Alps.
Together, the telescopes work like this: as the Earth spins, the target object rises over the horizon for some of the telescopes, they all lock onto it with millimetre precision, and track it across the sky. As more telescopes in the network are able to join in, they do, while those passing beyond the point where they can see the target cease observations until the Earth’s rotation brings the object back into view.
This effectively turns Earth itself into a massive radio telescope using Very Long Baseline Interferometry (VLBI), with all of the telescopes gathering an immense amount of data at resolutions far in excess of anything the individual telescopes could achieve. So much data, in fact, that the images of Sgr A* released by the EHT actually don’t do genuine justice.
This is because the total amount of image data gathered by EHT amounts to 3.5 petabytes (that’s equivalent to 100 million Tik Tok videos for the young ‘uns out there!). In order to produce images that could be easily transmitted over the Internet, this data had to be compressed and altered. In fact, the data volume was so huge, it was easier to remove the hard drives containing it and shipping them to the various centres around the world wanting to analyse the data, rather than trying to transmit the data between different locations!
The data were gathered over the course of multiple nights of observations performed by the telescopes in the network in 2017, and it has taken 5 years of analysis using a batch of super computers for the researchers to reach a consensus. This was in part due to the nature of Sgr A* itself. The EHT team had cut their teeth observing M87*, but in terms of imaging, Sqr A* is completely different, as EHT team member Chi-kwan Chan explains:
The gas in the vicinity of the black holes moves at the same speed – nearly as fast as light – around both Sgr A* and M87*. But where gas takes days to weeks to orbit the larger M87*, allowing us to gather consistent images over days. The material around the much smaller Sgr A* it completes an orbit in mere minutes. This means the brightness and pattern of the gas around Sgr A* were changing rapidly as we were trying to image it, so it was a bit like trying to take a clear picture of a puppy quickly chasing its tail.
– Chi-kwan Chan, Steward Observatory, University of Arizona
However, one thing did emerge as processing continued: despite being very different in almost every respect, both M87* and Sgr A* have produced images that are remarkably similar. That they do is seen as a further proof of Einstein’s theory of general relatively, with both accretion disks conforming to his predictions of what should be seen, despite the – no pun intended – massive differences in their nature.
And that’s the key factor in studies like this: they do much to help increase / confirm our understandings of the cosmos around us (or at least, reveal what we theorise to be the case is actually the case). With M87* and Sgr A*, the data gathered are allowing scientists to formulate and model a “library” of different simulated black holes. This library in turn enables researchers test the laws of physics under different domains and offer opportunities to better understand the formation, life and death of galaxies and the very nature of SMBHs themselves, which are believed to be the “powerhouses” of massive galaxies.
One of the things the EHT observations of Sgr A* have confirmed is that it is actually quite “tame”. In contrast to the idea of the black hole “sucking in” any and all material straying too close to it, it does nothing of the sort – and this appears to be typical for black holes of all sizes.
If Sagittarius A* were a person, it would consume a single grain of rice every million years. Only a trickle of material is actually making it all the way to the black hole. Sagittarius A* is giving us a view into the much more standard state of black holes: quiet and quiescent. M87 was exciting because it was extraordinary in size and power. Sagittarius A* is exciting because it’s common.
– Michael Johnson, Harvard/Smithsonian Centre for Astrophysics