Photographing Sagitarius A*: first images of the Milky Way’s supermassive black hole

By Leo Li

Three years ago, the world gasped at the first direct image of a black hole in awed amazement. It was like staring into the eye of a celestial being – a surreal, fiery ring of hot gas surrounding a pitch-black, bottomless singularity. We named it Messier 87*, a supermassive black hole with a mass 6.5 billion times that of our Sun, staring back at us from 50 million lightyears away.

However, what was truly astounding and historical was our ability to directly image what, decades ago, was thought to be a mathematical abstraction and a cosmological impossibility.

Messier 87*’s image was taken by the Event Horizon Telescope (EHT). It blatantly derives its name from the boundary of a black hole, at which not even light could escape due to the eater’s prodigious gravity.

Contrary to its name which suggests a singular super-telescope, EHT is actually an array of still sizeable radio-telescopes spread across the globe at 8 sites. More than 60 institutions in over 20 countries work collaboratively to cross-correlate and analyse obtained data on supercomputers. With as little artistic embellishment and as much truthful naturalism possible, the world in 2019 got to see Messier 87* in all its staggering strangeness.

Sagittarius A* is a mere 26,000 lightyears away from our even more dwarfish planet

As if falling down this rabbit hole did not satisfy astronomers’ curiosity, the EHT took the first picture of our friendly neighbourhood Sagittarius A*, the supermassive blackhole sitting in the centre of our galaxy.

At first glance, Sagittarius A* is less impressive than Messier 87* – it is about 4.2 million times the mass of our Sun, making it a thousand times less massive – a dwarf compared to the distant giant. However, Sagittarius A* is a mere 26,000 lightyears away from our even more dwarfish planet; it is also the centre around which our solar system spirals.

The image of Sagittarius A* is strikingly similar to that of Messier 87*, despite their disparity in mass and distance. Yet, its picture appears a bit blurrier – at least according to the staff at the EHT – there are three primary bulges with more severely bleeding edges, as if the white-hot, gaseous accretion disk can’t contain itself from spreading out.

In fact, since gases in all black hole accretion disks move at similar speeds, it takes only days for the gases’ patterns and brightness to change for Sagittarius A*. For the same exposure time, Messier 87* gave a more precise and definite result – Sagittarius A* essentially blinked a few times under burst mode.

Now one may ask: why do we need two, let alone many more to come, very similar images of black holes? – why do we choose to fall in the same rabbit hole twice? See, as we never step in the same river twice, all rabbit/black holes differ by a bit, and that bit of difference opens up a whole new world of astronomical possibilities.

The extent and precision to which upcoming imaging of black holes will be possible are unimaginable

First, we get to compare and contrast (every examiner’s favourite phrase) how the mass of a black hole affects the behaviours of hot gases rotating around them – especially for Doppler boosted beams, moving towards us at lightspeed, which constitute the bright bulges in the images taken; and how different black holes dictate the formation and evolution of the galaxy it is at the centre or a part of.

Second, it is a confirmation that what looms in the shadow at the pit of the Milky Way is indeed our much beloved and expected supermassive black hole, instead of some arbitrary ‘compact object’ otherwise uncertainly conjectured, leaving no doubt that Sir Penrose & Co.’s Nobel Prize was well-deserved.

Third, it is a tribute to the common father of all modern black hole scholars, Albert Einstein, who in his seminal General Relativity first predicted the mathematical existence of singularities in curved spacetime. Karl Schwarzschild then gave black hole its name and legitimacy. If, say, Einstein rings, due to gravitational lensing, are General Relativity’s engagement ring to black holes, then the recently procured images are the wedding ring that marry our forebears’ theoretical fancies to reality.

The EHT is still a work in progress and forever will be. With four more observatory sites under planning and construction, the extent and precision to which upcoming imaging of black holes will be possible are unimaginable. Also unimaginable are the imminent challenges and uncertainties, which will test the limits of our instrumentation, the human ingenuity, and our truth-driven passion for collaboration every second. But until then – see you down another rabbit hole.

Image: EHT Collaboration

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