Sagittarius A* – Our backyard’s black hole

There’s nothing in the whole universe as mysterious and fascinating as black holes. First theorised by John Michell in 1783, and also predicted by Einstein’s Theory of General Relativity in 1916, these dense, dark and elusive objects have been researched and studied for decades, capturing the interest of some of the world’s greatest physicists.

Among these, are three laureates who shared a Nobel Prize in physics in 2020 for their studies on black holes. Roger Penrose, one of the most brilliant minds of our times, demonstrated mathematically that black holes are indeed a direct consequence of the Theory of General Relativity, whereas Reinhard Genzel and Andrea Ghez predicted the existence of the supermassive black hole at the centre of the Milky Way and calculated its mass with fair precision: about 4.3 million solar masses!

Now, the idea of having such a supermassive black hole right at the core of our galaxy could be for many a cause of nightmares but in reality, there is nothing to fear. In fact, this is very common in the universe: it is estimated that there is a supermassive black hole in the centre of virtually every large galaxy.

Do you want to know more about black holes? Read my article: Black Holes: The dark side of gravity.

The supermassive black hole at the core of the Milky Way is called Sagittarius A* (abbreviated as Sgr A* and pronounced “Sagittarius A star”) and is located near the border of the constellations Sagittarius and Scorpius; it has a diameter of around 23.5 million km (14.6 million miles), and it is mostly dormant as it only occasionally feeds on nearby objects.

In May 2022, Sgr A* has made the headlines after its first-ever picture was produced and released to the public by the Event Horizon Telescope (“EHT”), a combination of eight large, ground-based telescopes that had already produced the first image ever of black hole in 2019 (in that case it was the black hole at the core of the Galaxy M87).

First ever image of Sagittarius A*

But how did they manage to achieve this? Obviously, imaging a black hole is a very complex business: while these objects have huge densities, in fact, they are very tiny regions of space. Besides, to observe Sgr A* one must look into the centre of the Galaxy through thick lines of dust, so the best options for observing the Galactic centre are in radio waves and in infrared light.

That is why the EHT used a technique known as Very Long Baseline Interferometry, which allowed astronomers to combine data from eight radio telescopes across the world as though they were one large telescope. Owing to this configuration, the EHT’s maximum baseline — the equivalent to a telescope’s aperture — for observing Sagittarius A* was 10,700 km (6,650 miles) across, effectively making the EHT the largest telescope on Earth!

The Event Horizon Telescope

Since the observation of Sgr A* and M87 were made, three more telescopes have been added to the network, but is it enough to get sharper images or even capture farther away black holes? After all, Sgr A* and M87 were the EHT’s top two targets especially because of their relatively large angular size in the sky whereas supermassive black holes in other galaxies appear far smaller from our viewpoint.

For higher resolution and more imaging power the EHT would need to widen its aperture even further. But how to achieve that given Earth’s limited size? Well, one solution would be to have a space-based addition to the EHT, which would significantly increase the overall angular resolution. How cool would that be?