THE DARK HEART of our Galaxy

Jane Green looks into how eight telescopes came together to capture the shadow of Sagittarius A*, the supermassive black hole at the centre of the Milky Way

- Jane Green

At last, the mystery at the heart of our Milky Way Galaxy has been uncovered. On 12 May at 13:07 UT, the first-ever direct visual image of our closest supermassive black hole was unveiled by the Event Horizon Telescope (EHT) team. It was a thrilling, profound moment.

Though not the first black hole picture presented by the EHT collaboration - that accolade belongs to M87* first presented in April 2019 - it was one long sought after. As nothing, not even light, can escape beyond a black hole's event horizon, they are impossible to see directly. Instead, for decades, astronomers have attempted to track it down by looking for giant stars circling an otherwise invisible point in space. Now we have its image or, more accurately, its silhouette. Our Galaxy's 'central engine' exists.

Over five nights in April 2017, astronomers observed Sgr A* (pronounced "sadge-ay-star") in the constellation of Sagittarius with eight radio telescopes, at six sites from Arizona to the South Pole and Spain to Hawaii. Part of the EHT collaboration - a global network of synchronized radio dishes yielding an Earth-sized virtual telescope of planet-wide aperture - it was the result of decades of work involving more than 300 people and 80 institutions. Using the technique of very long baseline interferometry, the EHT offered the highest possible resolving power from the surface of Earth, capturing objects as small as 20 micro-arcseconds on the sky - that's equivalent to spying a doughnut on the Moon.