1st Image of Our Galaxy's Black Hole Heart

Live Science

The Event Horizon Telescope captured the first image of the Milky Way galaxy's supermassive black hole Sagittarius A* — our galaxy's "black hole heart."  Credit: ESO

Transcript

00:00Imaging a black hole seems like an impossible dream.

00:12After all, they are black and do not emit light.

00:16So how can we see them?

00:19Well, with a telescope big enough,

00:21we could at least see the immediate surroundings of the largest black holes.

00:25The supermassive ones that are millions or even billions of times heavier than our sun.

00:32Then we would be able to unveil some of the mysterious secrets these monsters hide.

00:39Except that when you do the maths,

00:41you find that to observe even the closest supermassive black holes,

00:45you'd need a telescope the size of the Earth.

00:48Something beyond our wildest dreams.

00:51Or maybe not.

00:55A few years ago, 300 astronomers from nearly 80 institutes across the globe

01:00joined forces and found a way to create a telescope as large as our planet.

01:05And they did it without using new mirrors, screws or steel.

01:09Their Event Horizon Telescope, or EHT, is not a real telescope, but a virtual one.

01:16The stroke of genius of the EHT collaboration was in using powerful radio telescopes that already exist,

01:31including ALMA and APEX, co-owned by ESO.

01:35They combined their observations in a way no one had ever attempted before,

01:41with a technique called Very Long Baseline Interferometry.

01:45This may sound like sci-fi, but it actually works, as the EHT team showed back in 2019.

01:54That's when they revealed the supermassive object at the center of the M87 galaxy to the world.

02:06The very first image of a black hole.

02:14To understand exactly how hard that was, let us drop in some facts.

02:18First of all, you should know that the EHT telescopes could not see the black hole itself, as it is invisible.

02:27Rather, they picked up the radio signals from the hot glowing gas around it and imaged the shadow the black hole casts on it.

02:39To do this, the telescope antennas in the EHT array had to be pointed to exactly the same position in the sky at exactly the same time.

02:47The EHT can tell if one of these antennas is off by just a millimeter,

02:51and if the timing is shifted by a trillionth of a second, even though the telescopes are located thousands of kilometers apart.

03:01Imaging the black hole in M87 then required combining the observations of all telescopes in the network using interferometry.

03:12This technique works best if you have many telescopes, which wasn't the case.

03:16The team had eight observatories, though now the network has grown to 11.

03:27So the EHT researchers had to develop special algorithms to be able to fill in the gaps and reconstruct their image.

03:35It was like staring at a puzzle with most pieces missing, trying to figure out what the whole image would look like.

03:41To determine if the result was scientifically bulletproof, they used a variety of methods.

03:48Computer simulations to identify errors introduced by their telescope network,

03:53different teams working in isolation on reconstructing the image in different ways,

03:58new techniques and software.

04:00It took years of work, until they were sure they had done it right.

04:03Only then did they show their image to the world.

04:06The result was like peering at the black hole in M87 with a telescope almost the size of the Earth,

04:24an instrument so powerful that it could see details as small as a doughnut on the Moon.

04:36So what's next for the EHT?

04:40The team have already pointed their telescopes to a new target.

04:43Sagittarius A star, the supermassive black hole at the heart of the Milky Way.

04:51Our black hole.

04:56Sagittarius A star is much closer to Earth than the supermassive black hole in M87.

05:02So you may think that imaging it is a piece of cake by comparison.

05:06Sorry to disappoint you.

05:08It's even more difficult.

05:09First, the centre of the Milky Way is obscured to us by clouds of dust and hot gas

05:17that scatter the radio signals coming from around the black hole.

05:22Furthermore, because Sagittarius A star is about 1500 times less massive than its cousin in M87,

05:29its radio signals change far more rapidly in time.

05:33Blobs of plasma orbited in just a few minutes,

05:35whereas those in M87 orbit the black hole every few days.

05:42This forces astronomers to adapt their algorithms and to develop new techniques to get stable images.

05:48A bit like trying to read the brand on a basketball while spinning it on your finger.

05:53In the end, the EHT team did manage to overcome all these obstacles.

06:01So here it is.

06:03The first image of Sagittarius A star, the black hole at the centre of the Milky Way.

06:08The first image of the Milky Way.

06:09The first image of Sagittarius A star is the best of the Milky Way.

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