1st Image Of Our Galaxy's Black Hole Heart

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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.

00:13After all, they are black and do not emit light.

00:16So how can we see them?

00:18Well, with a telescope big enough, we could at least see the immediate surroundings of

00:23the largest black holes.

00:26The supermassive ones that are millions or even billions of times heavier than our sun.

00:33Then we would be able to unveil some of the mysterious secrets these monsters hide.

00:39Except that when you do the maths, you find that to observe even the closest supermassive

00:44black holes, you'd need a telescope the size of the Earth.

00:48Something beyond our wildest dreams.

00:52Or maybe not.

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

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

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

01:10The Event Horizon Telescope, or EHT, is not a real telescope, but a virtual one.

01:25The stroke of genius of the EHT collaboration was in using powerful radio telescopes that

01:30already exist, including ALMA and APEX, co-owned by ESO.

01:37They combined their observations in a way no one had ever attempted before, with a technique

01:42called Very Long Baseline Interferometry.

01:49This may sound like sci-fi, but it actually works, as the EHT team showed back in 2019.

01:56That's when they revealed the supermassive object at the center of the M87 galaxy to

02:01the world.

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

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

02:21First of all, you should know that the EHT telescopes could not see the black hole itself,

02:26as it is invisible.

02:28Rather, they picked up the radio signals from the hot glowing gas around it and imaged the

02:33shadow the black hole casts on it.

02:38To do this, the telescope antennas in the EHT array had to be pointed to exactly the

02:43same position in the sky at exactly the same time.

02:47The EHT can tell if one of these antennas is off by just a millimetre, and if the timing

02:52is shifted by a trillionth of a second, even though the telescopes are located thousands

02:57of kilometres apart.

03:02Imaging the black hole in M87 then required combining the observations of all telescopes

03:06in the network, using interferometry.

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

03:18The team had 8 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

03:32and reconstruct their image.

03:36It was like staring at a puzzle with most pieces missing, trying to figure out what

03:40the whole image would look like.

03:44To determine if the result was scientifically bulletproof, they used a variety of methods.

03:49Computer simulations to identify errors introduced by their telescope network, different teams

03:54working in isolation on reconstructing the image in different ways, new techniques and

03:59software.

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

04:06Only then did they show their image to the world.

04:18The result was like peering at the black hole in M87 with a telescope almost the size

04:24of the Earth.

04:25An 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, Sagittarius A star, the supermassive

04:48black hole at the heart of the Milky Way, our black hole.

04:55Sagittarius 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, it's even more difficult.

05:11First, the centre of the Milky Way is obscured to us by clouds of dust and hot gas that scatter

05:18the radio signals coming from around the black hole.

05:21Furthermore, because Sagittarius A star is about 1,500 times less massive than its cousin

05:27in M87, its radio signals change far more rapidly in time.

05:32Blobs of plasma orbit it in just a few minutes, whereas those in M87 orbit the black hole

05:39every few days.

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

05:47images.

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

05:56In the end, the EHT team did manage to overcome all these obstacles.

06:01So here it is, the first image of Sagittarius A star, the black hole at the centre of the

06:08Milky Way.

06:47Transcribed by ESO. Translated by —

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