How Our Galaxy's Black Hole Was Captured
Caltech’s Katie Bouman explains how the Event Horizon Telescope Collaboration captured the first imager of the Sagittarius A* Supermassive black hole at the core of the Milky Way galaxy - Milky Way vs M87.
Credit: Caltech
Credit: Caltech
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00:00supermassive black hole at the heart of our own Milky Way galaxy, known as Sajay Star.
00:05But this image from the Event Horizon Telescope, or EHT, is unlike most other familiar astronomical
00:10images. It's the product of technically challenging telescope observations and innovative data
00:15processing that tackles the unique complexities in EHT data. The fundamental challenge is one of
00:21scale. The Sajay Star black hole is about 4 million times more massive than our sun,
00:26extending over an area almost as large as Mercury's orbit. That may sound large,
00:31but at a distance of 27,000 light years, this is like trying to take a photograph of a single
00:36grain of salt in New York, all the way from Los Angeles. You would need a radio telescope as big
00:42as the entire Earth to take a picture of something that small. Constructing a telescope dish that
00:47big is of course impossible, so astronomers got creative. They developed algorithms that
00:52combine radio telescopes across the globe into a single virtual Earth-sized telescope.
00:57This computational telescope, the EHT, doesn't work like a regular telescope. Instead,
01:02the radio telescopes work in pairs, with each pair contributing a little bit of information
01:07to the entire image. Telescopes that are far apart can detect the smallest, sharpest features.
01:13Orientation is also important, with each angle picking up different parts of the hole.
01:17With enough samples, you can recover all the sharpest features.
01:21Telescopes that are closer together become sensitive to broader features that the wider
01:25pairs can't see. Combined, these components of the image can provide a good representation
01:30of the target being observed. Making a perfect image would require telescopes at all orientations
01:36and separations, but EHT's eight telescopes scattered around the globe only measure some
01:42of these possible pairings. Luckily, as Earth rotates, the separation and orientations between
01:47the telescopes change, providing more, but not all, of the information we need to make a perfect
01:52picture. In essence, taking a picture with the EHT is a bit like listening to a song being played on
01:58a piano that has a lot of broken keys. Since we don't know when the broken keys are being hit,
02:03there are an endless number of possible tunes that could be playing. Nonetheless, with enough
02:08functioning keys, our brains can often fill in the gaps to recognize the song.
02:13And on top of all this, in the case of Sajay Starr, there was another daunting challenge.
02:18The material swirling around the black hole moved so quickly that its appearance could change from
02:22minute to minute while the data were being collected. This is a bit like changing the
02:27key of the song as it's being played on the broken piano. To tackle this and other challenges,
02:32scientists and engineers have spent years developing computational imaging algorithms
02:37that allow us to capture images of the black hole with incomplete data.
02:42These algorithms can intelligently fill in the missing information in a number of different ways.
02:47To capture the range of possible Sajay Starr appearances, the EHT team produced thousands
02:52of images with different methods. Each of these images is slightly different, but they all are
02:57consistent with the EHT data. By averaging these images together, the team emphasized the common
03:03features appearing in most of the images while suppressing features that appear infrequently.
03:08Here, a bright ring clearly pops out! But it's important to note that not all the possible
03:13images look alike. In fact, the team found they could cluster the recovered images into
03:17four categories based on similar visual features. Three of the clusters contained a ring-like
03:22feature with different intensities around the ring. A much smaller fourth cluster contained
03:27images that did not appear ring-like. Although the non-ring images can't be fully rolled out,
03:33the vast majority of the images contain a ring of exactly the same size predicted by prior
03:38observations and theory. Through the power of computational imaging, the EHT team overcame
03:44seemingly impossible hurdles to capture the first image of Sajay Starr. In the future, with more
03:49telescopes and better algorithms, we aim to get an even clearer picture and a deeper understanding
03:54of the beastly black hole lying in the heart of our galaxy.