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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TechTranscript
00:00 supermassive black hole at the heart of our own Milky Way galaxy, known as Sajay Star.
00:04 But this image from the Event Horizon Telescope, or EHT, is unlike most other familiar astronomical
00:10 images. It's the product of technically challenging telescope observations and innovative data
00:15 processing that tackles the unique complexities in EHT data. The fundamental challenge is one
00:21 of scale. The Sajay Star black hole is about 4 million times more massive than our sun,
00:26 extending over an area almost as large as Mercury's orbit. That may sound large,
00:31 but at a distance of 27,000 light years, this is like trying to take a photograph of a single
00:36 grain of salt in New York, all the way from Los Angeles. You would need a radio telescope as big
00:42 as the entire Earth to take a picture of something that small. Constructing a telescope dish that big
00:47 is of course impossible, so astronomers got creative. They developed algorithms that combine
00:52 radio telescopes across the globe into a single virtual Earth-sized telescope.
00:56 This computational telescope, the EHT, doesn't work like a regular telescope. Instead, the radio
01:03 telescopes work in pairs, with each pair contributing a little bit of information to the entire image.
01:08 Telescopes that are far apart can detect the smallest, sharpest features. Orientation is
01:13 also important, with each angle picking up different parts of the hole. With enough samples,
01:18 you can recover all the sharpest features. Telescopes that are closer together become
01:23 sensitive to broader features that the wider pairs can't see. Combined, these components
01:28 of the image can provide a good representation of the target being observed. Making a perfect
01:34 image would require telescopes at all orientations and separations, but EHT's eight telescopes
01:39 scattered around the globe only measure some of these possible pairings. Luckily, as Earth
01:45 rotates, the separation and orientations between the telescopes change, providing more, but not all,
01:50 of the information we need to make a perfect picture. In essence, taking a picture with the
01:55 EHT is a bit like listening to a song being played on a piano that has a lot of broken keys. Since we
02:01 don't know when the broken keys are being hit, there are an endless number of possible tunes
02:05 that could be playing. Nonetheless, with enough functioning keys, our brains can often fill in
02:10 the gaps to recognize the song. And on top of all this, in the case of Sajay Star, there was another
02:16 daunting challenge. The material swirling around the black hole moved so quickly that its appearance
02:22 could change from minute to minute while the data were being collected. This is a bit like changing
02:26 the key of the song as it's being played on the broken piano. To tackle this and other challenges,
02:32 scientists and engineers have spent years developing computational imaging algorithms
02:37 that allow us to capture images of the black hole with incomplete data. These algorithms can
02:43 intelligently fill in the missing information in a number of different ways. To capture the range of
02:48 possible Sajay Star appearances, the EHT team produced thousands of images with different
02:53 methods. Each of these images is slightly different, but they all are consistent with the EHT data.
02:59 By averaging these images together, the team emphasized the common features appearing in most
03:04 of the images while suppressing features that appear infrequently. Here, a bright ring clearly
03:09 pops out! But it's important to note that not all the possible images look alike. In fact, the team
03:15 found they could cluster the recovered images into four categories based on similar visual features.
03:21 Three of the clusters contained a ring-like feature with different intensities around the ring.
03:25 A much smaller fourth cluster contained images that did not appear ring-like. Although the
03:30 non-ring images can't be fully rolled out, the vast majority of the images contain a ring of
03:36 exactly the same size predicted by prior observations and theory. Through the power of
03:41 computational imaging, the EHT team overcame seemingly impossible hurdles to capture the
03:46 first image of Sajay Star. In the future, with more telescopes and better algorithms, we aim to
03:51 get an even clearer picture and a deeper understanding of the beastly black hole lying in the heart of our galaxy.
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