In the News
An international team of astronomers has captured the image of a black hole, which was previously considered impossible. Evidence for the existence of black holes – mysterious places in space where nothing, even light, can escape – has been around for some time, and astronomers have long observed the effects of these phenomena on the environment. In the popular notion, it was assumed that taking a picture of a black hole was impossible because an image of something out of which no light could escape would appear completely black. For scientists, the challenge was to capture a picture of the hot, glowing gas falling into a black hole from thousands or even millions of light years away. Both have succeeded in an ambitious team of international astronomers and computer scientists. The team worked for over a decade to achieve the feat, enhancing an existing radio astronomy technique for high resolution imaging, and using it to recognize the silhouette of a black hole ̵
How They Did It
Although scientists had theorized, they could portray black holes by capturing their silhouettes against their images, glowing surroundings, the ability to image such a distant object escaped them. A team formed to meet the challenge and create a telescope network known as the Event Horizon Telescope or EHT. They wanted to capture a picture of a black hole by improving a technique that allows the imaging of distant objects known as Very Long Baseline Interferometry or VLBI.
Telescopes of all types are used to view distant objects. The larger the diameter or opening of the telescope, the greater the ability to collect more light, and the higher its resolution (or ability to image fine details). To see details in objects that are far away and appear small and weak from the earth, we need to gather as much light as possible with very high resolution, so we need to use a telescope with a large aperture.
Therefore, the VLBI technique was essential for capturing the image with the black hole. VLBI creates a series of smaller telescopes that can be synchronized to simultaneously focus on the same object and act as a giant virtual telescope. In some cases, the smaller telescopes also consist of several telescopes. This technique was used to track spacecraft and image distant cosmic radio sources such as quasars.
The aperture of a huge virtual telescope such as the Event Horizon Telescope is the same size as the distance between the two most distant telescope stations – the EHT has these two stations at the South Pole and in Spain. to create an opening that almost matches the diameter of the earth. Each telescope in the array focuses on the target, in this case the black hole, and collects data from its location on Earth, displaying part of the overall view of the EHT. The more telescopes in the array are far apart, the better the image resolution.
To test VLBI to map a black hole and a set of computer algorithms for sorting and synchronizing data, the Event Horizon Telescope team chose two goals: each of them presents unique challenges.
The closest supermassive black hole, Sagittarius A *, interested the team because it is located in our galactic backyard – at the center of our Milky Way, 26,000 light years (156 billion miles) away. (A star is the astronomical standard for referring to a black hole.) Although not the only black hole in our galaxy, it is the black hole that appears the largest on Earth. Being in the same galaxy as the Earth, however, meant that the team had to look through the pollution caused by stars and dust to image it, so there was more data to filter in processing the image. However, due to the local interest of the black hole and its relatively large size, the EHT team chose Sagittarius A * as one of its two objectives.
The second target was the supermassive black hole M87 *. One of the largest known supermassive black holes, M87 *, is located in the center of the gigantic elliptical galaxy Messier 87 or M87, 53 million light-years (318 miles) away. M87 * is much more massive than Sagittarius A * with 4 million solar masses and 6.5 billion solar masses. A solar mass corresponds to the mass of our sun, about 2×10 30 kilograms. In addition to its size, M87 * interested scientists because, unlike Sagittarius A *, it is an active black hole into which matter has collapsed and ejected in the form of particle beams that are accelerated to speeds close to the speed of light. But his removal made it even more difficult than the relatively local Sagittarius A *. As described by Katie Bouman, a computer scientist at the EHT, who led the development of one of the algorithms used to sort the telescope data during the processing of the historical image, it is almost like taking a picture of an orange on the lunar surface.  By 2017, EHT was a collaboration of eight locations around the world – and more have been added since. Before the team could begin collecting data, it had to find a time when the weather was likely to facilitate telescope observations at each location. In April 2017, the team tried to keep the weather good for M87 *, and of the ten days selected for observation, there were a whopping four days to spare at all eight locations!
Each telescope used for the EHT had to be synchronized with the M7, others within a fraction of a millimeter with an atomic clock linked to a GPS time standard. Thanks to this precision, the EHT is able to resolve objects around 4000 times better than the Hubble Space Telescope. As each telescope received data from the black target hole, the digitized data and time stamp were recorded on computer disk media. By collecting data for four days around the world, the team was able to process a considerable amount of data. The recorded media was then physically transported to a central location because the amount of data (about 5 petabytes) exceeds the current Internet speeds. At this central location, data from all eight locations was synchronized using the timestamps and merged into a composite image set revealing the unprecedented silhouette of the M87 * effect horizon. The team is also working to generate a picture of Sagittarius A * from additional observations from the EHT.
Adding more telescopes and considering Earth's rotation may result in more image resolution, and it is expected that future images will have a higher resolution. But we may never have a complete picture, as Katie Bouman explains here (under "Imaging a Black Hole").
To supplement the EHT results, several NASA space probes were at great expense in observing the black hole at different wavelengths of light. As part of this effort, NASA's Chandra X-ray Observatory, the Nuclear Spectroscopic Telescope Array (NuSTAR), and Neil Gehrel's Swift Observatory Space Telescope Missions, which were all designed to detect different types of X-rays, were looking at the black hole M87 at the same time with the EHT in April 2017. NASA's Fermi Gamma-Ray Space Telescope also observed changes in M87 * gamma-ray light during EHT observations. If the EHT observed changes in the structure of the black hole environment, data from these missions and other telescopes could be used to find out what was going on.
Although NASA observations did not directly track the historical image, astronomers used data from Chandra and NuSTAR satellites to measure the X-ray brightness of the M87 * jet. The scientists used this information to compare their models of the jet and the disk around the black hole with the EHT observations. Other findings could come as researchers continue to think about this data.
Why It Matters
Learning mysterious structures in the universe provides insights into physics and allows us to test observation methods and theories, such as Einstein's general theory of relativity. Massive objects deform space-time in their vicinity, and although theory of general relativity has been directly applied to smaller objects such as Earth and Sun, the theory for black holes and other dense-matter regions has not yet been directly proven
One of The main outcome of the EHT Black Hole Project is the more direct calculation of the mass of a black hole than ever before. Using the EHT, scientists were able to directly observe and measure the radius of the M87 * horizon or its Schwarzschild radius and calculate the mass of the black hole. This estimate was similar to that derived from a method using the orbiting stars – and thus validated as a method of mass estimation.
The size and shape of a black hole, which depends on its mass and spin, can be predicted from general relativity equations. The general theory of relativity predicts that this silhouette is approximately circular, but other theories of gravity predict something different. The image of M87 * shows a circular silhouette and lends credibility to Einstein's general theory of relativity near black holes.
The data also provide insight into the formation and behavior of black hole structures, such as For example, the accretion disk that gets matter into the black hole and plasma jets that emanate from its center. Scientists have hypothesized how an accretion disk forms, but so far they have never been able to test their theories with direct observation. The scientists are also curious about the mechanism by which some supermassive black holes emit powerful particle beams at near the speed of light.
These and other questions are answered when more data is collected by the EHT and synthesized in computer algorithms. Stay up to date and the next expected picture of a black hole – the own shooter of our Milky Way A *.
Capture your students' enthusiasm for black holes by encouraging them to solve them Math problems oriented to standards .
Model Black-Hole Interaction with this NGSS-Oriented Lesson:
See NASA Space Place Student Resources
for more information. TAGS: Black Hole, Teachable Moments , Science, K-12 Education, Teacher, Educator