Event Horizon Telescope (EHT)

The Event Horizon Telescope (EHT) is a worldwide collection of Earth-based radio telescopes designed to detect and collect data from supermassive black holes. A black hole is the collapsed remains of a massive dead star in which gravity is so strong not even light can escape. Since light cannot escape a black hole, it is very difficult for Earthbound astronomers to detect them. For many years, astronomers could only identify the objects by indirect means, such as observing the radiation emitted by matter as it is sucked into the black hole. The Event Horizon Telescope was designed to gather data from supermassive black holes and provide astronomers with the first-ever images of such an object. In 2019, the EHT succeeded when it successfully observed a black hole at the center of the galaxy M87. Three years later, in 2022, the telescope returned the first image of a supermassive black hole at the center of Earth’s own galaxy, the Milky Way. By 2024, the telescope had made high-resolution images of the black hole.

Background

Stars are born from giant clouds of gas and dust that collapse in on themselves until pressure and heat ignites nuclear fusion at their centers. This is the process that makes a star “turn on” and begin producing energy. Stars spend their lives in a constant state of balance. The nuclear fusion at their cores seeks to push the star outward, while gravity seeks to continue its initial collapse.

Stars will eventually run out of nuclear fuel, and when they do, their fates depend on their size and mass. When the star cannot create enough energy to fight off gravity, it begins to collapse further. A star about the size of our sun will collapse until it becomes a white dwarf, a star about the size of Earth. A star more massive than our sun will collapse until an ultra-dense object known as a neutron star, a star about the size of a city, results.

The collapse of more massive stars, however, cannot be stopped by any force. They will collapse into an infinitely dense point known as a singularity. The gravity surrounding a singularity is so strong that not even light can escape, forming a black hole. Many black holes are relatively small and were once singular stars crushed by their immense gravity. However, black holes can “feed” on nearby matter, such as gas and dust, and even merge with other black holes to grow larger over time. These supermassive black holes can be millions or billions of times the mass of our sun. Astronomers believe that supermassive black holes lie at the center of almost all large galaxies.

Overview

The idea of a black hole was first theorized in the late eighteenth century, but their existence was first predicted mathematically in the early twentieth century by physicist Albert Einstein and several of his contemporaries. The first black hole was discovered in 1971 when astronomers detected X-rays coming from a star that was orbiting an invisible companion. The X-rays were generated by the intense radiation given off by the star’s matter as it was being sucked into the black hole.

Despite confirming the distance of black holes by theoretical means and observing the effects they have on nearby stars, astronomers could not actually “see” what a black hole looked like. Black holes that form from single massive stars would be only a few miles across and almost impossible to view from Earth. However, supermassive black holes—an idea first theorized in the 1970s and confirmed in the 1990s—could conceivably be detected from Earth. Although the black hole itself can never be seen, astronomers believed that radiation from the glowing gas around the object could be detected.

The problem faced by astronomers was that the telltale evidence from supermassive black holes would be impossible to detect by a single telescope. They would need a very large telescope, one about the size of Earth itself. Astronomers solved the problem by combining data from several radio telescopes from around the planet. Their locations ranged from Greenland to Antarctica. The project, which officially began collecting data in 2009, was dubbed the Event Horizon Telescope (EHT) after the outer boundary of a black hole past which nothing can escape.

Each part of the EHT consists of a single radio telescope or an array of telescopes that collect data from a target site. The telescopes record radio waves emitted from electromagnetic radiation that surrounds a supermassive black hole. Each telescope can collect only a small amount of information. Their data is then sent to the Max Planck Institute for Radio Astronomy in Germany and the Haystack Observatory at the Massachusetts Institute of Technology where it is pieced back together and analyzed. The collaborative effort requires highly sophisticated data analysis, high-tech imaging computer systems, and more than two hundred scientists and engineers.

In 2017, the EHT began observing the center of the galaxy M87, which is about fifty-five light years away from Earth. Long-exposure observations from eight radio telescopes in Europe, the United States, Mexico, Chile, Greenland, and Antarctica were painstakingly collected and sent back for analysis. After two years of work, astronomers released the first-ever image of a supermassive black hole in 2019. The black hole was about 6.5 billion times more massive than our sun.

The image showed a donut-like ring of super hot radiation around a dark center known as a shadow. The image confirmed what Albert Einstein had predicted more than a century earlier, that a source of massive gravity would actually bend light waves. The ring-like image proved that the black hole’s gravity was bending the light around it, creating what scientists call a gravitational lens.

After the success of M87, scientists turned the EHT toward the center of Earth’s galaxy, the Milky Way. Although the Milky Way’s black hole was closer, it was obscured by gas and dust between Earth and the galaxy’s center, making it more difficult to detect.

The EHT’s telescopes again made long-exposure observations of the Milky Way’s core, which is about 27,000 light years from Earth. In March 2022, scientists unveiled the first-ever image of our galaxy’s supermassive black hole, called Sagittarius A*. Although similar in appearance to M87’s black hole, Sagittarius A* is about a thousand times smaller and has a mass of about four million times that of the sun. The EHT telescopes captured sharper images of M87's black hole in 2023. The following year, EHT detected magnetic fields spiraling from the edge of Sagittarius A*. These fields were very similar to those of the black hole at the center of the M87 galaxy. Also in 2024, astronomers determined how to use EHT telescopes to capture the highest resolution images ever obtained from Earth. These images were crisper than previous images and multicolored.

Bibliography

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