Event Horizon Telescope and the First Black Hole Image

The Event Horizon Telescope is a global very-long-baseline interferometry array that operates at a wavelength of 1.3 mm. By linking radio dishes across the planet, it forms a virtual Earth-sized telescope capable of resolving the innermost regions of supermassive black holes. This capability allowed the collaboration to produce the first image of a black hole in 2019 and, subsequently, the first image of the black hole at the center of the Milky Way.¹

The EHT observes at a frequency of 230 GHz, corresponding to a wavelength of 1.3 mm. At this wavelength, the array achieves an angular resolution of about 20 microarcseconds, sufficient to resolve the shadow of a supermassive black hole in the galaxy M87 and later that of Sagittarius A*. The observations rely on very long baseline interferometry to achieve this resolution.¹

In 2017, the EHT array consisted of eight telescopes. These stations were coordinated to observe the target sources simultaneously, effectively creating a single telescope with a diameter approaching that of the Earth. The resulting data provided the basis for the first black hole image, which was produced by the Event Horizon Telescope Collaboration.¹

Very-long-baseline interferometry is a technique used in radio astronomy. It combines signals from multiple radio telescopes to simulate a larger telescope. The resolution of VLBI is determined by the maximum separation between telescopes; by spanning the globe, the EHT achieves an effective aperture comparable to the Earth's diameter. This technique enables the EHT to reach the angular resolution needed to study black hole shadows.²

The EHT is a global network of radio telescopes. It links radio dishes around the world to form a virtual Earth-sized telescope. The array operates at a wavelength of 1.3 mm, where the atmosphere is relatively transparent and the emission from the immediate vicinity of a black hole is strong. The EHT uses very long baseline interferometry to achieve its resolution.³

ALMA, the Atacama Large Millimeter/submillimeter Array, is a key component of the Event Horizon Telescope. Located in the Atacama Desert in Chile, ALMA observes at millimeter and submillimeter wavelengths. Its sensitivity greatly improved the EHT's imaging capabilities. As part of the global array, ALMA contributed to the detection and imaging of both M87* and Sagittarius A*.⁴

The EHT collaboration includes over 200 researchers. The project is supported by the National Science Foundation, among other organizations. The EHT observes at 1.3 mm wavelength, a band that allows the array to probe the synchrotron emission from relativistic electrons in the accretion flows around supermassive black holes. The collaboration's work has produced the first image of a black hole in 2019 and the first image of Sagittarius A* in 2022.⁵

The first image of a black hole was released in 2019. It shows the shadow of the black hole in M87, a supergiant elliptical galaxy in the constellation Virgo. The black hole, often referred to as M87*, is about 55 million light-years from Earth and has a mass of 6.5 billion times that of the Sun. The image was produced by the Event Horizon Telescope Collaboration.³

The EHT observations of M87* were made in April 2017, and the results were published on April 10, 2019. The image shows a ring with a diameter of about 40 microarcseconds. The observed image is consistent with the shadow of a black hole as predicted by general relativity. The measured diameter of the shadow is about 42 microarcseconds, which is about 2.5 times the gravitational radius.¹

The emission ring is brighter in the south. This brightness asymmetry is consistent with Doppler boosting from relativistic motion. The black hole in M87 has a mass of 6.5 billion solar masses and is located at a distance of 16.8 Mpc. The accretion flow around M87* is radiatively inefficient, and the synchrotron emission is from relativistic electrons.¹

The black hole is rotating, but the spin is not tightly constrained. The jet in M87 is launched from the black hole. The EHT image is consistent with a Kerr black hole. These findings, derived from the 2017 observations, were published on April 10, 2019, marking the first time a black hole's shadow had been directly imaged.¹

The Event Horizon Telescope has imaged the supermassive black hole at the center of the Milky Way, Sagittarius A*. Sgr A* is located in the constellation Sagittarius and has a mass of about 4 million solar masses. It lies at a distance of about 8 kpc from Earth. The results were published on May 12, 2022.⁶

The image shows a ring with a diameter of about 50 microarcseconds. The shadow of Sgr A* has a diameter of about 52 microarcseconds, which is about 2.5 times the gravitational radius. The image is consistent with the shadow of a black hole as predicted by general relativity. The Sgr A* observations were made in April 2017, simultaneously with the M87* observations.⁶

The Sgr A* ring is brighter on one side. The brightness asymmetry is likely due to Doppler boosting. The accretion flow around Sgr A* is radiatively inefficient, and the synchrotron emission is from relativistic electrons. The black hole is rotating, but the spin is not tightly constrained, and the EHT image is consistent with a Kerr black hole.⁶

The Sgr A* image required new calibration techniques because the source is more difficult to reconstruct than M87* due to rapid variability. The EHT uses very long baseline interferometry to achieve its resolution, which was essential for capturing the shadow of this comparatively small and rapidly changing black hole. The first image of Sagittarius A* was released in 2022.⁶

The M87* image provided a direct test of general relativity in the strong-field regime. The observed image is consistent with the shadow of a black hole as predicted by general relativity. The measured shadow diameter is about 42 microarcseconds, which agrees with theoretical expectations for a black hole of 6.5 billion solar masses at a distance of 16.8 Mpc.¹

The emission ring is brighter in the south, an asymmetry that is consistent with Doppler boosting from relativistic motion. This indicates that the emitting plasma is moving at relativistic speeds around the black hole. The synchrotron emission is from relativistic electrons, and the accretion flow around M87* is radiatively inefficient, meaning that most of the gravitational energy is not radiated away as light.¹

The black hole is rotating, but the spin is not tightly constrained by the current observations. The jet in M87 is launched from the black hole, linking the accretion flow to the large-scale relativistic jet. The EHT image is consistent with a Kerr black hole, the rotating black hole solution in general relativity. These results were published on April 10, 2019.¹

The Sgr A* image confirmed that the compact object at the center of the Milky Way is a black hole with a shadow matching general relativity. The shadow has a diameter of about 52 microarcseconds, and the image shows a ring with a diameter of about 50 microarcseconds. The mass of Sgr A* is about 4 million solar masses, and it is located about 8 kpc away.⁶

The Sgr A* ring is brighter on one side, and the brightness asymmetry is likely due to Doppler boosting. The accretion flow around Sgr A* is radiatively inefficient, and the synchrotron emission is from relativistic electrons. The black hole is rotating, but the spin is not tightly constrained, and the EHT image is consistent with a Kerr black hole.⁶

Imaging Sgr A* required new calibration techniques because the source is more difficult to reconstruct than M87* due to rapid variability. The EHT uses very long baseline interferometry to achieve its resolution, enabling the collaboration to overcome these challenges. The results were published on May 12, 2022, providing a second direct image of a black hole shadow.⁶

The Event Horizon Telescope Collaboration is responsible for the first black hole image, released in 2019. The collaboration includes over 200 researchers. The EHT is a global array of radio telescopes, and it observes at 1.3 mm wavelength. The project is supported by the National Science Foundation, which has highlighted the EHT's achievements, including the first image of a black hole in 2019 and the 2022 image of the black hole at the center of the Milky Way.⁷

The EHT is a global network of radio telescopes. It uses very-long-baseline interferometry to create an Earth-sized virtual telescope. The collaboration released the first image of a black hole in 2019 and the first image of Sagittarius A* in 2022. These milestones were made possible by the coordinated efforts of observatories around the world.⁸

The EHT is a global network of radio telescopes that operates at a wavelength of 1.3 mm. It produced the first image of a black hole in 2019 and the first image of Sagittarius A* in 2022. The collaboration's work has been supported by the National Science Foundation, which provided critical funding and infrastructure. The results have opened a new era in the study of black holes.⁹

Very-long-baseline interferometry is a technique used in radio astronomy. It combines signals from multiple radio telescopes to simulate a larger telescope. The resolution of VLBI is determined by the maximum separation between telescopes. This principle underlies the EHT's ability to achieve an angular resolution of about 20 microarcseconds, sufficient to resolve the shadows of supermassive black holes.²

ALMA is a key component of the Event Horizon Telescope. Located in the Atacama Desert in Chile, ALMA observes at millimeter and submillimeter wavelengths. Its sensitivity greatly improved the EHT's imaging capabilities, particularly for the 2017 observations of M87* and Sgr A*. ALMA's participation was crucial for the success of the EHT's first imaging campaigns.⁴

The EHT is a global very-long-baseline interferometry array operating at a wavelength of 1.3 mm. In 2017, the array consisted of eight telescopes. The EHT achieved an angular resolution of about 20 microarcseconds. These technical capabilities enabled the first image of a black hole, showing the shadow of M87*, and later the image of Sagittarius A*.¹

The EHT observes at a wavelength of 1.3 mm, which corresponds to a frequency of 230 GHz. The EHT uses very long baseline interferometry to achieve its resolution. The black hole shadow is about 2.5 times the gravitational radius. The synchrotron emission is from relativistic electrons. These parameters define the observational framework for both the M87* and Sgr A* results.¹

M87* and Sgr A* are both supermassive black holes, but they differ significantly in mass, distance, and environment. M87* has a mass of 6.5 billion solar masses and is located 16.8 Mpc away. Sgr A* has a mass of about 4 million solar masses and is about 8 kpc away. Despite these differences, both images show rings and shadows consistent with general relativity.¹

The shadow of M87* has a measured diameter of about 42 microarcseconds, and the image shows a ring of about 40 microarcseconds. The shadow of Sgr A* has a diameter of about 52 microarcseconds, and its ring is about 50 microarcseconds across. In both cases, the shadow is about 2.5 times the gravitational radius. The brightness asymmetries in both rings are consistent with Doppler boosting.⁶

Both black holes have radiatively inefficient accretion flows, and in both cases the synchrotron emission is from relativistic electrons. Both are rotating, but their spins are not tightly constrained. The EHT images of both are consistent with Kerr black holes. The Sgr A* image, however, required new calibration techniques and is more difficult to reconstruct than M87* due to rapid variability.⁶

The first image of a black hole was revealed on April 10, 2019. The image shows the black hole at the center of the galaxy M87. The black hole's shadow is about 40 microarcseconds across. The image was captured by the Event Horizon Telescope. This achievement marked a turning point in observational black hole physics, providing direct visual evidence of the shadow predicted by general relativity.¹⁰

The EHT made the first image of a black hole in 2019 and imaged the black hole at the center of the Milky Way in 2022. These results were supported by the National Science Foundation. The EHT is a global network of radio telescopes that links dishes around the world to form a virtual Earth-sized telescope. Its success has spurred further developments in millimeter-wavelength VLBI.⁷

The Event Horizon Telescope is a global network of radio telescopes that uses very-long-baseline interferometry to create an Earth-sized virtual telescope. It released the first image of a black hole in 2019 and the first image of Sagittarius A* in 2022. These observations have confirmed key predictions of general relativity and opened new avenues for studying the physics of supermassive black holes and their accretion flows.⁸