Brussels, Washington, Santiago, Shanghai, Taipei, and Tokyo · 10 April 2019
The Event Horizon Telescope (EHT) collaboration unveiled the first direct visual evidence of a supermassive black hole and its shadow, captured at the centre of the galaxy Messier 87 (M87). The image — a luminous orange ring surrounding a dark central region — was presented simultaneously in coordinated press conferences across six cities on four continents, marking the culmination of more than a decade of planning, engineering, and data analysis.
The results were published in a series of six papers in a special issue of The Astrophysical Journal Letters.
A Planet-Scale Virtual Telescope
Capturing an image of a black hole required an instrument of extraordinary resolving power. No single telescope could achieve it. The EHT linked eight ground-based radio observatories across the globe — from volcanoes in Hawai'i and Mexico to mountains in Arizona and the Spanish Sierra Nevada, from the Chilean Atacama Desert to Antarctica — using a technique called very-long-baseline interferometry (VLBI). By synchronising their recorded data with atomic clocks (hydrogen masers) and exploiting the rotation of the Earth, the array functioned as a single virtual telescope the size of the planet, observing at a wavelength of 1.3 millimetres.
The participating telescopes included the Atacama Large Millimeter/submillimeter Array (ALMA), the Atacama Pathfinder Experiment (APEX), the IRAM 30-meter telescope, the James Clerk Maxwell Telescope, the Large Millimeter Telescope Alfonso Serrano, the Submillimeter Array, the Submillimeter Telescope, and the South Pole Telescope.
The Target: M87's Supermassive Black Hole
The black hole imaged by the EHT sits at the heart of M87, a giant elliptical galaxy in the nearby Virgo cluster, approximately 55 million light-years from Earth. With a mass estimated at 6.5 billion times that of the Sun, M87's black hole was predicted to be one of the largest whose shadow is viewable from Earth, making it an ideal target.
The visible glowing ring in the image is composed of superheated gas — plasma at temperatures of billions of degrees — spiralling inward toward the event horizon at nearly the speed of light. The ring appears brighter on one side due to the Doppler effect: the material rotating toward the observer appears boosted in brightness, while the receding side appears dimmer.
The dark central region is the black hole's shadow — not the black hole itself, which is a completely dark object from which no light can escape. The shadow is caused by gravitational bending and capture of light by the event horizon. The event horizon itself measures just under 40 billion kilometres across, approximately 2.5 times smaller than the shadow it casts.
Confirming Einstein
The image provided the first direct visual confirmation of a prediction made by Albert Einstein's theory of general relativity more than a century ago: that a black hole, when immersed in a bright region such as a disc of glowing gas, would create a dark region — a shadow — caused by the gravitational bending and capture of light.
"If immersed in a bright region, like a disc of glowing gas, we expect a black hole to create a dark region similar to a shadow — something predicted by Einstein's general relativity that we've never seen before." — Heino Falcke, Chair of the EHT Science Council, Radboud University, the Netherlands
The observations matched extensive computer models incorporating the physics of warped spacetime, superheated matter, and strong magnetic fields with surprising fidelity.
"The confrontation of theory with observations is always a dramatic moment for a theorist. It was a relief and a source of pride to realise that the observations matched our predictions so well." — Luciano Rezzolla, EHT Board member, Goethe Universität, Germany
Data at an Unprecedented Scale
The data used to construct the image were collected during a global observing campaign in April 2017. Each telescope in the EHT array produced roughly 350 terabytes of data per day, stored on high-performance helium-filled hard drives. These petabyte-scale datasets were physically flown to two specialised correlator supercomputers — at the Max Planck Institute for Radio Astronomy in Bonn, Germany, and at MIT Haystack Observatory in Massachusetts, USA — where they were combined and painstakingly converted into an image using novel computational algorithms developed by the collaboration.
The EHT links telescopes around the globe to form an Earth-sized virtual telescope. Credit: ESO/EHT Collaboration
A Global Collaboration
The EHT collaboration involved more than 200 researchers from Africa, Asia, Europe, North America, and South America. Thirteen stakeholder institutions worked together, using both pre-existing infrastructure and support from a variety of funding agencies. Key funding was provided by the US National Science Foundation (NSF), the EU's European Research Council (ERC) — including a €14 million grant for the BlackHoleCam project — and funding agencies in East Asia.
The European Southern Observatory (ESO) played a pivotal role through its leadership in two of the EHT's component telescopes in Chile: ALMA and APEX. ALMA, with its 66 high-precision antennas, was the most sensitive facility in the EHT array.
"We have taken the first picture of a black hole. This is an extraordinary scientific feat accomplished by a team of more than 200 researchers." — Sheperd S. Doeleman, EHT project director, Center for Astrophysics | Harvard & Smithsonian
Looking Ahead
The success of the EHT opens new avenues for studying the most extreme objects in the universe. The collaboration plans to extend observations to other black holes, improve image quality with additional telescopes, and even produce time-resolved movies of black hole accretion flows. The technique also holds promise for testing general relativity in the strong-field regime, where gravitational effects are most extreme, and for understanding the role of supermassive black holes in galaxy formation and evolution.
"We have achieved something presumed to be impossible just a generation ago. Breakthroughs in technology, connections between the world's best radio observatories, and innovative algorithms all came together to open an entirely new window on black holes and the event horizon." — Sheperd S. Doeleman, EHT project director
Sources
- ESO Press Release eso1907, "Astronomers Capture First Image of a Black Hole," 10 April 2019 —
- Event Horizon Telescope Collaboration, "First M87 Event Horizon Telescope Results," The Astrophysical Journal Letters, Vol. 875, No. 1 (2019) — six papers published 10 April 2019
- NASA, "Black Hole Image Makes History," 10 April 2019 —
- Event Horizon Telescope project —
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