Niruj Mohan Ramanujam in The Wire:
What has the Event Horizon Telescope actually seen?
The Event Horizon Telescope (EHT) has imaged the silhouette – or shadow – of the black hole at the centre of the M87 galaxy. To create this image, astronomers combined data from eight different telescopes across the world in an observation run conducted in April 2017. The data in question was electromagnetic signals with a frequency of 230 GHz, or a corresponding wavelength of 1.3 mm. Using this, astronomers formed the image of a black hole for the first time.
The event horizon of a black hole is the ultimate boundary. Nothing from inside it can escape to the outside. The ring of fire in the EHT image is light from the gas falling into the event horizon, and its shadow is the dark hole in the centre. The exact shape of the ring is due to the way the incredible gravity of the black hole bends the light around it, and the extreme speed at which the gas is falling in. The ring is not seen to be uniformly bright because these numbers are uneven.