42 ± 3 μas
M87* · Ring 42 μas
The bright ring around the black hole at the center of the galaxy M87, measured by the Event Horizon Telescope from its April 2017 campaign at a diameter of 42 ± 3 microarcseconds.
In April 2019 the Event Horizon Telescope showed the first picture of a black hole. Radio dishes across the Earth worked together as one telescope the size of the planet. The black hole sits in the galaxy M87, about 55 million light years away. Around its dark center they saw a glowing ring of light bent by gravity. The discovery paper gives the ring's diameter on the sky: 42 ± 3 microarcseconds. A microarcsecond is a millionth of an arcsecond, a sliver of sky far too small for any eye.
What it supports
A measured value printed by the discovery paper itself (EHT Collaboration 2019, ApJL 875, L1): "an asymmetric bright emission ring with a diameter of 42 ± 3 μas." Paper VI finds the same 42 ± 3 μas across all methods and, with a distance of 16.8 Mpc, a mass of about 6.5 billion Suns. The ring kept its size over four nights, and again in the 2018 campaign, as general relativity predicts for a Kerr black hole.
What it does not
Not exactly 42. The error is plus or minus 3 μas, and the April 2018 campaign measured 43.3 (+1.5, -3.1) μas, the same ring within the error. Not the size of the black hole itself: the ring is light bent around its shadow. Not the Kerr ISCO 42.3%, a different object. No unit conversion, no chosen catalogue row. Not Adams.
ExactnessRange / bound
Form42
Unitmicroarcseconds (μas)
Strengthhard
When2017 · 2019
Hung6 October 2026