Astronomers Map the Color of M87's Black Hole Ring and Pinpoint Where Its Light Breaks Free

The orange doughnut that became the first image of a black hole in 2019 has a color, and astronomers have now mapped how that color changes across it — a measurement that turns a famous picture into a probe of the plasma physics happening a few billion kilometers from an event horizon.

An international team led by the Shanghai Astronomical Observatory published the first spatially resolved dual-frequency spectral study of M87*, the 6.5-billion-solar-mass black hole at the center of the galaxy M87, in The Astrophysical Journal Letters on July 20. Shan-Shan Zhao is the first author and Ru-Sen Lu the corresponding author, with collaborators at the University of Tsukuba's Center for Computational Sciences, Italy's INAF Institute of Radio Astronomy, the Andalusian Institute of Astrophysics and the Max Planck Institute for Radio Astronomy.

The trick was to combine two sets of images taken in 2018 at different wavelengths: the Event Horizon Telescope's 1.3-millimeter observations, which produced the celebrated ring, and Global Millimeter VLBI Array data at 3.5 millimeters, which sees a slightly larger, fuzzier structure. Comparing brightness at two frequencies point by point yields the spectral index — a number that says whether radiation at a given spot is climbing or falling with frequency, and therefore whether the plasma there is transparent or is swallowing its own light.

The map shows a clear transition. Close to the black hole, the spectral index is positive, the signature of synchrotron self-absorption: electrons spiraling through magnetic fields at nearly light speed reabsorb the photons they themselves emitted, so the region is optically thick and radiation cannot escape freely. Farther out the index turns negative, meaning photons stream away unimpeded. The changeover happens at about 30 microarcseconds from the center — a scale that matches the ring seen in the 3.5-millimeter images.

That coincidence is the result's real payload. It means the ring astronomers have been staring at for seven years is not just a geometric shadow cast by gravity, but a boundary tied to the physical state of the gas — where the accretion flow stops being opaque to its own radiation. As the researchers put it, the ring structure is closely connected to the plasma near the event horizon rather than being merely a morphological feature.

Thirty microarcseconds is roughly the angle a poppy seed on the Moon would subtend from Earth, and M87 sits about 55 million light-years away, which is why the measurement required telescopes scattered across continents working as a single Earth-sized instrument. The dual-frequency method now gives modelers something they have long wanted: a way to test simulations of magnetized accretion flows against a spatial map rather than a single averaged spectrum, and a template for doing the same for Sagittarius A*, the far more variable black hole at the center of our own galaxy.