What 7 Years of Imaging M87* Reveals About Black Hole Physics

Multi-Epoch Images (2017 and 2018)

Observations one year later confirmed that the ring persists. The median diameter in 2018 was 43.3(^{+1.5}_{-3.1}) μas, consistent within uncertainties with the 2017 measurement . However, the brightest part of the ring shifted — from the southeast in 2017 to the southwest in 2018, a rotation of about 30 degrees .

Polarimetric Images: Magnetic Field Structure

In 2021, the EHT published first polarimetric images of M87(^*), revealing a coherent spiral pattern in the magnetic field near the event horizon . New polarimetric images from 2025 show that this pattern changes over time: the magnetic field orientation in 2017 spiraled one way, had settled by 2018, and then reversed by 2021 . These flips indicate a highly dynamic magnetosphere surrounding the black hole.

86 GHz (3.5 mm) Images from GMVA+ALMA

In 2018, observations with the Global Millimeter VLBI Array (GMVA) combined with ALMA at 86 GHz simultaneously resolved a ring structure and the extended jet emission . These images show a larger ring (diameter about 61 μas) and a triple-ridged jet emerging from the core, directly connecting the accretion structure near the black hole to the base of the relativistic jet .

The First Spectral-Index Map (July 2026)

In the most recent major advance, researchers combined 1.3 mm EHT data with 3.5 mm GMVA+ALMA data to produce the first spatially resolved spectral-index map on event-horizon scales . The spectral index — which measures how radiation intensity changes with frequency — reveals a clear gradient with distance from the black hole . Near the center, the spectral index is positive, indicating that synchrotron self-absorption still dominates. Farther out (beyond roughly 30 μas), the index becomes negative, as the plasma becomes optically thin . The transition point aligns with the 3.5 mm ring structure, demonstrating that the ring reflects not just emission morphology but also a genuine change in the physical state of the plasma .

How the Images Were Created

All these images were produced using Very Long Baseline Interferometry (VLBI), a technique that links radio telescopes across the globe to form a virtual Earth-sized telescope . In April 2017, eight radio observatories on four continents — including ALMA in Chile — synchronized observations of M87 at a wavelength of 1.3 mm (230 GHz) . Hydrogen maser atomic clocks at each site precisely timed the recordings, and the raw data (totaling petabytes) were shipped to supercomputers at MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy for correlation and imaging . For the 86 GHz images, the GMVA array (including ALMA) used a similar approach but at the longer wavelength of 3.5 mm . The recent spectral-index map required combining data from both the 1.3 mm EHT and the 3.5 mm GMVA+ALMA arrays — a dual-frequency approach that had been predicted theoretically years earlier .

Key Findings: Plasma Behavior

A Turbulent, Magnetized Accretion Flow

The region immediately surrounding M87(^*) is a hot, magnetized accretion flow. Turbulence in this flow is the primary driver of the observed variability in ring brightness and asymmetry . New simulations show that the turbulence is consistent with a two-temperature plasma (electrons and ions at different temperatures) in a magnetically arrested disk . The plasma radiates via synchrotron emission from relativistic electrons spiraling along magnetic field lines .

Dynamic Magnetic Field Orientation

Between 2017 and 2021, the magnetic field orientation near the black hole flipped direction not once but twice, as seen in polarization images . This indicates that the magnetosphere is not static — it evolves on timescales of years, which is fast for a 6.5-billion-solar-mass black hole where the orbital timescale near the event horizon is about a month .

A New Probe: The Spectral-Index Gradient

The 2026 spectral-index map provides direct observational evidence for a systematic change in plasma conditions . In the innermost region (spectral index positive), synchrotron self-absorption is strong; farther out (spectral index negative), the plasma becomes optically thin. This gradient constrains the mechanisms of particle acceleration and electron heating — long-standing open questions in black hole astrophysics .

Key Findings: The Ring Structure

Persistent but Dynamic

The ring diameter has remained stable at about 40 μas across the 2017 and 2018 observing epochs, consistent with the predicted shadow of a 6.5-billion-solar-mass Kerr black hole . Yet its brightness peak moves. The shift of roughly 30 degrees between 2017 and 2018 is now understood to be driven by turbulence in the accretion flow, not by a change in the black hole itself .

Brightness Asymmetry and Black Hole Spin

The ring asymmetry is caused by Doppler beaming of relativistic plasma in a rotating accretion disk combined with gravitational lensing . The amplitude and position of the asymmetry fluctuate with the turbulence, and statistical comparisons of these fluctuations to GRMHD simulations can encode information about the black hole's spin . However, spin remains unconstrained from current data .

Direct Link to the Jet

At 86 GHz, the ring-like accretion structure is directly connected to the base of the relativistic jet . The images show a compact radio core with a triple-ridged jet emerging from the ring . This provides the strongest evidence yet that the jet is launched from the immediate vicinity of the event horizon — within a few gravitational radii of the black hole .

What Comes Next

The First Black Hole "Movie"

Expanded EHT arrays and improved sensitivity should produce time-lapse observations (a true movie) of the accretion flow around M87(^*), directly connecting accretion dynamics to jet launching on timescales of months to years .

Stronger Tests of General Relativity

Every new epoch of imaging continues to confirm that the shadow's size and shape match the predictions of general relativity for a Kerr black hole . With more epochs and higher signal-to-noise, these data will provide ever more stringent tests of gravity in the strong-field regime.

Spectral-Index Mapping Becomes a Standard Diagnostic

The 2026 dual-frequency breakthrough establishes spectral-index mapping as a powerful tool for probing plasma microphysics on event-horizon scales . This technique is now being applied to Sgr A(^*) and other targets, and will be routine with the next-generation EHT (ngEHT) .

Space-Based VLBI

Proposed space-VLBI missions, such as the Event Horizon Imager, could achieve month-long temporal resolution for M87(^*), capturing variability and jet dynamics with unprecedented detail .

Bottom Line

Seven years of EHT observations have moved M87(^*) from a single iconic image to a rich, multi-wavelength, time-domain dataset. The ring is persistent but dynamic, the magnetic field is restless, and the first spectral-index map has opened a new window into the plasma processes that govern black hole feeding and feedback. The next few years, with expanded arrays and improved techniques, promise to turn this still-growing dataset into the first movie of a black hole — and perhaps a definitive test of gravity at the edge of a singularity.