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The ring of light around the black hole M87* flickers unevenly, and that flicker reveals how fast the hole is spinning.
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In 2019, astronomers unveiled the first-ever image of a black hole's shadow — a dark void encircled by a lopsided orange ring. This is the supermassive black hole at the heart of galaxy M87, 55 million light-years away. The ring isn't just glowing: one side is noticeably brighter than the other, like a campfire burning unevenly, with flames leaping higher on one side.
Black hole M87* weighs as much as 6.5 billion Suns, and its shadow on the sky is the size of a tennis ball as seen from Earth on the Moon.
It turned out that a very slow spin poorly explains what we see. A too-weak black hole couldn't accelerate the gas so strongly or make one side of the ring consistently brighter. The images best match models where the hole spins fast. This aligns with the fact that M87 shoots out a giant plasma jet thousands of light-years long, such jets, according to calculations, are launched by the hole's rotation.
Future observations will pin down exactly how fast it's spinning. That matters not just for astrophysics: the spin rate reveals how the black hole grew — whether it gulped matter chaotically or neatly along a spiral. The black hole's shadow, like a speedometer needle, reveals the hidden mechanics.
Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring's peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spin $$a_{*}$$. We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor $$|a_{*}| \lesssim 0.2$$. This is consistent with the Blandford-Znajek model for M87's jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints, and discuss how improved spin constraints could distinguish between differing jet-launching mechanisms and black hole growth scenarios.
Black hole M87* is so massive that its shadow is visible from Earth, even though the hole itself is 55 million light-years away, like spotting a coin on the Moon.
In the movie Interstellar, the black hole Gargantua is shown with a bright ring, lopsided due to rotation — exactly what we see at M87*.
