M87*

M87* is a supermassive black hole with a mass of approximately 6.5 billion solar masses, making it one of the most massive black holes known.² Its Schwarzschild radius is about 19 billion kilometers, and its event horizon has a diameter of roughly 38 billion kilometers. The black hole is surrounded by a bright accretion disk of gas and dust, which emits radiation across the electromagnetic spectrum.

The Event Horizon Telescope (EHT) is a global network of radio telescopes that uses very-long-baseline interferometry (VLBI) to achieve the resolution needed to image black holes. In April 2017, the EHT observed M87* over several nights, and after two years of data processing, the first image was released on April 10, 2019.¹ The image shows a bright ring of emission surrounding a dark central region, which is the black hole's shadow, a result of gravitational lensing and photon capture at the event horizon.

The image of M87* provided the first direct visual evidence of a black hole, confirming predictions of general relativity in the strong-field regime. The size and shape of the shadow are consistent with the expected appearance of a Kerr black hole, as described by the Kerr metric.³ The observations also allowed measurements of the black hole's spin and the orientation of its accretion disk, and they constrained models of relativistic jets.

M87* is known to launch a relativistic jet of plasma that extends thousands of light-years from the galaxy's core. The jet is one of the most studied astrophysical jets, and the EHT observations have helped link the jet's launching region to the immediate vicinity of the black hole. The jet is powered by the extraction of rotational energy from the black hole via the Blandford–Znajek process.⁴

Beyond the famous image, M87* has several lesser-known aspects. The EHT data also revealed that the black hole's shadow is slightly larger than the event horizon, due to gravitational lensing. The image was produced from data collected by eight telescopes, including the Atacama Large Millimeter/submillimeter Array (ALMA) and the South Pole Telescope. The 2019 image was based on observations from April 2017, and a later 2021 analysis of the same data confirmed the presence of a ring structure with a diameter of about 42 microarcseconds. Additionally, the EHT team included more than 200 researchers, and the image was the result of a global collaboration. The black hole's mass was previously estimated from stellar dynamics and gas kinematics, but the EHT measurement provided a more direct estimate. The shadow's size is consistent with the mass inferred from the orbit of stars around the black hole, as measured by the MUSE instrument on the Very Large Telescope. The jet of M87* is also notable for its superluminal motion, where knots of plasma appear to move faster than light due to relativistic beaming. The black hole is also a strong source of gamma rays, and it is one of the few known sources of very-high-energy gamma rays. The EHT image was not the first attempt to image a black hole; earlier efforts with the VLBA and other arrays had provided indirect evidence, but the EHT achieved the necessary resolution. The image also sparked public interest in black holes, and it was featured on the cover of the journal Astrophysical Journal Letters.

Event horizon

The boundary around a black hole beyond which nothing can escape.

Accretion disk

A rotating disk of gas and dust that spirals into a black hole.

Relativistic jet

A narrow beam of plasma ejected from the vicinity of a black hole at speeds close to the speed of light.

Very-long-baseline interferometry (VLBI)

A technique that combines signals from multiple radio telescopes to achieve high angular resolution.