Discovery of Black Holes Timeline

First noticed as something unusual in images of the center of the Milky Way in 2002, the gas cloud G2, which has a mass about three times that of Earth, would later be confirmed to be likely on a course taking it into the accretion zone of Sgr A*.

G2 has been observed to be disrupting since 2009, and was predicted by some to be completely destroyed by its encounter with the black hole, which could have led to a significant brightening of X-ray and other emission from Sgr A*.

The gas cloud G2 was confirmed to be likely on a course taking it into the accretion zone of Sgr A* in a paper published in Nature in 2012. Predictions suggested it would make its closest approach (a perinigricon) in early 2014, at just over 3,000 times the radius of the event horizon from the black hole.

It was thought that the passage of G2 in 2013 might offer astronomers the chance to learn much more about how material accretes onto supermassive black holes, making it one of the most anticipated events in Galactic Center research.

It was proposed in May 2013 that, prior to its perinigricon, G2 might experience multiple close encounters with members of the black-hole and neutron-star populations thought to orbit near the Galactic Center, offering some insight to the region surrounding the supermassive black hole at the center of the Milky Way.

Researchers suggested in 2014 that G2 is not a gas cloud but rather a pair of binary stars that had been orbiting the black hole in tandem and merged into an extremely large star, explaining why the cloud survived its close passage.

Astronomers from the UCLA Galactic Center Group published observations obtained on March 19 and 20, 2014, concluding that G2 was still intact (in contrast to predictions for a simple gas cloud hypothesis) and that the cloud was likely to have a central star.

An analysis published on July 21, 2014, based on observations by ESO's Very Large Telescope in Chile, concluded alternatively that the cloud might be a dense clump within a continuous but thinner stream of matter, acting as a constant breeze on the disk of matter orbiting the black hole rather than sudden gusts.

On January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sgr A. The total luminosity from such outbursts is estimated to be a million times stronger than the current output from Sgr A, comparable with a typical active galactic nucleus.

In 2019, measurements made with the High-resolution Airborne Wideband Camera-Plus (HAWC+) mounted in the SOFIA aircraft revealed that magnetic fields cause the surrounding ring of gas and dust, with temperatures ranging from −280 to 17,500 °F, to flow into an orbit around Sagittarius A*, keeping black hole emissions low.

On May 13, 2019, astronomers using the Keck Observatory witnessed a sudden brightening of Sgr A*, which became 75 times brighter than usual, suggesting that the supermassive black hole may have encountered another object.

In June 2023, unexplained filaments of radio energy were found associated with Sagittarius A*, adding a new mystery to the environment around the Milky Way's central supermassive black hole.

A third associated gas cloud, called G3, was discovered in 2026, joining G1 and G2 as objects tracked in the vicinity of Sagittarius A* and offering further opportunities to study accretion onto the supermassive black hole.

In 2017, the Event Horizon Telescope, a world-wide network of radio observatories, collected the radio interferometer data that would later be used to produce the first image of the accretion disk around the event horizon of Sagittarius A*.

Image source: Event Horizon Telescope

A 2018 paper predicted an image of Sagittarius A* that is in agreement with later observations, anticipating what the Event Horizon Telescope would eventually capture of the shadow of the Milky Way's central black hole.

In 2019, the Event Horizon Telescope Collaboration released the first confirmed image of a black hole: the supermassive black hole at the center of Messier 87. This paved the way for the later imaging of Sagittarius A*.

Reinhard Genzel and Andrea Ghez were each awarded a quarter-share in the 2020 Nobel Prize in Physics for their discovery that Sagittarius A* is a supermassive compact object, for which a black hole was the only explanation.

Image source: Nobel Prize in Physics

On May 12, 2022, the first image of Sagittarius A* was released by the Event Horizon Telescope Collaboration. The image confirms that the object contains a black hole and is the second confirmed image of a black hole, after Messier 87's supermassive black hole in 2019.