Short answers, pulled from the story.
Radio astronomers Barry Balick and Robert L. Brown identified Sagittarius A in 1974 using the National Radio Astronomy Observatory's baseline interferometer. They published their findings on February 13 and 15 that year, marking the first clear detection of what we now call Sagittarius A. The object sat near the border of the constellations Sagittarius and Scorpius.
Researchers determined the mass of Sagittarius A* to be approximately four million solar masses confined within a radius no larger than 17 light-hours by examining the Keplerian orbit of star S2. Later observations of star S14 showed the mass was about 4.1 million solar masses within a volume with a radius no greater than 6.25 light-hours.
The Event Horizon Telescope Collaboration released the first image of Sagittarius A* on the 12th of May 2022 based on radio interferometer data collected in 2017. This marked only the second confirmed image of any black hole following Messier 87's supermassive black hole in 2019. Eight radio observatories across six geographical sites contributed to the dataset.
Astronomers recorded star S2 traveling at 30% of the speed of light during its pericenter approach to Sagittarius A* in July 2018. The closest point occurred in May 2018 at roughly 1,400 Schwarzschild radii from the black hole. Observations confirmed gravitational redshift alongside velocity redshift within 10 percent measurement precision.
Nothing dramatic happened during or after the closest approach of the gas cloud G2 to Sagittarius A*, described by Daryl Haggard as a lack of fireworks and a flop. Observations published March 19 and 20 2014 concluded G2 remained intact despite predictions for simple gas clouds. Andrea Ghez's team later suggested G2 might actually be binary stars merged into an enormous star.