Short answers, pulled from the story.
Sagittarius A* is the supermassive black hole at the center of the Milky Way galaxy. Viewed from Earth it lies near the border of the constellations Sagittarius and Scorpius, about 5.6 degrees south of the ecliptic, at a distance of roughly 26,000 light-years.
The current best estimate of the mass of Sagittarius A* is approximately 4.297 million solar masses. Two independent research groups measuring stellar orbits arrived at figures of 4.31 million solar masses (the German group led by Reinhard Genzel) and 4.1 million solar masses (the American group).
The first image of Sagittarius A* was released on the 12th of May, 2022, by the Event Horizon Telescope Collaboration. The underlying radio-interferometer data were collected in 2017 by eight observatories at six geographical sites, and the image required five years of calculations to process.
Sagittarius A* was discovered on the 13th and the 15th of February, 1974, by Bruce Balick and Robert L. Brown using the baseline interferometer of the National Radio Astronomy Observatory. Brown gave it the asterisk name in a 1982 paper because the source was "exciting", and excited atomic states in chemistry are denoted with asterisks.
Reinhard Genzel and Andrea Ghez each received a quarter share of 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. Sir Roger Penrose received the other half for proving that black hole formation is a robust prediction of general relativity.
Contrary to predictions of a dramatic brightening, nothing notable was observed during or after the closest approach of G2 to Sagittarius A* in early 2014. Observations by the UCLA Galactic Center Group in March 2014 found the cloud still intact, and later analysis suggested G2 may be a dense clump within a continuous gas stream, or possibly a merged pair of binary stars.