According to news from NASA's Chandra X-Ray Observatory:
Previously, the last known supernova in our galaxy occurred around 1680, an estimate based on the expansion of remnant Cassiopeia A. ...
The tracking of this object began in 1985, when astronomers ... used the [National Radio Astronomy Observatory's] Very Large Array to identify the remnant of a supernova explosion near the center of our galaxy. Based on its small size, it was thought to have resulted from a supernova that exploded about 400 to 1000 years ago.
Twenty-two years later, Chandra observations revealed the remnant had expanded by a surprisingly large amount, about 16 percent, since 1985. This indicates the supernova remnant is much younger than previously thought.
That young age was confirmed in recent weeks when the Very Large Array made new radio observations. This comparison of data pinpoints the age of the remnant at 140 years - possibly less if it has been slowing down - making it the youngest on record in the Milky Way.