A supernova obviously. But what made the supernova? Thermonuclear supernovae are very well studied. They have even been used to measure the expansion of the universe. But we still don’t know what these supernovae come from. We have found that the youngest supernova in the galaxy, SNR G1.9+0.3, was made by the merger of two white dwarfs. Read more at the NASA press release:

https://www.nasa.gov/mission_pages/chandra/trigger-for-milky-way-s-youngest-supernova-identified.html

Or our article:

http://adsabs.harvard.edu/abs/2016ApJ…819…37C