These are the remnants of the last supernova known to explode in our galaxy whose light has reached Earth. It lit up the night sky in 1604 and attracted the attention of Johannes Kepler himself. But it's only in the last couple of decades that we've developed the technology to understand exactly what kind of supernova this is.
Indeed, Kepler and his contemporaries didn't even have the benefit of telescopes when studying this supernova, let alone giant orbiting space telescopes that can see across all parts of the electromagnetic spectrum. Modern astronomers have all those resources, and so they have been able to determine just what sort of stellar explosion created the last Milky Way supernova visible to human eyes, as NASA explains:
The supernova produced a bright new star in early 17th century skies within the constellation Ophiuchus. It was studied by astronomer Johannes Kepler and his contemporaries, without the benefit of a telescope, as they searched for an explanation of the heavenly apparition. Armed with a modern understanding of stellar evolution, early 21st century astronomers continue to explore the expanding debris cloud, but can now use orbiting space telescopes to survey Kepler's supernova remnant (SNR) across the spectrum.
Recent X-ray data and images of Kepler's supernova remnant taken by the orbiting Chandra X-ray Observatory has shown relative elemental abundances typical of a Type Ia supernova, and further indicated that the progenitor was a white dwarf star that exploded when it accreted too much material from a companion Red Giant star and went over Chandrasekhar's limit. About 13,000 light years away, Kepler's supernova represents the most recent stellar explosion seen to occur within our Milky Way galaxy.
For more on this and other amazing images, check out NASA's Astronomy Photo of the Day.
Image Credit: X-ray: NASA/CXC/NCSU/M. Burkey et al. Optical: DSS