A supernova is an exploded star. This particular explosion was first observed by people on Earth on October 9, 1604. Famous astronomer Johannes Kepler began observing it on October 17. When the star exploded, it could be seen with a naked eye. At the peak of its explosion, it was brighter than any other star in the night sky, and any planet (except for Venus). At the time of this writing, it has been the last supernova to have being unquestionably observed in our galaxy.
The explosion occured 20,000 light-years away, and the gas cloud is expanding at 4,000,000 mph.
What you see in the images are a shell of iron-rich material surrounded by a shock-wave that is sweeping up space dust.
In his book about the star, Johannes Kepler, draws an analogy between the star of 1604 and the star of the Magi, and settles for 5 B.C. as the year of birth of Christ.
Sources:
http://en.wikipedia.org/wiki/SN_1604
NASA images
http://chandra.harvard.edu/photo/2004/kepler/
http://www.nysoclib.org/collections/winthrop/kepler_johannes.html