Four hundred years ago Johannes Kepler (the same Kepler who gave us Kepler’s laws) saw a new bright star in the sky: a supernova. Obviously, Kepler couldn’t take a photo of it, but he did take detailed notes. Those notes have allowed us to determine where in the sky we should look to see the remains of “his” supernova. Interestingly, this remnant has been puzzling current astronomers as much as it puzzled Kepler – even with all our new instruments.
The puzzle is this: what kind of supernova was it? Supernovae come in two basic flavors: Type Ia and Type II, and scientists can’t tell which Kepler’s was.
Type Ia:
Type Ia supernovae occur when you have two stars orbiting in a binary system: a white dwarf star and a larger companion star. If they’re close enough together, gasses fall from the bigger star onto the white dwarf, and if this happens too fast, the star explodes
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http://imagine.gsfc.nasa.gov/docs/introduction/supernovae.html