Kepler’s Supernova is the most recent supernova observed in the Milky Way Galaxy. Kepler’s Star, a star approximately 20,000 light years from Earth, went supernova, and the light from the explosion reached Earth in 1604, remaining visible even in broad daylight for over three weeks.
My Google Fu is failing me, though: Are there any paintings of what it actually looked like back then? I’m thinking of proposing a supernova of a distant star for the roleplaying site I participate in, and I’d like an idea of what observing one would be like. Both during the day, and at night.
If you’re asking about what kind of detail would be visible, the answer is none. It would look like a bright star. The angular diameter would be far too small to resolve with the naked eye. At a distance of 20,000 light years, it would need to be about 6 light years in diameter to reach the ~1 arcmin resolution of the human eye, and bigger than that to show any detail. Even today, after expanding for 400 years, it is only about 4 arcmin across, although it now too dim to be seen.
With the naked eye at great distance, a supernova is just going to look like a very bright star, or a non-moving planet. It may be visible by day for a few weeks during the initial outrush and radiative expansion but would dim after that. Telescopes were not widely available inn 1604– Hans Lipperhey applied for the first known patent for a telescope in 1608 (although it wasn’t granted because it was similar to other optical instruments) although other people were building and using telescopes at the same time. However, reflecting telescopes capable of collecting enough light to image an astronomical body had to wait until Newton in the 1670s.
If you were up close—within a few dozen light years—you’d see actual structure from naked eye or a small aperture telescope. You wouldn’t have to worry about the gamma ray burst unless you happened to be very close to the axis of rotation, but the X-ray and ultraviolet radiation released by the secondary nebula could be detrimental at close distance. If a supernova occurred within a few parsecs of Earth, the ultraviolet alone could significantly degrade the ozone layer.
Be aware that a lot of the images of supernovas and planetary nebulae are generally ‘false color’ images; that is, they shift radio spectrum, infrared, and ultraviolet wavelengths into the visual range to make a more vibrant image. Astronomers and astrophysicists can actually interpret these images to understand the evolution of the supernova and nebula, and it just happens that they also really look spectacular. Even a close distance, the visible wavelengths will just look like various versions of “bright” or “dim” (e.g. more ‘whiteish’ or ‘orangish’) because the human eye just can’t resolve the variational structure adequately to make color distinctions. Among astronomers, the way in which false color transformations are used to create images is an issue of violent disagreement, sometimes to the point of lightsaber duels,
In seriousness, for an roleplaying game, make it look however you want. Hewing too close to technical reality just for the sake of accuracy isn’t really serving anything, and it isn’t as if your players are going to be impressed with “a particularly bright point in the sky.”