In the 'False Sun' appendices (or notes at the bottom, whatever), the Nail is given a Cunuroi name which means the 'Newborn' (or something like that). That would suggest the star didn't exist before a specific time, but did after. So, it wasn't a star that went nova (a dim star to a bright star), but rather a 'star' that didn't exist which suddenly did. Of course, the star could have been too far from Earwa to have been visible before it went nova, thus appearing to have... appeared (ahem) out of nowhere. I've always found it strange that this 'Newborn' star is the pivot around which all the other stars wheel. Again with the astrophysics...
EDIT: Also, according to a google search (take that for what it's worth), at best a supernova would be visible for a few months, gradually losing the intensity of its brightness over time. So probably not a supernova, since the nonmen observed and named it thousands of years in the past.