[teraflop]

The speed of light pretty much precludes anything happening to an entire star on a nanosecond timescale.

In the other direction, it's basically a tautology. A "nova" or "new star" is, by definition, a relatively fast change. Much slower changes to stars also occur, but they tend not to get news articles written about them when they happen gradually over millions of years, and we don't call them novas.

[dredmorbius]

For a main-sequence star, yes.

For a neutron star, at ~12 -- 30 km diameter, light-speed timescales are on the order of 40--100 microseconds, which would be the lower bound on whole-star cataclysmic events.

(Time dilation might extend this somewhat, perhaps by a factor of two or so.)

For a white dwarf, ~10,000 km, lightspeed events would be about 30 milliseconds (~300 times longer than on the neutron star). That's about 1/10 of an eyeblink.