The movie itself does that. Spock's account indicates that there was time between the supernova and Romulus's destruction for him to promise the Romulans help, arrange for the red matter, and launch the Jellyfish. That must've been a matter of days at least. The only way the supernova star could be within the same system is if it was a wide binary system with Romulus and Remus around the smaller, main-sequence member of the pair. It can't have been Romulus's own primary star, or the planet would've been destroyed within minutes.
I thought this was one of the worst parts of the film. I think I have a fair understanding of what science tells us about the life or death of stars. I thought the 09 Star Trek film version of the threat to Romulus (and beyond) was catoonish at best. The only way I have been able to rationalize it is that there is a MASSIVE amount of untold story that would need to be fleshed out in a few novels or comics stories. (Whatever the future license agreements are).
I would post a couple of my crazy rationalizing of those events (I do have a couple of ideas that I think work better than what we saw on film) but I know there are rules about not posting story ideas for writers....
However, the overall point you are making TOTALLY aligns with my reaction to that portion of the film. I remember sitting in the movie theater and thinking. "What?" So I haven't read any of the IDW stuff, I would love for them to at least touch on Red Matter, the star explosion, etc. I look forward to reading that continuity and a time when some of the writers who hang out here can give us background on what was scene on screen.