If a star is a couple of light years from a planet and not in the solar system it can affect a planet. For example if one of the stars in the Centuri system went nova(which isn't likely) it would affect earth, It wouldn't destroy the earth or anything but it would cause a power outage Supposedly the star on Romulus was probably a blue star(even though it was yellow on screen) that's really the only way it could possibly destroy Romulus if it's close enough. None of it seems likely.
You are quite incorrect. If any star (capable of going nova) within a thousand lightyears of earth went supernova, or even nova, all life on this planet would be annihilated. When a star goes supernova, it burns more fuel in those few days than it does in all the preceding millions of years (stars that are capable of supernova, generally are super huge and burn for only a few million years). Indeed, when a star supernovas, it burns brighter than all other stars in the galaxy combined. The energy released is massive beyond any comparable maxim. In fact, a thousand LY might be cutting it close. Five thousand (or more) LY is really the safest distance. Understanding that the UFP at this time couldn't be more than a few hundred LY across, and that Romulus couldn't be more than five hundred LY from Earth, this nova actually stands to destroy all life in the UFP, Klingon and Romulan Empires (though, with hundreds of years of escape time for Earth and Qo'nos).
If it wasn't the Romulan star system and a neighboring one--say at between 1 and five LY--Romulus would have anywhere from a year (slightly more, in fact) to five years to evacuate to safe ground.
Figure Romulus's population of 6 billion. Figure every transport and shuttle within the empire, as well as a number from the UFP, and I'd say that the Romulan people stand a pretty good chance of avoiding any casualties, if any from the supernova.
This was, indeed, the biggest plot hole in the movie, especially, since the Romulans would certainly posess the technology to detect any star approaching a nova stage several hundred years off (we have this technology now).
~String