I don't believe Nero had the red matter until he captured Spock, and he only got Spock recently before he imploded Vulcan into a black hole, if he imploded a star, that would have attracted the attention of Star Fleet and would have ruined his surprise when he attacked Vulcan.
Actually, we have the timeline of these things down pat, thanks to meticulous Starfleet bookkeeping! Kirk and Chekov give us the exact hours at which various things took place.
Chekov's PA: "At twenty-two hundred hours, telemetry detected at an anomaly in the neutral zone. What appeared to be a lightning storm in space." (That's Spock arriving.)
Kirk's attempt to convince Pike: "at twenty-three hundred hours last night, there was an attack. Forty-seven Klingon warbirds destroyed by a Romulan, Sir." (That's Nero having had red matter for one hour. Also note "last night", meaning Vulcan is dying some time after San Francisco dawn, just as the shuttle scenes suggested, but before the
next San Francisco dawn.)
And no, imploding stars generally do
not attract the attention of Starfleet. Strangely but consistently enough, a starship has to visit a star system to see even the most massive changes in real time, as in "Doomsday Machine" or ST2. It would take some years for the lightspeed evidence of the disappearance of a star in Romulan or Klingon border regions to reach the Federation. And apparently even the faster-than-light telescopes of the UFP aren't constantly looking out for things like this with military alertness; any warning would have to diffuse down to Starfleet through cohort upon cohort of lab-coat nerds, easily giving Nero the less than full day he needed to act.
Of course, Starfleet might have found out about the star - and immediately slapped "Top Secret" on the knowledge, like they did on the information about the loss of Klingon warships. It's quite unlikely they could see a connection between the astronomical phenomenon and an impending attack on Vulcan. An incorrect reaction would be more likely than a correct one in any case. Something did prompt them to send their warships to Laurentius...
This star would have had to have been fairly close to Romulus to destroy the planet when it exploded
Indeed, the only realistic alternative here is that it was the very star Romulus orbited. Otherwise, Spock should have had enough lead time to intercept the wave of destruction as it moved between star systems. Even if it was a FTL wave, Spock was supposedly in a fast FTL ship...
But I guess we could make do with a star exploding in the next system over. We know Klingons and Romulans constantly contest the ownership of border systems (such as Khitomer), and Romulus itself appears to lie very close to the border, judging by travel times in several episodes. A neighboring system thus could well be infested with Klingons like specified in the
Kelvin walla for 2233, or in the message from 2258...
Timo Saloniemi