Also, the TSFS novelization mentions that the Genesis device did, in fact, create a star out of the remnants of the Mutara Nebula, in addition to the planet -- indeed, it's mentioned that this protocol was what largely contributed to the Genesis planet's eventual disintegration.
Basically, according to the novel, there were two different pre-programmed protocols in the device -- the first, used when the detonation-site occurs close enough to a pre-existing star (i.e., no star needed), with substantially more resources directed into the formation of the actual planet itself.
The second protocol was the one seen in the film, with the Reliant detonating too far away from a sun for a potential Genesis planet to be habitable; therefore, the parallel star-formation matrix was also activated by the Genesis device at detonation, repurposing nearby matter formerly belonging to the Mutara Nebula (and the former starship and its crew).
However, in the novel Saavik theorizes that because this second protocol diverted so many resources into stellar-accretion, the Genesis planet itself got shortchanged slightly, contributing to its instability and abruptly-shortened lifespan. She says something to the effect that, had the Reliant exploded close enough to a pre-existing star, the Genesis planet likely never would have disintegrated.
