Starfuel is the best chemical to perform siderignition. It is mostly unreactive outside this particular reaction. It behaves like a gas under any temperature or pressure.
Starfuel is an assemble of starlite, helium and traces of other elements as impurities; the lattice is essentially of the starlite as a frame, while many bacillar, orthogyric helium bits are laid in a tricrossed, straight spaced display. Some of the bacillar bits are naturally replaced by baccal bits of impurities, mostly aurora and azote. The starlite will typically have aurora impurities bound to it too. The purer the assemble, the more efficient the starfuel.
Helium is an extremelly unreactive substance. As such, it won't normally assemble to form starfuel. It is naturally assembled within cold asteroids through the Sideral Space. As they break, they may release dust impregnated with starfuel.
The exact process through which this substance forms is unknown, as they helium bits are naturally replaced by aurora and azote bits. It was made artificially by the Artisans, however, who knew how to create it, perhaps transmutating the aurora and azote into helium by means of philosopher's salt.