The Star Hammer a.k.a "The poor mans Solution to intergalactic warfare"
Perhaps the most deadly and cheapest device ever constructed, the "Iron hammer, is quite literally several tons of pure iron, encased in a heat resistant thermal shroud and a rudimentary sublight propulsion system and power supply.
This giant mass of metal is then directed at the sun of an enemies solar system and let loose, it's impact causing destabilization of the sun from it's core outward. Often this reaction results in a supernova, black hole, or the suns conversion into a neutron star.
No matter the outcome the civilizations surrounding the sun are either entirely destroyed or their planets rendered uninhabitable.
This works by simply speeding up the natural life and death processes of a star. Once you have an iron nuclei in the core there's a problem.
Any reactions involving iron nuclei be they fission or fusion. are endothermic, REQUIRING energy. This places the star in big trouble, as elements merge with iron nuclei taking energy from the core.
The core itself rapidly contracts, driving the temperature still higher, which actually makes the iron reactions proceed even faster similar to the rolling snowball effect. With no energy to support the outer layers, they all come crashing down (by gravity) onto the super-hot (I mean 100-900 billion degrees hot by then) core, and basically the star implodes.
All that incoming plasma onto the super hot core immediately fuses, the core rebounds, and you achieve a supernova as the implosion becomes an explosion. In short production of or introduction of iron nuclei in the core is the key to the supernova process.
There can be 3 outcomes to the core in a supernova. One, the core can be completely disrupted by the implosion, leaving nothing but hot gases and dust. (Read big badda boom!)
Or two, the core can collapse down to a neutron star, or three, in some cases, the neutron star can even be crushed down to a black hole.
Even more frightening, as little as two tons of iron is needed to start this chain reaction.
(special thanks to Tom Whiting for the quick straightforward explanation.)
History
This weapon was first invented by a Venusian space marine with a background in astronomy and stellar physics, who was seeking a way to eliminate the home planet of the Galzrith, a dangerously aggressive species that was driving the humans from their colonized worlds without remorse.
Using her knowledge of astronomy and basic physics he managed to convince a small asteroid mining company to help supply the necessary materials to fund his project of salvation.
When first displayed to skeptical fleet admirals, Captain Heather Davion convinced them to at least allow her to attempt to prove the effectiveness of her weapon by launching the prototype against the sun of the Glazrith's home system.
Although the device worked as expected, it had the unfortunate side effect of driving the surviving Glazrith fleet and out of system colonies into a suicidal rampage against the human occupied worlds.
In the aftermath of the battles and repeated deployment of Star Hammers where appropriate, several dozen otherwise habitable solar systems were utterly destroyed. The few resulting black holes created dangerous obstacles to hyperspace travel from then forth, marking the graveyards of battles to costly to fight with ships and men.
For her part, Captain Davion earned the nick name "System Killer" and retired at the end of the war to mixed fame.
The future of the human empire would record this weapon being kept in reserve as the "ultimate response" to any attempt by outlying colonies to rebel against their authority.