A weapon developed by the astrophysicist Dr. Alkad Mzu, the Alchemist is capable of either snuffing out a star or causing it to go supernova. This has been her quest for 30 years ; she plans to use it for revenge against the Omuta star system, whose government murdered the entire population of her native world Garissa with antimatter. Near the end of the second book from the second part of the trilogy, appropriately named The Neutronium Alchemist, Hamilton has a long scene describing the effects of the supernova, as it is used to destroy a pair of Adamist cruisers sent by the possessed. This weapon has two settings, “humane” and “violent”. In both cases, zero-tau technology is used to freeze an energy patterning node as it reaches or approaches singularity.
In the humane setting, the effect freezes the energy patterning node as it becomes a singularity, but before it can form a wormhole. This has the effect of turning the weapon into a black hole. When fired into a star, this cleanly siphons away the fusing mass, eventually absorbing the entire star. However, there is no direct risk to the occupants of the star system from radiation, and they can evacuate their planet before the atmosphere starts to condense. This was the setting Dr. Mzu intended to use on Omuta's star, condemning the Omutans to a life as homeless wanderers.
The violent setting actually freezes the energy patterning node before it becomes a singularity, producing an object of the highest possible density. This has the effect of compressing all matter it comes into contact with into neutronium, in the process releasing huge amounts of energy. When fired into a star, this extra energy leads to the star (or gas giant) to go nova.
Note: In real-world physics, the so-called "humane" setting of the Alchemist would actually be the most destructive of the two. It has been proven that almost 50% of all matter falling into a black hole (event horizon) is converted to pure energy (mostly gamma rays), as it is being accelerated to near light speed. This is believed to be the driving force of a hypernova, the most violent explosion of a supermassive star and believed to be the birthplace of a new black hole.
So, in the real world, using a black-hole weapon against a star would obliterate that star, all its planets (and sterilize some neighboring star systems as well) in an explosion that could be seen across the visible universe.