I'm trying to engineer a scenario where a nation wipes out another's home solar system by vaporizing it into a cloud of gas nearly instantly and it engineers a huge controversy over how it happened and would cause extreme chaos within the galaxy over the war crime. What realistic(ish) theoretical weapon could achieve this effect?
EDIT: By vaporizing a solar system I meant turn the whole thing to a cloud of gas or plasma...
A Jupiter-sized mass of antimatter
Based on some quick Googling:
Based on these numbers, if you sent a lump of antimatter with the mass of Jupiter on a collision course with the sun, it would annihilate with 3.4 x 1044 joules of energy, or more energy than three supernovae. This should be more than enough to vaporize the entire solar system, and probably a few neighboring systems as well.
Solar systems are big and hard to vaporize. Fortunately, when we're trying to do so, we can basically ignore all the planets and moons. On a relative scale, planets are easy to vaporize when compared to vaporizing the star at the center of the system.
What's difficult about this is that a star has a lot of gravity. The gravitational binding energy of the Sun is $2.276\cdot10^{41} J$. That's a lot of energy, that that's what you'd need to convert a nice tidy ball of fusion supplies and byproducts into something more resembling an expanding cloud of gas. For that, we turn to one of my favorite Wikipedia pages: Orders of Magnitude (energy)
How much energy? Canyon Runner mentions in a comment that you could accomplish it with a Gamma Ray Burst, which has $5\cdot10^{43} J$. If about 1% of the energy of the burst went into your star, that would be enough to add the kinetic energy needed to break the binding energy of the star.
How realistic? Well, let's just say that wikipedia includes the phrasing "... gamma-ray bursts (GRBs), [which are] now recognised as the most violent events in the universe." These are not small events.
Or you could just set off a supernova next to it. That outputs $10^{44} J$ (aka 1 foe). 1/10th of 1% of a supernova would be enough to do the trick. However, we do have to question whether the process of hauling a large star nearby and then somehow causing it to go nova qualifies as instantaneous.
In all senses, you will find whatever weapon that gets used will not be realistic(ish). The weapon has to emit that much energy, and its simply hard to make devices that have energies on those scales. But if you like, you can use that wikipedia page to see what sorts of events might be applicable. One of them might be "realistic(ish)" within the scope of your story.
As for the rest of the galaxy, war crimes may be difficult to level against a foe so dangerous that they wield this kind of power on a whim.
In terms of technological weapons with a sort of realistic hand wave, you could build a Dyson swarm around a star and use it to power a Nicoll-Dyson beam.
Nicoll-Dyson beam in operation
The energy of the beam would be sufficient to vapourize the planets and small bodies orbiting the star, and if focused on the star for a sufficient length of time, could provide enough energy to essentially "evaporate" the star as well. It is also possible to accelerate the rate of fusion reactions by dumping energy into the star, and essentially creating a flare star or perhaps even a false Nova, energized from without.
The downside of this is the beam propagates at the speed of light. A Nicoll-Dyson beam launched from Tabby's star today would vapourize the Earth in @1700 years from now. The energy could also be used indirectly to propel a swarm of RKKV's at the Solar system, with each one capable of adjusting its flight path to impact a target, or if no target was available to strike the star.
From a political POV, it s difficult to imagine what sorts of conflicts would last literally tens of thousands of years (know where the enemy is, locate the target, fire the beam, wait for the effects to be known i.e. another 1700 years to see the results on the Solar System, then relay, re-aim and fire again.....). And of course, since space is 3 dimensional, you are shooting at one planet while their allies off the plane of the ecliptic are shooting at you from a totally different direction...
In Children of The Lens by E.E. Smith (1950), The Galactic Patrol fired a planet from another universe at the target star. In the other universe, the vector of the planet was greater than our speed of light.
This sheerly fabulous handwave allows the attack to contain as much energy as you wish, for it to be a complete surprise, and for the speed of light in our universe to be effectively irrelevant for the attack.
Here's what happened:
"What happened? Even after the fact none of the observers knew; nor did any except the L3's ever find out. The fuses of all the recorder and analyzer circuits blew at once. Needles jumped instantly to maximum and wrapped themselves around their stops. Charts and ultra-photographic films showed only straight or curved lines running from the origin to and through the limits in zero time."
"Ploor's sun became a super-nova. How deeply the intruding thing penetrated, how much of the sun's mass exploded, never was and perhaps never will be determined. The violence of the explosion was such, however, that Klovian astronomers reported--a few years later--that it was radiating energy at the rate of some five hundred and fifty million suns."
Of course, this attack was also the penultimate battle of the war - no defense against such an attack was conceivable, so the Galactic Patrol had to finish the war within days after first use - before their enemy discerned the principles used, engineered the mechanisms, and began using this game-changer themselves.
You will need a portal gun:
... Or any other thing capable of building stable wormholes on demand.
One mouth ot the hole goes on a planet in the solar system you wish to cook. The other gets sent towards TON 618.
TON 618 is a black hole with a mass estimated at 66 billion times that of the sun. At that tonnage, the notion of margin of error becomes ridiculous. Anyway, one of Tony's most striking features is its accretion disc. Its temperature is in the order of dozens of millions of K. Yes, millions.
Compare with the Sun's surface temperature of ~5,770 K. To give you an idea of how hot that is... I'll just quote the wiki:
The surrounding galaxy is not visible from Earth, because the quasar itself outshines it. With an absolute magnitude of −30.7, it shines with a luminosity of 4×1040 watts, or as brilliantly as 140 trillion Suns, making it one of the brightest objects in the Universe.
So there you have it. Once the mouths of the portals/wormhole connect the target solar system to TON 618's accretion disc, you get an expletive amount of energy coming towards the target, as well as enough gravity to shred the star into pieces.
A controlled False Vacuum over the area
This is not supposed to be possible, but let's admit that your engineers invented a weapon capable of producing a false vacuum, and control its expansion. Basically, a way to erase from existence a part of the universe cleanly and in the most horrible way possible.
This would cause controversy.
Why ? For the same reason that, in today's world, people fear that an uncontrolled black hole would suddenly pop from a scientific experience and delete Earth. Imagine that your mass weapon of destruction fails, and the false vacuum continues his way through the designated area. That would be really bad for the WHOLE universe.
But also for another reason : because the area where the weapon has have been used is empty afterward. Which means it has some impact on the other galaxies around ! Imagine all the orbits changing, or the trajectory of comets taking different destination. The calculations of all your galactic empire become obsolete, and they have to be done again.
Here is a cool video that explains it further: https://www.youtube.com/watch?v=ijFm6DxNVyI
Your rogue space nation might be wise to consider nano-disassemblers. These could be manufactured relatively cheaply and could be smuggled (or launched) into the target system relatively easily.
Though the effect wouldn't be instant, it would accelerate as the nano-disassemblers produced more of themselves from the matter present in the star system. Though the entire system wouldn't be reduced to gas, the targeted planets and other objects would be reduced to dust.
The nano-disassemblers could be programmed to shut off after a calculated time, or perhaps even to dive toward the sun to clean up the evidence. Or if the nano-disassemblers don't shut off, there's a horrible booby trap waiting for anyone foolish enough to investigate.
You need a way to make the star go novae.
A wormhole adding several solar masses to de star or something converting part of the core to antimatter. The resulting blast should vaporise the star system in less than a day.
As an addendum to the other answers...
nearly instantly
That could be a problem. Using our solar system as an example, Neptune orbits at about 250 light minutes, so you have 3 options:
For option 1 my vote's for lobbing some antimatter or a singularity in to the star, depending on how much destruction you need.
For option 2 my vote's for a nice Von Neumann grey goo, the nano disassembler approach, or some repurposed mining equipment.
For option 3, just grab a nice wormhole generator and see how many event horizons you can pack in to the system, and connect the other ends to somewhere fun like a black hole, some kind of powerful energetic source like an engineered gamma burst or a high speed quasar. A nice combination of gravity and destructive radiation. Even if you don't vaporise it completely, you can probably sterilise the system and detonate the star. Certainly the result of something like that would cause you plenty of chaos even if the destructive amount is a little less.
It won't turn the star itself entirely into a cloud of gas or plasma — there will be a strange star remaining at the center of the ensuing explosion — but the energy released should do the trick for the rest of the solar system. I have no way of actually calculating the energy released, but it should easily be enough to vaporize the entire solar system. Given that any potential drone ship used to deliver the bomb would be obliterated along with the rest of the solar system, the identity of the perpetrator nation would be difficult to determine.
The effects of a strangelet bomb will propagate at the speed of light outward from the central star — the solar system is vaporized not by the bomb itself, but by the extremely intense and energetic radiation emitted, which travels at the speed of light. Short of simultaneously bombing different parts of the solar system, this is as close to "nearly instantly" as it is possible to achieve.
Of course, the compounding issue for anything that vaporizes a solar system is what will happen to surrounding systems in a few years when the radiation released by the event arrives. If the event is too energetic, it will cause disruption and damage to unintended solar systems nearby, potentially resembling the effects of a nearby supernova. This may or may not be advisable for the perpetrator nation, depending on whether they or their allies occupy any nearby systems.
Do you know the difference between matter and antimatter? No. Well, now that you got ultra violet secret 3 security clearance... it is simply the direction of the 4th level quantum spin. Energy content is the same.
This is very relevant because - you know the work we did with artificial wormholes and hyperjumps? The dead ends. Happens if you create a hyperfield with a very small calibration error, it can invert the 4th level quantum spin on all matter within it. Takes little energy and depending how you set it up, it converts up to nearly 199% of the matter in it's range into antimatter.
And yes, this can be weaponized. The warhead is actually quite small - you dont really need to get into hyperjump level energy levels. And you just need to keep the field up for a nanosecond. The biggest joke is that if you want to build a star buster, the shielding to get the warhead into the core of the sun is like 100% as heavy as the warhead. How you think we got rid of the rebels in UAX-249? That sudden sun eruption was... not exactly unexpected given that we are now short one sunbuster projectile.
Get the idea?
Matter and Antimatter have the same energy level, so come up with some technobabble and voila, free matter to antimatter conversion. Possibly in a large scale. Which makes this anything, from a posibly solar busting projectile to actually free energy - the old problem that antimatter is NOT an energy source (because you need to generate it first,something star trek never cleared up). I used that many many years ago in a SF role playing campaign. Very slow FTL (unless you used up ridiculous amounts of fuel, very ineconomic, or antimatter, very expensive), but there was this military secret that... if you just manipulated a hyperdrive a little, you could acutally make free antimatter. Ultimate WMD. Use a lot more energy and a lot more complex system and you got a 99.999% conversion in a way that you could actually syphon off the antimatter - for use in military starships. How did noone realize it? Well, they imported it from a black ops company they ran (which supposedly had those huge close to the star antimatter generation plants) - and used all the budget saved for not exactly official purposes ;)
If somebody repurposed an Alcubierre drive to send a black hole into a solar system, that would effectively accomplish the same. Plus, if you make alcubierre drives ubiquitous, this possibility is demonstrated to the whole galaxy's population, who could readily do the same, which makes controversy much bigger than the fate of just one system.
As far as vaporization goes, you could use the same mechanism to move the black hole away later, leaving bare space in its wake. If you did it fast enough, it would look like one huge projectile passed through and vaporized it, achieving (at least in appearance) the same effect as having a single weapon vaporize it all.
And if you're looking to leave no trace, you have to put everything behind an event horizon somehow, due to the conservations of mass and energy. Other than sending it faster-than-light beyond the observable universe, a black hole is the only way to achieve true tracelessness.
Your scientists and engineers have developed a material that can (temporarily; it doesn't need to be that long) withstand the temperatures in the star.
Make a (or more) large missile out of it, with a magnetic containment field full of antimatter. Once the outer shell melts and the anti-matter is exposed... BLAMMO!!
Kinectic projectile
There's literally not a limit to the amount of energy you can put into one. It just asymptotically approaches the speed of light as you add more energy. Just keep going until it's enough to destroy the star. The star shrapnel will take care of everything else.
Be warned that, at near the speed of light, collisions with tiny objects can seriously dent or destroy your projectile. Even individual atoms in the way may be a problem. Better to send a handful of projectiles to be sure.
Time dilation device.
I lifted this scifi concept from Stargate. It works for your purposes and offers narrative possibilities too.
http://stargate.wikia.com/wiki/Time_dilation_device
The Time dilation device was a device of Asgard design, conceived to artificially change the normal passage of time... Within the bubble, time could be slowed down by a factor of ten to the fourth power. This means that one year within the time dilation field would be 10,000 years for everyone else ... During the Asgard-Replicator war, the Asgard summoned the Replicators to the planet Hala and trapped them in a time dilation field... However, the Replicators managed to stop the time-dilation device and used it to increase time for themselves, thriving and replicating many thousands of times.
Time moved faster inside the bubble once the Replicators reverse engineered it. In your scenario this is what is used against the solar system. Time moves much faster in the bubble. When the bubble turns off, quintillions of years have passed inside. All that remains is a gas and loose elementary particles - a solar system dying the heat death at the end of time. The detectives can figure out what happened by first realizing that not only has there not been an explosion, absolutely nothing is radioactive in this system any more. They then find evidence of proton decay - something which should not happen in our universe for a long time yet. They realize that all the matter in the system is extremely old.
The perpetrators of the event counter that no crime has been committed - from the standpoint within the time dilation field, the worlds went on as normal, and all inhabitants, and their descendants, and their civilizations, and the stars their plants orbited lived out their natural lives.
The Outer Limits twist: within the time dilation field, some of the inhabitants figured out what was going on. They could not turn off the time dilation field they were in but they could duplicate a smaller one and esconce themselves inside, slowing the passage of time within to nothing. When the big fast field turns off, the small inner field (later in the story) turns off too. They emerge. They have had hundreds of thousands of their years to improve their technology before turning on their inner field. These beings are very different from who lived there before.
One possible method would be artificial wormholes. It wasn't specified how much of the target solar system had to be obliterated. Was it just the area containing the planets, or all the way out to the outer comets halfway to the closest stars?
If you could generate an artificial wormhole with a mouth a few billion miles wide just ahead of the solar system as it orbits the galactic center, the solar system will enter the mouth of the wormhole and exit from the other mouth of the wormhole far, far away and unable to bother you.
Unfortunately, since the orbital speed of the solar system will be just a few hundred kilometers per Earth second and there are only about 31,000,000 seconds in an Earth year, it might take several years for the target solar system to completely fall into the wormhole mouth.
So you may have to give the mouth of the wormhole a very fast velocity toward the target solar system in order to envelope it in weeks or days.
Or you will have to create many, possibly hundreds, of different artificial wormholes at once and position their mouths much closer to the orbiting planets, moons, dwarf planets, etc. to make them enter the wormhole mouths in minutes, hours, or days.
Another technique would be to create wormhole mouths that are only ten, twenty, or thirty percent as wide as the planets and moons they are formed in front of. So the wormhole mouths will transport cylinders of matter from the onrushing planets to distant regions of space, leaving the spherical planets and moons with missing cylinders through their centers if the wormhole mouths are positioned correctly.
Spherical planets and moons are spherical because their gravity is stronger than the strength of their materials and pulls them into a spherical shape. So if the centers of the planets and moons are removed, the matter in their outer layers will no longer be supported and will fall inward toward the mathematical centers of the planets and moons. And when the tremendous masses of matter falling at great speeds collide with each other in the central regions of the hollowed out bodies, the collisions should be many orders of magnitude greater than the greatest asteroid impacts in Earth's history.
I guess that most of those astronomical bodies will be totally turned into plasma by the energy released by those collapses, plasma that might be moving fast enough to escape from the gravity of the former bodies.
Type II supernovas are caused by core collapses in massive stars. So forming a wormhole mouth in front of a moving ordinary star that transports a cylinder shaped section out of the star should cause the star to collapse. That should result in a much, much smaller version of type II supernova.
But of course even a tiny insignificant fraction of a type II supernova should be a very devastating event that further ensures the vaporization of all the remaining matter in that solar system and will probably heat up all that plasma to escape velocity.
Then, of course, shut off all of the artificial wormholes so no evidence remains.
You could destroy all life within a system if you could create a gamma ray burst of sufficient intensity. Normally gamma ray bursts occur when stars collapse to form neutron stars or black holes - for example the infall of material into a black hole drives a pair of relativistic jets out along the rotational axis. This can occur when two stars spiral into each other to become an object with too much mass to remain a normal star, and must collapse to degenerate matter or a singularity. Gamma ray bursts are really one of the most deadly phenomena in the universe, and even though one would not notice much change in the appearance of the earth, it would destroy all life and possibly strip off a lot of the atmosphere.
Gamma ray bursts are kind of the galactic scale EMP weapons. Gamma ray bursts also can be beamed long distances in one specific direction in a narrow jet. You can imagine purposely using this to focus on a particular star system. However energetically it takes major cataclysms to create them, e.g. formation of black holes.
Vibrational energy, and a lot of it!
If we're talking about a solar system roughly the size of our own, with the intended effect being effectively crumbling it into gas and dust, the best I can think of would be a local and extraordinarily strong tidal pull from a passing body. This would have to be extremely massive, like a pair of orbiting neutron stars or black holes. The vibration felt by the entire system as they moved in and out of phase could easily crumble any solid matter within it, particularly if the orbit was at an extreme speed.
One such system, PSR J1311-3430 (a millisecond pulsar), completes a full orbit every 93 minutes. When both stars are in alignment with the system, it would feel the pull of all present mass at once; whereas when 90° out of alignment, a decent portion of the pull from each star would be canceled by the other.
Of course, actually getting your hands on the requisite mass and transporting a couple of neutron stars is no small feat; but it should be doable with some canon scifi technology. Wormholes, mass effects, and such; maybe even some kind of 100 AU wide pulsating artificial gravity beam. In any case, after only a couple of hours (maybe substantially less), nothing living would be left in that system, and as a warcrime-worthy bonus it would likely be an extremely painful death; after a day it would be complete chaos and gaseous debris.
Stellar muon bomb.
Muon catalyzed fusion allows meaningful rates of fusion of deuterium-tritium at room temperature (and lower). Stars are hot enough and dense enough to fuse "normal" hydrogen. (Citation needed?) At room temperature, muons increase the rate of proton-deuteron fusion about 38 orders of magnitude.
One might be concerned that there is very little molecular hydrogen in stars. This is both true and false. In Sol, the largest fraction of (fluorescent) molecular hydrogen occurs in the chromosphere, around 150 km above the top of the photosphere. The number density exceeds $10^{13} \,\mathrm{cm}^{-3}$ in a 200 km thick layer over the entire surface of the Sol. (ibid., p. 4. See also the pretty pictures in fig. 2 of an associated paper showing the complicated distribution of molecular hydrogen in the chromosphere.) The total number of $H_2$ molecules known to be "on the surface of the sun" is $$ 6 \times 10^{12} \,\mathrm{km}^2 \cdot 200 \,\mathrm{km} \cdot 10^{15} \frac{\mathrm{cm}^3}{\mathrm{km}^3} \cdot 10^{13} \,\mathrm{cm}^{-3} \approx 10^{42} \text{.} $$ (There are about $10^{63}$ atoms in the sun, so this number isn't wildly too big, a good thing to check for such unfamiliarly big numbers. The "real" number of hydrogen molecules could easily be 1000-times larger.)
These hydrogen molecules have roughly the mass of Mars's moon Phobos. So even if you were to fuse all of it at once, although the CME would be quite spectacular, the star would hardly notice.
Something that is not known is what muons would do to the metallic hydrogen phase at the core of stars. It is plausible that muons would cause the lattice spacing of the molecules to decrease substantially, catalyzing fusion in a manner similar to that at room temperature but, instead of a single muon catalyzing fusion of the two members of a single molecule, a single muon would accelerate fusion over an entire delocalized region of the metal. That is, a sufficiently intense beam of near-light-speed (muon half-life is 2.2 microseconds in their comoving frame) muons could cause the portion of the core they reach to engage in fusion about $10^{30}$-times faster (or more, possibly much more than the $10^{38}$-times for room temperature fusion).
The mass of the core is about one-third the mass of Sol, roughly $10^{63}$ hydrogen masses. If you could saturate the entire core, you could induce the Sun to perform all the of the rest of its lifetime hydrogen burning in a few femtoseconds. Typical numbers for the power of the sun are around $10^{26} \,\mathrm{W}$. Increasing by $30$ orders of magnitude for $1 \,\mathrm{fs}$ gives $10^{41} \,\mathrm{J}$, the gravitational binding energy of the sun. So, this process is releasing about the right amount of energy. (I'm not convinced one needs to actually impart escape velocity to all particles. Even one-tenth on average would produce a nebula that would persist for a very long time on human scales.) This would also dump about $10^{15} \mathrm{J}/\mathrm{m}^2$ of energy on the Earth, about the energy of $1 \,\mathrm{Mt}$ of TNT per square meter. The presented cross-section of the Earth is about $10^{13} \,\mathrm{m}^2$, so the initial flash would only carry about $0.1\%$ of the gravitational binding energy of Earth. (However, the rather large uptick in solar wind will ablate the remainder rather quickly.) To be clear, this process is roughly the least energetic process to achieve your stated goal. Only exciting a few percent of the core or only exciting a beam or narrow cone through the star will not produce the desired effect.
Each muon has rest mass of $100 \,\mathrm{MeV}/c^2$ and it is likely we need more than $10^{50}$ of them if they are imparted with zero velocity relative to the star. (At room temperature, each can participate in 100-1000 fusions before being captured by an alpha -- at core densities, this number should be higher and delocalization should make it higher still. Until someone builds a high intensity muon gun and fires it at a working fusion reactor, we don't any of these numbers.) However, to excite the entire core at once, we postulated relativistic muons. Determining the effect of the heating coming from stopping all those muons exceeds my stamina for this problem, but suffice to say that each muon dumps a few $100 \,\mathrm{MeV}/c$ of momentum when it is captured, which is a few thousand times the activation energy for fusion in the core. So it would be reasonable to describe this as a heat gun which instantly completes stellar fusion. Equivalently, we could reduce the flux of muons by up to 3 orders of magnitude and still get the desired effect. This still necessitates converting about $10^{-10}$ Sol masses (about $10^{20} \,\mathrm{kg}$) to energy 100% efficiently. This is roughly the mass of a rock having a $300 \,\mathrm{km}$ diameter (roughly the mass of $200$ Death Stars). What is not known and could make this process much easier: muons are easily produced by the decay of charged pions, pions are easily produced by hadron-hadron collisions (i.e., fusion), the rate of pion production depends on the energy of the fusion reactions, which we can now control by the intensity of our relativistic muons. So, can we arrange for our muon catalyzed stellar fusion to produce copious quantities of muons? If so, we could use a much smaller rock and/or a more realistic muon production efficiency.
The Lost Fleet series deals with this somewhat, (spoilers):
A hyper gate collapse could theoretically result in a super-nova like energy release and annihilate the system. Hyper gates have an extreme amount of energy held in tension, and a chaotic release would be devastating.
Everyone here is answering your question quite litteraly offering you weapon concepts, which is great.
Now if what you want is the destruction (regardless pieces left floating in space) you could take this problem in a different and more simple way.
If you know a little about Newton's laws one of them could help you : Newton's law of universal gravitation.
This law states that a particle attracts every other particle in the universe with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
Now to speak in simple terms, if earth (for exemple) stopped rotating around the sun, it would (fast enough for us to be unable to notice), crush into the it because of the attraction between them. That's where I wanted to drag your attention. If you could stop (or at least highly slow down) planets rotation around the sun they would crush into the it and get destroyed.
This article explains stuff about the orbiting of the earth around the sun and the consequences if it stopped.
A few other answers have suggested dropping a black hole into the Sun. This would trivially certainly destroy the Sun by consuming it, but there's another, much more spectacular, mechanism at play. And that is accretion.
When matter falls into a black hole, it won't fall straight in unless aimed incredibly precisely. Instead, an object dropped into a black hole will more often be shredded by the black hole's tidal forces, then form into a disk of gas orbiting the black hole. The gas's angular momentum keeps it from falling inward right away, but it can bleed off that angular momentum by knocking into particles orbiting just a bit farther outward, transferring momentum and letting the inner edge of the disk creep ever closer to the event horizon. This whole turbulent momentum-exchange process produces heat- a lot of heat. There's a lot of potential energy stored in a mass suspended above a black hole, after all, and when the mass falls in, that energy has to go somewhere. In fact, an accretion disk can covert around 10% to over 40% of the mass of the infalling matter into energy that can radiate away from the hole- far more than the 0.7% attainable by nuclear fusion.
Imagine that- a weapon capable of converting 10% of the sun's mass into energy. If Jupiter's mass-energy is comparable to a supernova, then 10% of the sun's... is just insane.
Now, there are a few caveats here. For one thing, it's not instantaneous. Black holes can only consume matter at a certain rate. The accretion disk emits radiation, and that radiation exerts a force on any matter that it hits, pushing it away. When that force equals the force of gravity pulling matter inward, they balance out- and the hole can't eat any faster. Some matter will continue to slip in, as that's powers the radiation pressure; but the rest of the star will be kept out. This is called the Eddington limit. It increases linearly with the mass of the black hole, which means that bigger black holes can eat faster. And since black holes grow by eating, a black hole dropped into a star will grow exponentially until there's nothing left for it to consume.
So you could kick-start the process by using a large black hole, or use a small black hole and let it act as a sort of apocalyptic time bomb. Over time, the star would grow brighter, larger, and redder as the black hole outstrips the thermonuclear reaction in its core (which will happen around when the hole has absorbed about 0.1% of the star's mass), until the rate at which the hole's consuming matter exceeds the rate at which that matter can heat up and expand, whereupon the star will become smaller, hotter, and ever more luminous until there's nothing left to shield the rest of the solar system from being scoured clean by the incredibly intense radiation from the accretion disk.
And I have absolutely no idea over what sort of time scale this would take place. I've made a couple of attempts at the math, but I'm fairly sure I messed it up somewhere.
Another caveat is that much of that radiation may be directed directly out of the plane of the solar system. We might wind up essentially creating a gamma-ray burst. Sometimes, when massive stars collapse into black holes, much of the energy of the supernova is directed into two narrow beams of gamma rays perpendicular to the accretion disk. The same thing might happen when a black hole fully consumes a smaller star, depending on how quickly the star is spinning.
The third caveat is that this won't fully vaporize the solar system- there will for sure be a stellar-mass black hole left behind, and perhaps also some planets. I have no idea whether or not they could survive this kind of event, although their biospheres almost certainly wouldn't. And if the biospheres somehow did survive the blast, there would be no sun afterwards, and the planets would freeze.
I'm going to go for something that's real and might be possible with current technology. You put a space telescope in orbit around the solar system you want to destroy. The space telescope looks closely at all the stars around. When you see the light of some stars shift because a primordial black hole is between you and the that star, you can figure out the distance and location of that primordial black hole. Then, you fly your space telescope close to the black hole. When the space telescope starts to be sucked in by the black hole, its engines turn on and pull away from it. The black hole is slowly being pulled in the direction of the space telescope. Over time, you can pull the black hole in a trajectory that causes it to eventually hit the sun of the solar system you want to destroy. Once the black hole is attracted by the sun's gravity, it should start to move toward the sun faster and faster. It might take a few weeks, but it's a sneak attack that nobody expects. Although this is a bit of a stretch as far as really being possible, it makes for good science fiction.
I assume you have FTL. Create a microscopic black hole on a ship that is in hyperspace right before it drops out of it, right in the middle of the star. Due to radiation pressure and extreme temperatures it generates, it might be enough to send star's matter flying into the planets, causing them to turn into molten pieces of rocks.
Use iron to make the star die. Iron is unique--when it's created, it begins the countdown to the star's death. With hydrogen, helium, lithium, etc., the star can still use nuclear fusion to create higher and higher numbers of elements. Iron, however, absorbs the energy of the impact. But it won't fuse into heavier elements, so it decreases the fusion of the star. A star's life is marked by a balance between gravity and fusion. If fusion decreases, gravity wins, the star collapses and then . . .
A: Goes super/hypernova. B: Blows off its outer layer and fizzles. C: Goes supernova and then collapses into a black hole (note: only for massive stars.) You'd need a lot of iron to do it that quickly, but a planet- or two's worth would probably work.
A kuelblitz is a black hole made by focusing light on a specific location in space. When enough light is focused on a specific point, it collapses into a black hole proportionate in size to the amount of energy used to create it.
Several incomprehensibly powerful lasers could be directed at the different astral bodies in the solar system designated for destruction. When the lasers converge on the planets and so on that are being targeted, a black hole will form and consume the body.
This could be done for every body, turning the solar system into a black hole system.
