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Lets assume we are dealing with a futuristic setting set several thousand years in the future with major galactic empires controlling thousands of systems and producing all sort of things that we can only dream of as easily as we can produce nails. This includes all sorts of Dyson spheres and super advanced FTL space ships.

This of course includes manipulating metals at a level far surpassing our own. Or maybe discovering all sorts of crazy elements or exotic materials and stuff of dreams, or nightmares...

Let's assume that an ambitious engineer or someone who owns a sun is bored and asks his people if he can use the sun as a weapon.

What does that even mean? Either by breaking parts of it off and throwing it at something, or making it explode or go supernova. (I guess, ok I'm bad at science.)

It is probably insanely impractical and the resources needed would be too high and will pose a danger to the system and it might have no value. But this is a question of could it be done, not should it be done This project is a question of the possibility. We are not worried about anything else.

The answer should rely on some sort of scientific or speculative science basis: up-scaling what we have or modifying certain elements or materials or expanding our current knowledge but with a clear connection to actual science.

I don't want complete fiction like: drop two tons of element contrivium and enjoy your desired effect.

And if it can't be done then please explain why.

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    $\begingroup$ Does attaching a solar panel to a laser count? Because if so, you wouldn't even need futuristic technology... $\endgroup$ – user3153372 Nov 11 '20 at 14:48
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    $\begingroup$ Is turning the sun into a black hole acceptable? $\endgroup$ – Mast Nov 11 '20 at 14:53
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    $\begingroup$ In addition to the other answers, there's also the star-powered laser described in Larry Niven's Ringworld series. Quoting from Wikipedia: "The defense system uses the superconductor grid embedded in the Ringworld's scrith floor to manipulate the magnetic field of the Ringworld's sun to trigger stellar flares that power a titanic gas laser." $\endgroup$ – GrumpyYoungMan Nov 11 '20 at 15:26
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    $\begingroup$ @user3153372, Yes. But honestly it is not insane and unpractical enough. I was thinking of theatrical stuff like scoping parts of the sun and catapulting it at the enemy. But in reality the setting is a lot more boring. Like star forts and fleets and Matrioshka brains and all those boring stuff. But you are welcome to add an answer with your suggestion. I'd love to hear it $\endgroup$ – Seallussus Nov 11 '20 at 17:20
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    $\begingroup$ For ages, boys have been weaponizing the Sun in their fights against ants $\endgroup$ – Hagen von Eitzen Nov 11 '20 at 20:16

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Weaponizing a star might not be as implausible as you imagine.

First, we have the "tried and tested" method of luring your opponents close enough to a star so that they cannot escape it's gravitational pull. Then just let nature do its work. However, this might not be the weaponization you are looking for.

But I am going to try and answer your actual suggestions here:

"Either by breaking parts of it as throwing it at something."

This could prove to be tricky. Stars are mostly made of super-hot materials in a fluid/gaseous state, all bound together by huge amounts of gravity. It's not like you could just scoop a chunk off the surface of the sun - imagine it like trying to scoop some helium out of a balloon with a spoon that is quickly melting.

It's not impossible however. If you have something with a greater amount of gravity (like a black hole), parts of the star will be slowly siphoned off into the black hole. You could collect parts of the star during this siphoning. It should be noted that if you have tech to bring a black hole to a star, you may as well just throw the black hole at your target as it will do far more damage.

"Or making it explode or go supernova"

Now we are talking, this could be done. Stars go supernova when they have too much matter, so providing that you have enough matter you could achieve this. We're talking a lot of matter however, possibly in the range of several million Earth-sized planets would be needed to make the star go supernova. Or maybe another star would do, throw one star at another and then boom.

Moving Stars

But if we can move stars, then that could be weaponization enough for you.

Being able to move a star has several interesting features for a would-be weaponsmith. Not only can you throw the star at your target, simply moving a star slightly would effect the orbits and climates of all it's planets. It could be possible to render a world completely uninhabitable by giving its host star enough of a nudge.

The biggest downside to moving stars is that their gravity is so huge that, even if your tech levels were so advanced that moving a star was trivial, the movement of the star would be detectable from anywhere relatively nearby (if Alpha Centauri moved significantly, for example, we'd be able to detect that from Earth with today's tech). It wouldn't be a stealth operation, and your enemy would no doubt have time to react and respond.

Deathrays

Stars output a lot of energy, huge amounts. You could capitalize on this and position a spacestation near a star and have it collect as much energy as it could. The spacestation could then use this energy to power some kind of deathray to eliminate the enemy.

This spacestation could take many forms, from something resembling the Icarus II from the film Sunshine, all the way up to Dyson Spheres.

To destroy the Earth, you need somewhere around 100,000,000,000,000,000,000,000,000,000,000 joules of energy. The sun’s luminosity generates about 380,000,000,000,000,000,000,000,000 Joules a second, so if you could collect all the suns energy for just over 3 days you'd have enough to deathstar the hell of out the Earth and reduce it to rubble.

This is probably the most practical method of weaponizing a star, unless of course you have super advanced technology, and you could play around with Neutron Stars...

Neutron Stars

Everything mentioned before this has been assuming you are talking about most stars in the spectral range. But if we are talking Neutron Stars and we have technology so advanced it could be considered magic, then we have some new options for weaponization.

Neutron stars are the remnants of of a star after it has gone supernova - it's effectively the remaining core of a star and they are super dense and have insane amounts of gravitational pull.

Imagine a ball about 10 kilometres big that has considerably more mass than our Sun. That's a neutron star.

Moving or manipulating a neutron star would be bordering on the impossible, but supposing we could do something with it, throwing a neutron star at a planet could potentially end up taking out the entire solar system.

Returning to "breaking parts of it as throwing it at something."

To return to one of your suggestions, neutron stars are incredibly dense, and scooping a bit off them is suddenly way more feasible than scooping bits off a gaseous/fluid star. Dropping a part of a neutron star the size of a football at a planet would definitely wipe out all life on that planet (and the planet might get vapourized in the process).

Just bear in mind that scooping parts off a neutron star would be virtually impossible, and require such ridiculous amounts of energy that it simply becomes inefficient. Effectively you'd already have enough energy to vaporize several planets, so why then bother scooping bits off a neutron star?

You could have this football sized chunk of neutron star left behind by some super advanced but now extinct race - as the tech levels required to do this is far beyond anything we could currently imagine. Or at least, anything I can imagine.

Personally, I'd go with the deathrays shot from an orbital spacestation approach, as this would give you a reusable weapon that would be very efficient. But unfortunately this might not be as dramatic as what you were originally after, if that's the case go for the neutron-star-football left behind by some long dead aliens. But be prepared to deal with how something with such an insane gravitational pull could be moved/stored and used as a weapon (this is almost worth another question all of it's own).

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    $\begingroup$ BTW, an (American) football's worth of a nutron star would weigh approximately 973,844,000,000 tons...yeesh $\endgroup$ – MarsNebulaSoup Nov 12 '20 at 14:49
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    $\begingroup$ And a whole star weighs between 20-50 nonillion (10^30) kilios $\endgroup$ – MarsNebulaSoup Nov 12 '20 at 14:57
  • $\begingroup$ It's amazing how many times I've read "if you have the technology to move a black hole...". We really ought to figure out negative mess entities that are gravitationally reversed. $\endgroup$ – Sidney Nov 12 '20 at 20:36
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    $\begingroup$ @Cyrad of course, there is plenty more that could be detailed here - but I just wanted to give an overview of the options as I saw them. The OP can delve further into these things themself. I felt like the answer was already long enough. $\endgroup$ – Jimmery Nov 13 '20 at 0:04
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    $\begingroup$ This death ray you speak of. It would be some sort of military base that basically kills stars to power it, yes? Some sort of Star Killer Base? $\endgroup$ – Ed Marty Nov 13 '20 at 16:01
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Matrioshka Brain

Nobody said a weapon had to be a gun you fired or a bomb you exploded. Knowledge is a kind of weapon all its own.

A human brain draws about twenty watts of power, and nature came up with that by mashing cells together until something survived. So it's clearly possible to develop a computer at least as energy-efficient as that, even if the human brain is somehow the most energy-efficient thought-generator in the universe and your approximation of it is just to replicate it with silicon instead of meat jelly.

You've got a Dyson sphere's worth of power (~ 3.8 x 1026 watts for a Sunlike star) and presumably future-grade heat sinks, information processing systems, and support infrastructure. This means you can bring to bear a minimum of septillions of brains' worth of raw thought off of your star.


Model a conflict before it even begins, and deduce exactly how to act in order to win it with the least material resources expended on the part of the galactic empire. Use your sun as an economic weapon, ensuring you win the cost war before your enemy tries to win a gun war.

Come up with an idea so compelling, a psyop so thought-consuming, that humans cannot resist its power over their minds. Deploy a memetic weapon against your enemy, tailor-made to convince them all to surrender, to kill themselves in shame, whatever suits your purposes.

A Matrioshka Brain is the ultimate cyberweapon. Set your computers up on pinging duty and conduct the greatest DDOS the universe has ever known. Bring your enemy's networks crashing down, destroy their ability to coordinate and communicate, and mopping them up is easy. Or design the perfect hack, infiltrate and shut everything down as stealthy as a legion of legions of legions of legions of minds can think up.

Harness your ideatic weapon; spearhead your technology forward a billion years all at once. Test every new concept of physics that the population of an entire galaxy of inhabited worlds would be able to think up.

This is how you use a sun to win a war.

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    $\begingroup$ the level of out-of-the-box thinking to come up with using a star to outwit your opponents makes me suspect you already have your own personal Matrioshka Brain ;) $\endgroup$ – Jimmery Nov 11 '20 at 15:58
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    $\begingroup$ Doesn't this run into the issue that in a civilization that can build such a thing all major powers have something at least comparable in scale. Sure it is mindblowingly more powerful than anything we currently have but if you adversaries all have something similar, the power advantage is not that big even if yours is currently the biggest computer out there. $\endgroup$ – quarague Nov 11 '20 at 18:35
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    $\begingroup$ @quarague The question didn't imply that everyone was having this idea — it was that one side had this idea first. Both sides could develop a Nicoll-Dyson beam and they'd also be evenly matched. And I'd argue that the first side to develop such a brain has an unstoppable advantage over anyone who might be able to get one later, but that might be a different point. $\endgroup$ – parasoup Nov 11 '20 at 18:46
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    $\begingroup$ This is a cool idea, but with my mere mortal brain, I'm still fairly sure the super-sun-brain is gonna say "just use 99% of this energy for a few minutes to evaporate the enemy completely". $\endgroup$ – Mark Nov 11 '20 at 19:52
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    $\begingroup$ Thank you. It's creative no doubt. Not exactly what I would consider an answer but that's another thing. Anyway I appreciate the well made detailed answer. Certainly something to consider. $\endgroup$ – Seallussus Nov 11 '20 at 22:52
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This can definitely be done if you can build a Dyson Sphere. Imagine you have built a Dyson Sphere. Then a war breaks out between you and a neighboring solar system. If you open your Dyson sphere on the side towards your enemy, most of your sun's energy will be focused in that direction like a deathray. This is called a Nicoll-Dyson Beam or Laser

Here is a good video explaining the Nicoll-Dyson Beam.

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  • $\begingroup$ Isaac Arthur's video was the first thing I thought of when I read this question. He's always a good place to start for this sort of topic. $\endgroup$ – Harabeck Nov 12 '20 at 17:45
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Ramming Speed!

No, seriously. If you can build Dyson spheres, then you have everything you need to MOVE your star.

Just put a ring around it, run a significant electric current around your ring. The produces magnetic field causes your star to preferentially jet its corona material at the poles, producing asymmetric thrust.

Your star (with ring) is now a slightly slow, clumsy, but HUGE battering ram.

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  • $\begingroup$ +1 and I wish I could upvote again, but I do feel sorry for the people living in the planets orbiting the battering ram. $\endgroup$ – The Square-Cube Law Nov 11 '20 at 12:21
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    $\begingroup$ Yeah, after a few million years you can ram some other system... Stellar engine $\endgroup$ – Alexander Nov 11 '20 at 18:23
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    $\begingroup$ Also called a Shkadov thruster. $\endgroup$ – Ryan_L Nov 11 '20 at 18:50
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    $\begingroup$ it is veeerrrryyy slooooow $\endgroup$ – MolbOrg Nov 11 '20 at 20:24
  • $\begingroup$ If its really that slow, it seems like its thrust would be unlikely to counteract its gravitational forces enough to cause serious problems for bodies in orbit about it. They'd just move along with it. $\endgroup$ – T.E.D. Nov 11 '20 at 21:41
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At least according to legend, the sun has already been weaponized, and what's more, in the distant past. It's well known that focusing enough of the light of the sun on a small enough point will cause it to burst into flames.

http://www.hellenicaworld.com/Greece/Science/en/Mirrors.html

It has been proven that this CAN work, although it remains controversial whether it actually DID work.

There's no reason your future society couldn't use a version of this. Just put some lenses or mirrors in space, and focus all that light on a target. The easiest target would be something relatively small on a planet in the local solar system, but in theory, this could also work at longer distances. Of course they'd have to have unbelievable accuracy for distance work, but this is SF, right?

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    $\begingroup$ brilliant answer! $\endgroup$ – Ross Presser Nov 11 '20 at 20:07
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    $\begingroup$ Focusing the light on greater distances is not that easy without really large mirrors (that's why there is doubt about Archimedes), but that should not be a problem for a super power. $\endgroup$ – Paŭlo Ebermann Nov 12 '20 at 23:23
  • $\begingroup$ This was my first reaction upon seeing the question in HNQ - Archimedes has done it already :-) $\endgroup$ – Jyrki Lahtonen Nov 14 '20 at 5:35
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Of course! There are lots of ways to do that, here are several:

  • Stellar Engines
  • Supernova
  • Nicoll Dyson beams
  • Stellar Lifters
  • Mirrors
  • Blackhole bombs
  • Ascending Toposophic Levels
  • Tippler Cylinder

Build a Stellar Engine

Move your star to a more strategic location, or cause problems by moving it in unhelpful ways.

Kursgesazt has a video about stellar engines, here's several possible designs:

enter image description here

This is assuming you can't just apply the FTL technology your universe has to the star. Many scifi novels talk of entire planets being moved through hyperspace junctions etc, but stellar engine theory is based in our own universes physics.

The shield version is called a Shkadov thruster and works by reflecting the stars output in one direction to generate net thrust

A faster version would be the caplan thruster, which uses a stellar lifter to gain mass for a giant fusion rocket and a secondary bumb to push the star forwards

Gravitational Perturbation

Move your star through the viscinity of populated enemy systems so that orbits are perturbed and catastrophe ensues.

If your FTL allows it, move the entire star into the system on top of the targets, then move it elsewhere after a short while.

Here's a graphic from a Kursgesazt video demonstrating a red dwarf displacing asteroid orbits to cause bombardment:

enter image description here

Ramming Speed

You could also ram the target, though if the target is another star this would destroy your setup

Supernova

You can blow up the star if it's large enough and destroy all life within a significant range, if not planets themselves.

Here's Romulus being destroyed by the Hobus star supernova in Star Trek:

enter image description here

Planets further afield can be devastated by radiation alone

enter image description here

Here is an enlightening question on this stack that asks how to artificially trigger a supernova.

If your star is a white dwarf you can dump a large amount of fusable hydrogen on to its surface to trigger a type Ia supernova ( the target will need to exceed 1.4 solar masses ).

Alternatively, adding non-fusable mass to a star so that it exceed its Chandrasekhar limit. Moving a large amount of iron into the stars core may do this. As would elements that are beyond the stars ability to fuse such as other heavier elements.

Nicoll Dyson Beams

Partially enclose the sun and focus its output into a Nicoll-Dyson Beam or Laser, and aim at the target:

enter image description here

Any large dyson swarm that's reflective enough can be repurposed for this use, becoming a phased array emitter.

Keep in mind though that the construction of a Nicoll Dyson weapon will be very apparent to neighbouring systems.

Most scifi Nicoll Dyson weapons combine a wormhole to allow fast targeting and pinpoint destruction of smaller objects such as planets and fleets. However in a real life setting one would point the output at a starsystem and everything within it would be razed after several years of transit time, assuming it is within range. Beyond 100 light year ( likely less ), the beam will have diverged enough that it can no longer cause enough damage, though still visible

Build a stellar lifter

You could do this to get materials and build huge numbers of automated weapons

Star lifting is any of several hypothetical processes by which a sufficiently advanced civilization (specifically, one of Kardashev-II or higher) could remove a substantial portion of a star's matter which can then be re-purposed, while possibly optimizing the star's energy output and lifespan at the same time. The term appears to have been coined by David Criswell.

https://en.wikipedia.org/wiki/Star_lifting

This has the added benefit of increasing the lifespan of the star.

There are several methods of doing this but the simplest method is to build a dyson swarm of mirrors and target the reflections on an area of the star to induce heating and thus emission.

Mirrors

Even without all that technology, large arrays of mirrors act as a superweapon of sorts, such a weapon could be achievable within the next 100 years, and already fries birds in solar concentrator generators.

In Gundam MS such a weapon was built to get around treaties by the federation and named the Solar Flare system:

enter image description here

See Ep35 glory of solomon

Blackhole Bombs

Compress the star down to blackhole size and use it to create a blackhole bomb

A black hole bomb is the name given to a physical effect utilizing how a bosonic field impinging on a rotating black hole can be amplified through superradiant scattering. If the amplified field is reflected back towards the black hole, the amplification can be repeated, leading to a run-away growth of the field, i.e. an explosion. One way this reflection could be realized in nature is if the bosonic field has mass. The mass of the field can then cause the amplified modes to be trapped around the black hole, leading to an endless cycle of self-amplification. The mechanism by which the black hole bomb functions is called superradiant instability. It can also refer to one such method of creating such a runaway effect, a Penrose sphere with no means for energy to passively escape.

https://en.wikipedia.org/wiki/Black_hole_bomb

Ascend the Sophonce Scale and reach higher Toposophic levels

Currently you are but a mere human, with human level intelligence, how small and boring.

A star has a lot of computing potential, we can build various forms superstructure to ascend the toposophic scale, starting with a singularity level intelligence at S-1, then repurposing more of the stars output to reach S-2 and beyond.

A matrioshka brain is one method of doing this.

The orions arm project has a primer on toposophic levels here, as well as several theoretical mechanisms for doing so

Tippler Cylinders

Take your star and compress it into a hollow cylinder 10km long, then spin it as fast as you can around its longitudinal axis. This should produce powerful frame dragging effects.

If such a cylinder appeared to have an infinite length, or if negative energy was provided, then ships following a spiral trajectory could follow closed timelike curves at sub-light speeds to travel into the past.

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    $\begingroup$ Just finding a black hole seems way easier than collapsing a star to build a black hole bomb, but it's creative. $\endgroup$ – Mark Nov 11 '20 at 20:11
  • $\begingroup$ true but stars are far more numerous, and naturally occurring blackholes have probably been catalogued and charted $\endgroup$ – Tom J Nowell Nov 11 '20 at 23:07
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    $\begingroup$ Black holes are, in general, very hard to find. Unless they have a bright accretion disk, they are invisible unless you happen to be close enough to see the gravitational lensing. This means that most black holes have not been charted at present (although a Type 3 civilisation should know where most of its black holes are). But stars are indeed much more numerous than black holes by several orders of magnitude according to current estimates science.nasa.gov/astrophysics/focus-areas/black-holes (up to 1 billion BHs vs 100-400 billion stars). $\endgroup$ – BBeast Nov 12 '20 at 4:48
  • $\begingroup$ @BBeast Much more numerous but there are still many millions in the galaxy. But yeah if you can't find one, and have something powerful enough to collapse stars, and that something can't just collapse the enemy :-) In any case it's a creative idea for storytelling! $\endgroup$ – Mark Nov 12 '20 at 7:55
  • $\begingroup$ @Mark I suppose it depends on what is harder: moving a black hole into position, or collapsing a conveniently local star. $\endgroup$ – BBeast Nov 12 '20 at 9:59
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Blast 'em through a wormhole!

If there are galactic empires then you have faster-than-light travel. Let's suppose FTL is done by using wormholes. You now have the most devastating weapon at your disposal. Open one end of the wormhole facing a city you want to destroy. Open the other end in or near to the local sun. Watch as radiation and hot plasma pour out and vaporise everything for miles around.

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  • $\begingroup$ so ez, yet so fun $\endgroup$ – Someone Nov 11 '20 at 16:23
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    $\begingroup$ I consider this a viable explanation for both the "Where is everyone?" Fermi paradox, and also the preponderance of binary star systems. Turns out it's really easy to accidentally open a wormhole to the core of your local star, and once it's open, it stays open long enough for stellar matter to form gravitational equilibrium between the two ends. $\endgroup$ – Tom W Nov 11 '20 at 17:04
  • $\begingroup$ This is truly interesting and it actually something I thought of. So. I have a universe with galactic travel being done through wormholes. However the only caveat is that there is a sort of blocked area around starts that is like a 3d bubble of no wormhole travel happening. It's only a half baked idea so far. The reasons are mostly story limitation because like you just said why not open a wormhole to the heart of the enemy's empire? So. I'm glad too see other people thinking that so I'm not totally crazy. $\endgroup$ – Seallussus Nov 11 '20 at 22:49
  • $\begingroup$ youtube.com/watch?v=vSYCmzZB-UE Or in case the link breaks: Farscape S3E15 $\endgroup$ – Glen Davies Nov 14 '20 at 10:59
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It's not just possible, I think it's almost certain.

If there is an interstellar civilization, it's extremely likely that they already have something akin to a Dyson sphere. Simply because the Sun is, by a very large margin, the most abundant source of energy in the solar system, and you don't build an interstellar civilization without a lot of power.

Having obtained many orders of magnitude more energy than we have now, it really doesn't matter that much what kind of weapon you create. Just find some way to point all that solar energy at the thing you want to go away, and it'll probably work.

If you redirect only 1% of the energy of the Sun, it's still like exploding more than 1.000.000.000 Hiroshima bombs per second. You can try fancy stuff, but a bunch of lasers, or a relitivistic railgun is good enough, anything else is just gravy.

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There are a variety ways a sufficiently advanced society could weaponize a star.

1. Harness the Star for a Superweapon

A society this advanced could easily harness the star's energy and mass using a Dyson Sphere or similar structure to fuel a superweapon. This is likely the simplest and most practical way to weaponize a star. Stars possess such unfathomably large amounts of mass and energy that their applications seem bound only to imagination. Each second, our Sun outputs more energy than all of our nuclear weapons combined.

Possible uses include:

  • Focusing the energy into a powerful energy weapon, like Starkiller Base from Star Wars: Force Awakens. Unlike the weapon in the movie, a star has enough energy and mass that such a weapon wouldn't need to destroy the star unless the device harnessing the star has an extremely inefficient conversion rate. Most proposed implementations of a Dyson Sphere are just a really big network of efficient solar panels and wouldn't actually harm the star. Focusing that energy into a powerful beam-like weapon could easily destroy civilizations and possibly planets. However, it would likely be more efficient to do so by incinerating an enemy planet's atmosphere or causing mass extinction climate change, instead of reducing a planet to dust.
  • Transporting a portion of the star's mass to a planet. Even a tiny piece of a star can create an explosion powerful enough to overshadow even the mightiest of nuclear weapons and bring about the end of all civilization on a planet. A larger piece could incinerate the atmosphere or outright annihilate the planet entirely.
  • Trigger a super solar storm, using the technology to direct a powerful solar flare to wipe out all electronics on an enemy planet
  • Fuel a doomsday device. Self-explanatory. Harnessing the power of a sun for any weapon of death and destruction.

2. Shoot the Star

Using a stellar engine, an advanced civilization can move the star and literally throw it at an enemy planet system.

This sounds fantastical, but this is actually quite feasible for a civilization with the technology to build a Dyson Sphere. A number of stellar engine designs have been proposed in peer-reviewed journals. The Caplan Thruster, for example, uses relatively tiny portions of the star's mass to fuel an engine that pushes the star using a beam of oxygen-14. Pushing the star would also have the effect of causing all of its orbiting bodies to move along with it with little adverse effect.

The real limitation of weaponizing a star with a stellar engine is distance and speed to the target. Solar systems are usually very far away from each other. Even if we had a stellar engine that could push our Sun at the speed of light, it would take more than 4 years for our Sun to reach Alpha Centauri, our closest neighboring solar system. So either the civilization would need FTL technology or be waging slow/patient wars against neighboring systems that last hundreds or thousands of years.

The damage a "star missile" could cause would be unfathomably devastating, especially as it also throws any other celestial body that orbits it. Even if none of the bodies directly collide with the bodies of another solar system, the weaponized star's gravity would utterly disrupt the target system. Fry celestial bodies. Fly-by kidnap planets or shove them out of orbit. Combine the star into a binary system or cause the suns to collide and explode. Numerous things can happen and none of them would be good for the victim.

3. Sabotage the Star

Another way to weaponize a star is sabotage a target system's sun or use it against them. You could harness the star for a weapon (like in #1) or kill/destroy the star. Probably the most efficient way to kill a star is to artificially age it. This can be accomplished by either adding an enormous amount of mass to it or by exhausting its fuel supply. Doing this would cause the star to progress to a red giant/supergiant and eventually to a nebula or supernova or neutron star. This would effectively destroy almost any civilization in entire solar system.

At a glance, this seems like one of the easiest and obvious ways to weaponize a star. However, even a very advanced civilization would encounter a lot of obstacles. Stars are massive and have a huge amount of fuel and energy. It would take an enormous amount of technology to transport enough mass and gas enough to prematurely kill a star. A Dyson Sphere might be a technological wonder, but it's essentially just a really big network of solar panels. Transporting its gas and mass would take significantly more effort.

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The idea of using solar output to power an epic-scale beam weapon has been used in science fiction. In the 1941 novel Second-Stage Lensman by E. E. "Doc" Smith, some sort of handwavy technology was set up on various asteroids, and the technology caused some large amount of solar energy to focus into a beam. This awesome weapon was called the "Sunbeam". According to Wikipedia, the amount was 100% so I guess the sun would appear to go dark when this weapon was used? In the climax of the book, a large invasion fleet comes to attack the Earth, and the Sunbeam destroys the ships easily.

The biggest objection to this one is the handwavy technology. I have no idea how the "Sunbeam" was supposed to work.

https://en.wikipedia.org/wiki/Second_Stage_Lensmen

The next paragraph has major spoilers for plot from the John Ringo book series Troy Rising. Read on only if you don't mind the spoilers.

In the Troy Rising series, Earth was contacted by sentient beings from other solar systems. One group decided to declare themselves the new rulers of Earth and Earth didn't really have any ability to resist. One clever guy got rich selling things to aliens and used his money to set up an asteroid mining operation in space; this used the old idea of reflecting sunlight from large mirrors to focus it on ore to melt it. IIRC, the new rulers permitted this as they expected to tax the income from mining and get fabulously rich. After a staggering initial investment the asteroid mining operation became self-bootstrapping and it built more and more of the mining mirrors. Once there were enough mirrors to have a reasonable chance of success, he activated a computer program that used all the mirrors together as a weapon: instead of focusing on ore to melt metals out, they would focus on a ship that was putting the Earth in danger. With enough mirrors focusing enough sunlight on a single ship, that ship would immediately melt and/or explode. This is an example of the idea that anything that can move large amounts of energy around can be used as a weapon.

This one is totally plausible. A bunch of giant reflectors that can be steered to focus sunlight on a target? Doesn't even require any new science, just some large-scale space engineering that seems possible. The hardest part would be scoring hits on ships at distances of millions of kilometres while the ships could be moving at rather high speeds.

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In Larry Niven's Ringworld, the ring and shadow squares -- much less than a full Dyson Sphere, though still an unreasonably large construction -- were able to use EM fields to cause a solar flare and to cause the plasma in that flare to lase. In the book this then disabled the spacecraft.

This happened directly in the substance of the sun, unlike the Nicoll-Dyson laser mentioned in other answers, where energy is first collected and used to power separately built laser machinery.

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The most plausible way I can think of is to develop nanotechnology that could replicate itself, harvest resources in space and the use those resources to build a giant lens. Then control systems could be added to that lens to position it however the operator chooses. That could then be used to burn the unfortunate target on the planet

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  • $\begingroup$ A giant mirror might be easier to build and steer. $\endgroup$ – Paŭlo Ebermann Nov 12 '20 at 23:26
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Countless generations of kids have weaponised the sun, either with a mirror- to the annoyance of anybody glancing in their direction, or with a magnifying glass- to the terminal displeasure of the neighbourhood ants.

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Since the technology to conquer galaxies, we can assume that they have enough power and resources to move stellar objects, then what needs to be done is to create a solar system with as much mass and energy as possible. This includes giant planets, multiple asteroid rings, and a neutron star(s) or a black hole(s). This wouldn't take (relatively) much energy, as you would need only to move the central body, making the rest of the system follow along. Depending on the size of the system being thrown, it may cause planetary damage to the target, temporary and destructive change to orbit paths, definitive changes to the central body of the target system, potentially disrupting the entirety of the target.

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Actually, really simple. If you want to blow up the sun, all you need to do is make it so that the Dyson Sphere aims the sunlight back at the sun. This causes the sun to expand because of a huge output of heat pressure and basically puts way too much energy into a small space, compromising the structural integrity of the sphere, which will break apart in an artificial supernova. Unfortunately, your Dyson Sphere will definitely need to be solid and it is a one time use. Plus, this is not a short and easy method. You will need to estimate when it should blow up. But if you can get past that, you now have a working time bomb. Simple.

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“... This includes manipulating metals at a level far surpassing our own.”

While the Sun is 98% hydrogen and helium, it also contains lithium, beryllium, boron, iron, nickel, calcium, sodium, and magnesium.

2% might not sound like much, but if you wanted to mine the elements to use those parts of the Sun for warfare, you would have something bigger and purer than a planet to draw off of as a resource.

(Source: Astronomy.com, solarscience.msfc.nasa.gov )

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    $\begingroup$ Please take a good read at our help center, I see that in all your answers so far you are mostly going tangential with respect to the OP's question. $\endgroup$ – L.Dutch - Reinstate Monica Nov 13 '20 at 7:18
  • $\begingroup$ This is true. Possibly better phrased as "mine the Sun as a source of minerals more abundant than all the planets combined". But even with that, it remains only a stub of an answer. $\endgroup$ – user79911 Nov 13 '20 at 12:07
  • $\begingroup$ Concur on the tangentiality of at least this response. "Weaponise", in context, means either to turn something into a weapon or else to make something that already dangerous into a more effective weapon. Mining doesn't make sense in context. $\endgroup$ – elemtilas Nov 14 '20 at 13:42
  • $\begingroup$ For the record, since the Sun makes for 99% of the mass of the solar system, all the metals in it combined outweight all planets, dward planets, asteroids and comets combined. $\endgroup$ – The Square-Cube Law Nov 15 '20 at 20:35
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I remember reading in Death's End, one of the Three Body Problem books by Cixin Liu, of another way to weaponise a star. Stop reading if you want to avoid SPOILERS.

In the book, this is called a "photoid", which is basically a rock launched towards a star at nearly the speed of light. The enormous kinetic energy makes the object quite massive. I remembered its effect in one way, but a quick internet search to brush up suggests a different effect.

  • When the photoid hits the star, it will travel deep (due to the mass and speed). The "crater" left behind will take some moments to close, and this short span of time exposes the sun's core to the outside universe for a brief moment. Given the core of the sun is (IIRC) 15 millions degrees Kelvin, the light escaping from it through the crater would be extremely powerful, both in terms of raw wattage and in terms of the energy of the average photon (which would be in the X-rays, again IIRC).
  • This summary I found says:

Dark forest strikes usually do this by shooting a star with a "photoid," which is essentially just a rock...fired at speeds just under the speed of light, so that its relativistic mass is a fraction of the star's mass. The impact literally blows the star open, spewing its contents onto surrounding planets before the star ultimately collapses.

a small piece of matter moving so close to light speed that it causes the star to go supernova on impact.

I remember finding it a pretty cool idea while reading the book.

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  • $\begingroup$ Hi Marco, welcome to Worldbuilding! Your answer poses a very interesting scenario, but it would be very nice if you could find some relevant sources on it or provide more information. $\endgroup$ – adrian Nov 19 '20 at 2:43
  • $\begingroup$ Hi @adrian, thanks! Is this enough detail, you reckon? $\endgroup$ – marcotama Nov 19 '20 at 5:50
  • $\begingroup$ It's very nice, I bet this will be some interesting reading! $\endgroup$ – adrian Nov 19 '20 at 6:37
  • $\begingroup$ You have no idea ;) $\endgroup$ – marcotama Nov 22 '20 at 22:31

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