There is an intergalactic war going on in the Milky Way, to get more resources. My company have been called upon to mine nebulae. There is no nebulae in the vicinity but I have got a few high mass stars. Is it possible to let them go supernovae without having to wait hundreds of thousands of years so that I can mine the nebulae? The technology level is Kardashev Type II and no Faster-Than-Light travel.
Surprisingly - Yes (though I wouldn't bother)
With FTL technologies off the table you have a serious problem when it comes to creating nebulas, but I think there may be a loophole for you.
First let's go over what WON'T work:
Distinct Note I: Distance / Time
Supernovas are the biggest explosions we have ever observed, and they can mess up you, your family, and your planet unless you're far enough away. We'll discuss distance with cases, but I want to be clear that we're talking about distances that are all measured in light-years. I don't know how fast your ships go, but I do know that you're going to be traveling for a long time to get to your nebula.
You will need to set a speed for your ships according to what you want to see as a result.
Distinct Note 2: Value
I could not in good conscience avoid mentioning this: the obtainable value from harvesting a nebula is minimal at best, particularly if the nebula is not pre-existing. If it is not pre-existing you have to cover huge distances with each "truck-load" of space dust. In that case it is trivial to show that your resources would be better spent strip-mining a rocky body.
Bad Candidate I: The Hypernova
Particularly large supernova's can fall into the non-scientific category of "hyper nova". I mention this because a very large supernova that is 8,000+ light-years away could still destroy all life on earth. Without FTL travel I can't imagine a scenario where a contract would be useful for the amount of time it would take to get to an event over 8,000 light-years away (and back).
Bad Candidate II: Type-II Supernovas
Let's look at this Wikipedia article for starters. Here we learn that a Type II supernova (a supernova with hydrogen present in its spectrum) would strip the earth of half of its ozone layer if it occurred within 8 parsecs (26 light-years). That's fairly damaging in and of itself, not to mention any other possible effects such as a shift of a number of bodies (planetary fragments, etc) in your direction or increased radiation from the explosion.
Keep in mind that as a space-faring species you also likely have other inhabited planets, ships, etc. So you'll want to be AT LEAST 10 parsecs away from anything habitable if you want a safe TYPE II supernova. Though I will leave it up to you whether your companies cares about "safe" for outlying habitations ;)
Given this basic boundary, let's consider what would happen if that same supernova occurred >10 parsecs away (~33.5+ light-years away). At that distance your home planet is probably safe, or at least safe enough. In terms of travel time though, without FTL you are talking about some LONG treks to get material. You are an advanced people and all, so let's say you could get ships moving at up to 0.3c... that's still a 100-year journey BOTH WAYS to capture material that realistically could have just mined elsewhere.
At the same time, the debris from a Type II supernova is traveling pretty quickly and in all directions. If you stayed 33.5 light-years away for the explosion then left to collect, by the time you got in the area I'm thinking the density of what you collect would be pretty low.
What you want to do is engineer a Type 1A supernova, preferably one in a system that has two orbiting white dwarfs. This scenario has a number of benefits:
- Type 1A supernovas are less explosive, so they could be closer (less transport time)
- A system with two white dwarves is already on the "brink" of collapse, so it would be easier to influence
- Nebula matter is ejected with less velocity in a scenario involving two white dwarves, so hopefully collection density will be higher when you get to the nebula
With all that said, given that you are a Kardashev Type II society there are a number of gestures that could be possible to you. Note: we are well outside the realm of "hard science" at this point. If you propose that your society uses Star Lifting to attain it's K-II status, for example, you have access to considerable amounts of matter, energy, and possibly even anti-matter. If you can harness it to push a dwarf planet into one of the white dwarves you might be able to trigger a supernova of this type. Note again that it would still take a lot of time for charged pushing material to make it to your white dwarf! Gravity from the other white dwarf can then help slow down the resulting nebula, and if the mated star survives it would obviously be a handy source of power for your vessels. Of course, it could also be blown apart or supernova itself - both are options!
You would need to find a carbon-oxygen white dwarf and manage to make it accumulate enough mass to reach Chandrasekhar limit, which equals 1.44 solar masses. That way, the white dwarf won't be able to support its own mass anymore and will collapse. Within a few seconds, a substantial fraction of the matter in the white dwarf undergoes nuclear fusion, releasing enough energy [...] to unbind the star in a supernova explosion. And, subsequently, the supernova creates a nebula.
But how could you give it so much mass? As we know, mass can neither be created nor destroyed, so you would need to transport it. I think your best bet would be to develop a REALLY powerful magnetic "beam" that lets you move another star and make it collapse into the other one. I don't see how humanity could have such technology if they haven't discovered FTL travel yet.
As Chris pointed out in a comment, it would just be easier to directly mine from the star, so if that's an option you could consider the Dyson sphere to collect energy from the star.
Stars in general do not produce elements with a higher atomic mass than iron and nickel. TLDR there would be nothing worth harvesting in any useful quantity from a supernova that you couldn't mine directly from rocky bodies (which were after all made from accretion of supernova material to begin with...). What is the point of making something LESS dense? It just means it takes more time to harvest the same quantity, and presumably the distribution of matter would be about the same, but would contain a much larger proportion of light elements, which you can obtain easily from a multitude of sources to begin with.
This question would make more sense if the goal was to produce a weapon of mass destruction, for example some kind of missile that would initiate a lethal reaction in a star's core.