I'm not quite sure of that. It seems to be based on the assumption that the only way to command star-like energy level is to actually have a star at your disposal. In fact stars are basically just fusion reactors, and very inefficient slow-burning ones, so I'm not so sure about that. Let's do some math.Xeriar wrote:Ultimately, whoever controls the star wins.
The sun radiates 3.86 X 10^26 watts. The energy density of hydrogen fusion is (IIRC) 6.2 X 10^14 joules per kilogram. Equalling the energy output of the sun therefore requires the fusion of approximately 6.23 X 10^11 kilograms of hydrogen per second.
Now let's say we want to command 4X the energy of the sun for 10 years. We need a total of roughly 7.85 X 10^20 kg of hydrogen.
By comparison the Earth's ocean is 7.61 X 10^23 kg of water (ref), which I calculate to contain ~9.5 X 10^22 kg of hydrogen based on comparison of the molar mass of hydrogen and oxygen. So we could get that much hydrogen by removing a mere ~.83% of Earth's water. Alternately, it's ~.394% of the low-ball estimate for the total mass contained in the Kuiper belt (ref).
Of course we need reactors to exploit this. Atomic Rocket suggests modern reactors can get as good as ~2277.9 watts/kg, although most designs are much less impressive, around 55.6 watts/kg (ref). Future technology would presumably be better than present technology so I don't think I'm being unrealistically optimistic by using the high-end estimate. We need ~6.778 X 10^23 kg of reactor. That's a little more than 2 Mercuries.
Now, to this you have to add lasers, cooling systems etc. so the figure might increase by an order of magnitude or two. But if you're planning to invade another solar system you'd want superiority of material anyway so you'd likely seed Von Neumanns in a number of empty systems and try to convert as much of their mass as possible to warships and lasers and stuff. With the resources of potentially dozens or hundreds of empty systems to draw on scraping together the mass equivalence of large terrestrial planets doesn't seem like an utterly impossible project.
Why go to all this trouble instead of just using lasers over interstellar distances? Targeting. If you're shooting at somebody over interstellar distances it will take years for your shots to get there, and the enemy could easily make things very difficult for your targeteers by changing the orbits of their space habitats, facilities etc. periodically. It's advantageous to try to get your lasers as close to the enemy as you can.
Of course, it goes without saying this sort of warfare will look very different from what we typically see in soft SF.
PS1: it's worth noting that if you have starships capable of getting up to significant fractions of c with mass of less than billions of tons you probably can get a lot better than 2277.9 watts/kg, at least in your engines.
PS2: sorry I couldn't find better references offhand, but the Wikipedia one is cited. Also I hope I didn't mess up any of my math.