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Q: Given humanity’s current knowledge and capabilities, is it possible to build a new star?
Randall’s response: A guy with a weapon labeled Sun Obliterator (Beta) says, “…I need to know by Friday.”
My response: Ah, an astrophysics question. The short answer is no.
Star formation is a hot topic of research in my field, and we still don’t fully understand it. But for our purposes, it’s not particularly complicated. To build a star, all you have to do in pile a minimum of 160 septillion tons of interstellar gas into one place. Now, if you want to find 160 septillion tons of interstellar gas, the only place to go is a giant molecular cloud, To pile it into one place, you need some way to move 160 septillion tons of gas. What’s the biggest thing we can move (a significant distance) with humanity’s current knowledge and capabilities? It’s a lot smaller than 160 septillion tons. It must just barely be possible to move planet Earth, which is only 6 sextillion tons.
So no, we can’t build a new star, and definitely not by Friday.
But let’s relax the question slightly. Is it possible to build a new star in principle, regardless of whether we can do it today. The answer to that is…kind of.
The way stars normally form is when a supernova slams into a gas cloud causing it to collapse into a star (or many stars). Theoretically, if you understood the structure of the gas cloud much better than we do now, and if you artificially set off a supernova at just the right place and time, it might be possible to cause a star to form of the type you want and in the place you want, and probably several stars.
Can you set off a supernova artificially? Yes! There are several ways you can do this, but the easiest is to crash two massive stars together. This requires moving stars, but that’s easy if you have a stellar engine. A stellar engine is basically a giant solar sail. Starlight bounces off the sail, pushing it away, but the sail’s gravity, feeble though it is, is just enough to drag the star along with it. In space, you really can blow your own sail.
A solar sail big enough to move a star–sort of like a Dyson sphere made of Mylar–could possibly weigh as little as 70 billion tons per trillion square kilometers. It sounds like a lot, but if you robotically disassemble a good-sized asteroid to make it, it’s entirely within the realm of possibility. And if you work it out, you could move two massive stars far enough to cause a supernova in as little as a hundred thousand years.
So yes, if you have a galaxy full of resources at your disposal, you can build new stars.