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In a science fiction story (Bobiverse) a nova is created by accelerating two small planets at relativistic speeds into a sun from opposite sides.The shockwave creates hydrogen fusion on opposite sides of the star, which then causes the center to have far more energy and be able to fuse higher elements, blowing the star up, ie an artificial nova.

This seems basically sensible, but the numbers matter. Would any other outcome be possible? Because the masses are coming in from opposite sides a jet seems impossible. With only a single mass perhaps the core of the star could be displaced along the line of impact rather than blowing up? A near miss could create two jets in opposite directions without the big blast?

Obviously gigantic energy required to accelerate planets to relativistic speeds. But even assuming that were possible, synchronizing two masses to hit diametrically opposite sides of a star at exactly the same time would be extremely difficult without ftl communication which does exist in the story. Are there any other problems with the idea? Given two masses as described, would a star go nova?

The kinetic energy of a mass the size of mars going at 0.9 c would be huge in the first place, far bigger than the energy of the 4000 tons/sec that get converted to energy in our sun. With that kind of energy at their disposal, it seems like there would be easier ways to kill the enemy, such as accelerating many, much smaller masses close to lightspeed, and letting them hit the target directly. Or for that matter, two masses collided together head-on could liberate that kind of heat without the star?

v = 0.9 * c = 2.7 x 10^8

1/2 m v ^2 = 0.5 * 6.0e23 * 2.7e16^2 = 2 x 10^40 J per mass

which, though not a supernova, isn't bad. Impacting two masses that fast should result in some fusion, so the next question would be how much fusion?

Last question, for the alternative of small masses, would an impact of a mass m=1000kg at 0.99c release radiation at point of impact in all directions, or would it be in the direction of the strike to preserve momentum? What would be the spectrum of such a collision?

I assume that the net momentum of the system would still be as before, but there would be high energy particles dopplered up going in the original direction, lower energy particles dopplered down going backwards, essentially a sphere of radiation, is that true?

In other words, With a giant target, a Dyson sphere in the making would such an attack destroy it or just neatly drill multiple holes in it?

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