Would it be possible to put hydrogen into a vacuum chamber and use something to turn it into a smaller version of a star? I am making a scene with a simulated planet environment in space and wanted the core to be a very, very small star.
$\begingroup$ You'll need magnetic field to trap all those hydrogen atoms and electric field to accelerate them to allow fusion, the stars are massive and hydrogen can easily tunnel to fuse into helium. $\endgroup$– user6760Dec 13, 2019 at 4:29
Yes, it is possible.
You can do it in an uncontrolled way in a hydrogen bomb,
Modern fusion weapons consist essentially of two main components: a nuclear fission primary stage (fueled by U-235 or Pu-239) and a separate nuclear fusion secondary stage containing thermonuclear fuel: the heavy hydrogen isotopes deuterium and tritium, or in modern weapons lithium deuteride. For this reason, thermonuclear weapons are often colloquially called hydrogen bombs or H-bombs
or in a controlled way in a reactor like ITER
The ITER thermonuclear fusion reactor has been designed to produce a fusion plasma equivalent to 500 megawatts (MW) of thermal output power for around twenty minutes while 50 megawatts of thermal power are injected into the tokamak, resulting in a ten-fold gain of plasma heating power.
Since you want it in the core of a planet, the ITER way seems the choice for you.
2$\begingroup$ However, if the OP is naively thinking of a self-sustaining star, then the answer is "no." There's a pretty-well-calculated minimum mass required for self-sustaining fusion - somewhere on the order of 20 Jupiters IIRC $\endgroup$ Dec 13, 2019 at 12:42
If your really want to simulate a star, you need enough gravity to hold the hydrogen (and "metals", the astromical term for anything that isn't hydrogen) together at sufficient density for fusion to take place.
Assuming you aren't going to drop a tiny black hole into the core of your synthetic star, you'd need a way to artificially generate gravity -- and then, probably, a way to shield against it, else the planet's own gravity, due to its mass, would be added to that used to contain the artificial star.
If all you're after is a heat source inside your planet, natural planet formation does this fairly well with the very large amounts (albeit tiny percentages) of natural radioactive elements incorporated into a planet's mantle and core. If you need your star to do something other than just heat the inside of the planet (say, act as the sun for a hollow world that breaks a bunch of physical laws), you'll have to do a lot of hand-waving.
$\begingroup$ I was going to use the small star as a source of heat and light. The inhabitants live on a halo-like ringworld, but with walls. $\endgroup$ Dec 13, 2019 at 14:47
$\begingroup$ Might check the habitable zone sizes for M6-M8 red dwarves. Contrary to popular belief, they aren't that red, surface temp is similar to the tungsten in an incandescent light bulb. $\endgroup$ Dec 13, 2019 at 14:59
$\begingroup$ A microscopic black hole also came to my mind. It wouldn't even have to be surrounded by fusable material (no idea where to even start the calculation of how that equilibrium looks like) but could just produce radiation due to the tidal forces in its accretion disk. The radiation could then be absorbed by a surface at the right distance which is heated to the right temperature to produce a solar-like spectrum. $\endgroup$– S. MoveDec 13, 2019 at 15:10
$\begingroup$ @GreenieE.-ReinstateMonica If you read some of the post-Ringworld articles, some MIT nerds proved that ring-type worlds are unstable. That is, they will not maintain the star at their center. $\endgroup$ Dec 13, 2019 at 19:33
$\begingroup$ Yep, I knew that long ago. "Just"requires active station keeping for the ring -- though if you're going to use Bussard ramjets for that job, there may be a lower limit to the size, because smaller rings will have lower rim velocity to produce 1 G centripetal gravity inside. $\endgroup$ Dec 13, 2019 at 19:55
No, because enough gravity is required to produce the pressure for self-sustained fusion. The amount required is absolute and fixed and does not scale with size of the star.
$\begingroup$ I don't think this is strictly accurate over all star and proto-star sizes $\endgroup$ Dec 13, 2019 at 19:33
$\begingroup$ @CarlWitthoft No? How does that work? You're saying it's possible for one a smaller mass of helium and hydrogen to undergo fusion while a larger ones does not? That doesn't seem right. Even if it was, it doesn't mean there is so suddenly no lower limit on the mass required to initiate fusion and that mass still seems far larger than whatever the OP could fit into an enormous vacuum chamber (planet-sized, even). $\endgroup$– DKNguyenDec 13, 2019 at 23:07