A long time ago in a galaxy called the Milky Way.....

An alien race had its planet thrown out of its solar system. It is now a rogue planet. They survived for thousands of generations in underground artificial environments. Then they achieve FTL and begin colonizing solar systems. A thousand years later the population of their planet is mainly too stubborn to leave since their race began there.

Since the government resides on their home planet and it does not intend on leaving (I know the following scenario is unlikely, but bear with me) they have decided to commission a hydrogen fusion project. This will build a miniature star orbiting their world via artificial means. I am not sure how big this star could be (or rather, how small?) and want it to supply the entire world with sunlight in a 26 hour cycle.

How can I go about doing this and how large would the 'star' have to be? Since we are dealing with a highly advanced civ, they can compress matter until it begins to fuse. The biggest problem I can foresee is supplying it with fuel.

You can use any technology level you want and are free to use the handwavium that such an impractical situation deserves. However, I do want answers to be descriptive and go into detail.

  • $\begingroup$ Possibly relevant: youtube.com/watch?v=TvoEAewmSds. Just ignore the guy in red-and-blue tights. $\endgroup$
    – Frostfyre
    Commented Apr 24, 2015 at 13:36
  • 4
    $\begingroup$ @Frostfyre You don't know how many times I hear that daily. $\endgroup$
    – Jax
    Commented Apr 24, 2015 at 13:56
  • $\begingroup$ The mass of even the smallest star is far larger than the mass of a planet; why not move the planet into orbit around an extant star? $\endgroup$ Commented Apr 24, 2015 at 15:16
  • $\begingroup$ @2012rcampion This isn't for an actual story....it's just a hypothetical scenario. I am more curious as to the 'how' and this might be useful for someone in the future. $\endgroup$
    – Jax
    Commented Apr 24, 2015 at 15:26
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    $\begingroup$ @DJMethaneMan My point was just that it may not make sense to create a star in this hypothetical. Any civilization that could move the required 20 million Earths of material to make a brown dwarf would certainly have the capability to move the planet. I think you need an additional stipulation as to why creating the star is the preferred choice. $\endgroup$ Commented Apr 24, 2015 at 15:41

5 Answers 5


As mentioned in other answers, other than using super-tech, keeping a tiny star operating with out massive gravity is a problem. Fuel is another.

I would suggest going more artificial and less pet-star. Normal stars, operate naturally due to gravity, and they hold together a solar system with that gravity.

Your Race/Government has no need for an actual star, just the star like effects on a single rouge planet. So just construct a moon like object, orbiting the planet. The '''Moon''' emits energy as light/heat only directed at the planet. Put the '''Moon''' in a lower than stable orbit to point that will counter the solar sail effect from the energy emission.

The orbital body only needs to generate a more-reasonable amount of power than it would take to keep a micro sun going. It also require orders of magnitude less fuel to operate.

As for size, that would mostly depend on the energy density available to the civ, sized to the power generation scale of the planet's solar needs. I assume earth's moon as a generic size guess. The Orbital period is set to whatever the civ considers a day. The Orbital height would be set to make the stable Orbit (with emissions) at the speed/period/average-mass. Then the moon/emitter is sized to be the same relative size as their original sun as seen from the planet, based off the Orbital height. The emitter can be bigger or smaller than the main body of the '''Moon''', as it will be so relatively bright the rest of the orbital body will be not observable from the planet.

This is also a safer option to the planet, as it a less massive, and less energetic solution. It's less likely to end the planet, if something minor to major goes wrong.

The biggest downside is that it isn't as much of a show piece as micro-sun would be to other races, but you didn't mention other races as a design point.


I'm not an expert on this but I hope I'm not telling complete BS.

So I think one problem with making a miniature star that actually orbits a planet is, as you stated, not getting it going, but keeping it going. Because for hydrogen to fuse inside a star's core the star's own gravity has to push the hydrogen cores close enough together to undergo fusion. So the star has to be big, very big.

As per wikipedia:

"Jupiter's upper atmosphere is composed of about 88–92%" and the interior contains denser materials, such that the distribution is roughly 71% hydrogen."

And jupiter is already very big. So, basically you'd need something bigger than jupiter with an even higher hydrogen composition to actually create a star.

At that point you'd have to be orbiting the actual star. Because it has a greater gravity than your actual planet, and you will eventually crash in to it because its orbital velocity relative to the earth is lower than the stars own orbital velocity. If you know what I mean. (correct me if I'm wrong)

Say it would be possible to have an object with greater mass than your planet to orbit your planet. And you don't want something that's almost as big as an actual sun to orbit your planet. And your civilization's tech is good to make REALLY dense materials I would opt for a neutron star. According to this website:

"If the mass of a normal star were squeezed into a small enough volume, the protons and electrons would be forced to combine to form neutrons. For example, a star of 0.7 solar masses would produce a neutron star that was only 10 km in radius. Even if this object had a surface temperature of 50,000 K, it has such as small radius that its total luminosity would be a million times fainter than the Sun."

A million times fainter, but at a much lower distance is still A LOT of light. But with an object the mass of the sun orbiting your planet, you'd have to deal with HUGE tides.

You could (and this is pure fantasy) also choose for creating some sort of very high point mass (maybe a very small neutron star). So this way you have a lot of gravity in a very concentrated place. This can be used to keep the hydrogen in place and compress around the point mass, and close enough to keep fusing. But I have no idea how you would indeed fuel something like this.

Hope this gives you some ideas.

  • $\begingroup$ See Interstellar. $\endgroup$
    – Jax
    Commented Apr 24, 2015 at 16:10
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    $\begingroup$ It doesn't matter whether an object is big or tiny, if it's mass is [a lot] greater than the planet's then the planet will orbit that object, not the other way around. The real question is: does it matter? Velocity is relative afterall... The position of the planet would get a little less exact from the point of view of outsiders, but other than that it doesn't seem such a big deal to me (though your point about tides is valid either way, in case the planet and the star are very close - the planet might become tidally locked or even be destroyed and turned into a ring around the star). $\endgroup$
    – mgibsonbr
    Commented Apr 25, 2015 at 18:04
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    $\begingroup$ Technically, regardless of the relative mass of the two objects, both will orbit their common center of mass. (Which in the case of, say, the Earth-Sun system is a point very near to the Sun's own center of mass) $\endgroup$ Commented May 27, 2015 at 1:33
  • $\begingroup$ You can't have a tiny neutron star. What makes a neutron star is all that mass being concentrated in a fairly small area. If you remove a small piece from the star and place it at a distance from the star, that piece would expand greatly because it would not feel the gravity from all the mass that used to be near it. $\endgroup$
    – Itsme2003
    Commented Nov 3, 2019 at 9:42

If they have FTL travel, it must offer some advanced physics to exploit. For example, open a wormhole with the other end inside a suitable star. Less dramatic: near a star, with the other end directed at the planet.

Control over bending space, even if not able to change topology, create an artificial gravity well to make a star that works like a normal one but doesn't need so much mass to self-gravitate.

How much light do they need? If they are adapted to living underground on a cold world, they might not want to melt their bedrock (H2O goes from a granite-like mineral to being an ocean) and disrupt everything.

So they may want to be showy, and produce what for them is a lot of light, but not much warmth. The energy requirements would be orders of magnitude lower than duplicating our insolation.

  • $\begingroup$ They aren't adapted for underground life. Probably should have mentioned in the question that they come from a world like Earth and have used technology to prevent evolution (not that it could conceivably happen in thousands of years....maybe millions). $\endgroup$
    – Jax
    Commented Apr 24, 2015 at 13:55
  • $\begingroup$ They came to high-level civilization and have not made comfortable living arrangements? I don't mean physically evolved, any more than humans are evolved to live in Canada. They have a successful culture and have for a long time, with the current situation. Where do they build their houses? $\endgroup$
    – JDługosz
    Commented Apr 24, 2015 at 14:05
  • $\begingroup$ They currently live underground. The environment is not exactly great in these underground shelters. Very crowded and they have trouble getting enough food (it is costly to import much from off-world colonies). I figured they would gradually move up to the surface as atmospheric conditions improved. Their surface cities have been frozen in near perfect stasis for thousands of years. They would simply begin to repair the decayed areas until they were decent. This would open up a lot of land to farming. $\endgroup$
    – Jax
    Commented Apr 24, 2015 at 14:58
  • $\begingroup$ If you can create wormholes, just pick a suitable star, decide how far out from it you'd need to be for comfort, and create a wormhole ahead of your rogue planet whose exit is at the right location, orientation and velocity relative to the star for the planet to fall through it and drop neatly into orbit. Congratulations, you are now a hermit planet. $\endgroup$ Commented Apr 24, 2015 at 15:50

I would create an artifical black hole whose hawking radiation power matched sun emission and the time it stays shining big enough to be economically viable. Unfortunatelly my math is not enough to calculate how much mass you'd need to do so.

  • $\begingroup$ Isaac Arthur does some good work on Youtube about this. $\endgroup$
    – Ash
    Commented Sep 11, 2017 at 14:01

Like others, I think [wave hands] gravity tech. will do. But you want details.

Put the machinery inside a spherical, one-way radiator. There is no gravity inside, so the machinery isn't crushed by its own weight. (It is inaccessible. This star contains no user-serviceable parts.) Also provide antigrav. bracing inside the radiator so it isn't crushed by the effective weight of the fuel above. (Maybe it is two-way: tractor outside, pressor inside. Maybe they know tricky ways to make gravitons spin, or something, so the two effects nearly balance and the net energy input is small for a star.)

The machine uses monstrous amounts of energy which just happen to be the amount of thermal radiation heading inward, so it stays cool. I suppose the radiator was sized to make this happen. No, I don't know how they achieve 100% conversion.

Sizing. Orbital period determines the altitude. Temperature of the photosphere determines the spectrum, which they will want to specify. I think temperature also determines the output per unit surface which, together with the desired level of insolation and the altitude, should tell you the necessary size of the photosphere. I have no idea what the math looks like for any of this.

Is it practical? What is the strength of tidal forces from an object with that surface gravity at that distance? How often do they have to drop another gigaton of H2 on it to keep it fueled? (How big is its fuel supply anyway? Where do they get it?) What are the safety margins you need in order to avoid roasting the planet due to a control hiccup? How much gamma and particle radiation does it deliver to the planet's surface?


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