Location: an interstellar gas cloud

Situation: an alien family stops for about half an Earth hour in the middle of the cloud to have a snack along their trip (by stops I mean they get null relative velocity with respect to the cloud's collective motion). The kids in the family, after playing for a while with a ball, the size of a tennis ball, filled with sand or lead, leave it next to their starship and forget it there when they leave.

Question: besides neglecting the "no litter" signs on the space highway, will this plausibly result in triggering the formation of a star?

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    $\begingroup$ This link may prove to be useful: abyss.uoregon.edu/~js/ast122/lectures/lec13.html $\endgroup$ – Nate Dukes Mar 6 '17 at 13:31
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    $\begingroup$ I have the impression that wake turbulence from their departure might have a much larger effect. $\endgroup$ – Burki Mar 6 '17 at 14:14
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    $\begingroup$ Are you sure it should be tagged reality check? $\endgroup$ – Mołot Mar 6 '17 at 15:45
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    $\begingroup$ I would tend to imagine you'd only ever "accidentally create a star" if what you were already doing was something like trying to make a black hole. "Woops, we didn't add enough mass fast enough, fusion has started and creating enough pressure to keep it from collapsing." $\endgroup$ – Shufflepants Mar 6 '17 at 21:41
  • $\begingroup$ Oops, I did it again, I played with some mass, got lost in the space, oh baby baby.. [Britney Stars] $\endgroup$ – frarugi87 Mar 7 '17 at 11:13

Either a star was about to form or it wasn't

A star forms when a sufficiently large mass of hydrogen and other interstellar media collects in a sufficiently small volume. Once this happens, the inexorable pull of gravity will cause a swirling proto-stellar disk, and once densities and temperatures are high enough, fusion ignition and a true star.

The mass required is something on the order of 1e30 kg. You ball is not going to have much of an effect, unless your aliens are playing with a ball the size of Jupiter.

Your tennis ball is insignificant compared to the power of the force (of gravity of the interstellar cloud)

Lets say your tennis ball is 0.1m across and 1kg (not really a tennis ball, I guess). The standard gravitational parameter $GM$ for this ball is $6.674\times10^{-11} \text{ m}^3\text{s}^{-2}$. If you multiply this by the mass of a nearby object and the distance between them, you get the force exterted on this object. Lets say you have a microscopic spec of space dust, 1e-6 kg at 1 meter away. The force on this speck from the tennis ball is $6.674\times10^{-17} \text{N}$, and the acceleration is $6.674\times10^{-11} \text{ ms}^{-2}$. At this acceleration it will take two days to pull the speck to the tennis ball, if nothing else is affecting it. At 10 meters it takes about 63 days.

But surely, the speck has some sort of velocity of its own. It is moving in whatever direction relative to the tennis ball, and the tennis ball's gravity barely affects it. In fact, the tennis ball is more likely to attract another particle through static electric charge than through gravity. It is the electric charge of small specks of space dust that probably formed the seed for planet and star formation.


You tennis ball will not have a major effect on the cloud. If it was going to form a star, then it still will in a few million years. If it was not going to, then your tennis ball is now part of the mysterious dark matter. Dark matter as space litter? So that's why they have all those signs on the highway!

  • $\begingroup$ I'm not trusting my math right now, but presumably the gravitational inertia from a spaceship parked in the cloud for a long time would overcome any effects of a little ball staying behind? $\endgroup$ – Black Mar 7 '17 at 0:42
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    $\begingroup$ I suspect that the OP may have been assuming that the tennis ball isn't contributing mass so much as creating a dense point that matter would start to condense towards. Pointing out that the cloud is already going to be drawn towards its center of mass, even if there's no actual matter at that point, would address that (potential) misunderstanding. $\endgroup$ – Ray Mar 7 '17 at 1:24
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    $\begingroup$ Maybe it was the parents' spaceship's departure that caused the star to collapse. Of course they'll blame the kids anyways. $\endgroup$ – John Dvorak Mar 7 '17 at 1:24

Trigger - probably not that easily.

Make it faster - yes.

If the gas cloud was massive enough to eventually form a star, more massive object might start attracting particles, and be nucleus of star formation, making it happen much earlier, removing chance and need for fluctuations.

I would go for engine. If they move using gravity, their engine might, during its start up, create quite strong gravitational field that would attract particles from a large volume. and then, instead of missing each other and flying away, particles would instead hit a ball.

Details needs to be figured out when you will decide about their propulsion.


It's not going to make much difference.

Nebula are many, many times larger than a star and many, many times more massive. If a nebula has the potential to form a star at all, it will have already formed "clumps" of denser gas, and these areas will be the places that attract more clumps together. If it isn't going to form a star, the ball isn't going to push it over the edge.

Think of it this way: if the ball was completely vaporized, how big would the cloud it forms be? Maybe the size of a large room? Practically nothing next to the rest of the nebula. The gravity from the ball as a solid isn't any higher than the gravity of the ball as a gas, and will have no more effect.

However, if the family is from a highly advanced civilization with a ship that moves at a significant percentage of light speed, the ship can potentially be leaving a high-energy wake behind it. This could compress areas of the cloud more than they would be otherwise, and may trigger star formation.


No. Consider that besides gas and fine dust, there are already bodies of asteroid, planetismal, planet, and brown-dwarf size floating around. So even if the ball, as kingledion speculates, is the size of Jupiter, it will not make a difference.

If this is a star-forming region, it is millions of times larger than a single star’s portion of that. Your peturbation might be like the butterfly that causes a hurricane—any change to a chaotic system substantially alters the results. So maybe this patch would have wound up forming a star with planets, but because of the change, one impact happened a little differently.


Reality check??? The premise of the question is totally divorced from reality. There is no reason whatsoever to stop the spaceship in the middle of nowhere to "have lunch". Assuming the aliens craft operates on known physics, it will take a huge amount of fuel to accelerate or decelerate to the relativistic speeds required to go anywhere quickly. Why would you waste fuel by stopping and starting up again? If the aliens don't want to be crushed with huge g forces then accelerating or decelerating to those sort of speeds will take months.

And NO a small lump of rock, floating in a gas cloud with many others could not possibly create a star, unless you are considering the butterfly effect, in which case it will collapse into a star as if it was going to happen anyway, and the effect is just as likely to stop a star forming. (in the same way that you breathing now could cause or stop a hurricane next year because the weather is very sensitive to tiny changes, but without vast computers and data on every air current across the world there is no way of telling what would have happened otherwise.)

The vast plume of hot exhaust gas could perhaps trigger star formation to happen slightly differently but I would need data on the engine and gas cloud to calculate that.(only of you have a large ship), Heat more likely to stop gas cloud collapsing.

  • $\begingroup$ Awww be nice. Could be jellyfish aliens who wanted to sight-see a dust cloud. No current power source provides what is needed so they could presumably have more than enough. And if they're jelly in water then gravitational gradient wouldn't hurt them. $\endgroup$ – Black Mar 7 '17 at 0:45
  • $\begingroup$ @Black I know right, where does all this hatred towards jellyfish aliens comes from (my best guess is sexual frustration)? If they want to stop in the middle of a gas cloud, then power to them. Lunch is delicious and gas clouds are pretty, it thus seems perfectly plausible to want to have lunch in the middle of a gas cloud. $\endgroup$ – AngelPray Mar 7 '17 at 3:54

Static electricity from the ball starts to attract matter from the gas cloud....over time that mass is big enough to attract more matter through gravity. In some millions years maybe the mass is so big that starts nuclear fusion of Hydrogen and becomes a star...

  • $\begingroup$ But is that plausible? All this does is add a suggestion of static being involved and repeat the OP’s scenareo. $\endgroup$ – JDługosz Mar 6 '17 at 19:29
  • $\begingroup$ @JDługosz You know we are talking about aliens playing with tennis balls while on pic nic, right? $\endgroup$ – papakias Mar 6 '17 at 20:47
  • $\begingroup$ Yes. How does that make repeating the OP’s scenereo a proper answer? $\endgroup$ – JDługosz Mar 6 '17 at 23:56
  • $\begingroup$ @JDługosz I did not repeat the OP's scenario. Sorry. If you have a problem with my answer just downvote it. $\endgroup$ – papakias Mar 7 '17 at 8:23

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