I have a civilization that lives on a planet located inside a nebula (and orbiting a normal, sun-like star).

Would this nebula somehow interfere with/ obscure the sight of a rogue neutron star/ black hole coming their way? I'm hoping the answer is yes, so that by the time they notice it, it's too late.

If not, is there somewhere else I can place the planet that would more convincingly obscure the sight of the intruding black hole/neutron star?

Thank you!

  • $\begingroup$ Related: worldbuilding.stackexchange.com/q/149991/627, worldbuilding.stackexchange.com/q/15111/627 $\endgroup$
    – HDE 226868
    Commented Jul 28, 2019 at 21:08
  • $\begingroup$ I've been wondering about a similar issue. Emitting nebulae are dim from here but I'm not sure how it would be being in them - ie. how much would be obscured potentially, or rather swamped by the emissions. Haven't sat down with the data yet. +1 $\endgroup$ Commented Jul 28, 2019 at 21:13
  • $\begingroup$ Interesting question. Nebula's density is the key here. The denser the nebula, the brighter would be accretion process of the intruder, but the more of this light would be absorbed by the nebula. $\endgroup$
    – Alexander
    Commented Jul 29, 2019 at 4:51

1 Answer 1


Cool idea!

I have to think an incoming black hole would be hard to detect. A big one will give off minimal radiation because it is a black hole. A small one might give off Hawking radiation you could detect if you were looking in the right way. We could detect a rogue black hole by new gravitation lensing of stars on the far side of it; as that distant light passed near the incoming hole it would bend in an uncharacteristic way and the stars in that region of the sky would seem to have shifted position relative to one another. If your people live in a nebula, the more distant stars might be already obscured / refracted by the nebula so it would be harder for them to notice the difference.

Nebulae have stars inside them and those close stars will be easier for your people to see. If the black hole interposes itself between the planet and a star that star will seem to disappear. They might start to catch on that something was happening.

An incoming black hole will certainly suck in the gas of the nebula. If this is an emission nebula the concentration of glowing gas whirling down the drain of the black hole will make that area of the sky brighter and give away the presence of the hole. If the nebula is just dust this will be more difficult to notice. Even dust might glow as it heats up during its descent into the hole.

Once the black hole gets close enough to the system to start messing with orbits your people will definitely detect it. Things are going to get bad fast for them.

  • 1
    $\begingroup$ The BH might not have much in the way of an accretion disk. If the BH is moving rapidly through the nebula then only small portions of it may get attracted to the BH, and it might not form much of a disk. The material might get swirled fairly drastically, but get left behind. If it's moving slowly enough to suck up a disk then probably the observers have a very long time to wait before it gets there. $\endgroup$
    – puppetsock
    Commented Jul 29, 2019 at 13:32

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