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Specifically, how does a civilization tell if their home planet - on which they have always resided - is orbiting a star that is inside a nebula? The way we know that we're not in a nebula is because we're not, and we can see what nebulae are - giant clouds of gas in space. But when I think about it (maybe it's just me), a civilization that evolved and grew inside a nebula without ever sending a ship outside of it would simply think of space as full of diffuse gas, with dark matter not only driving the strange behavior of galaxy rotation and universal expansion but also preventing the collapse of the Universe under its own immense weight (since the whole Universe being filled with diffuse gas greatly increases its mass).

So, how do civilizations who are born in nebulae determine that they are inside nebulae, and that not all space is filled with gas? The civilization I'm working with knows this, and not being able to pass the boundary of the nebula is part of the Major Handwavium Problem of the story, but I'm not quite clear on how one can scientifically and unequivocally determine that the nebula exists in the first place.

Details about the nebula:

  • Composition: 89% hydrogen, 6% helium, 1% carbon, 1% nitrogen, 1% oxygen, 2% iron and nickel (excuse the possible-anomalous composition, the star that formed the nebula was very strange)
  • Temperature: ~7,000 K in most places, since the supernova happened a while ago
  • Density: ~700 particles per cubic centimeter
  • Approximate size: 22 light-years in diameter, roughly spherical, with the civilization close to the center
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  • $\begingroup$ I'm not dead certain, but I think it may depend on the composition and temperature of the nebula (thickness/density too) too..... Is there any way you could be more specific in your description? $\endgroup$
    – Escaped dental patient.
    Commented Jun 21 at 22:07
  • $\begingroup$ Yes, sorry, that has been added to the question $\endgroup$
    – controlgroup
    Commented Jun 21 at 22:16
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    $\begingroup$ I suppose that you mean a diffuse nebula, such as the Orion Nebula. They are ephemeral objects, exactly because they are "giant clouds of gas in space", that is, raw material for the formation of stars. By the time planets form, cool down, and get a chance to develop even the most primitive kind of life, the nebula will have been long gone. (The alternative is a planetary nebula, but this is an extremely ephemeral object, spreading out into nothingness very quickly. And they are remnants of supernovas, so no life anywhere near.) $\endgroup$
    – AlexP
    Commented Jun 21 at 22:32
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    $\begingroup$ @AlexP given that controlgroup has specified that the star that formed the nebula was very strange, perhaps there was a significant closing velocity between the supernova-that-is-now-a-nebula and the solar system that is now within it, so the solar system was outside the destruction radius but has now entered. Could be an interesting separate question to look at the minimum safe distance from supernova vs requirement to enter nebula while it still exists and work out the minimum velocity and therefore maximum duration of solar system being inside the nebula. $\endgroup$
    – KerrAvon2055
    Commented Jun 22 at 5:01
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    $\begingroup$ If a nebula is a cloud of dust, Archimedes' principle comes into play: in addition to the gravitational pull of its primary, your home planet will experience a buoyant force opposing that pull. The determination of Newton's constant from local astrometry will not carry over to distant galaxies. $\endgroup$
    – François Jurain
    Commented Jun 22 at 11:19

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It's an interesting question. I think they'd first most likely notice differences in absorption between stars within the nebula and stars outside the nebula. In our local stellar neighborhood, the mean number density of stars is $n\sim0.1$ pc $^{-3}$. If the nebula is 22 light-years in diameter, there should be somewhere around 500 stars within it. That's enough to do a statistical analysis and notice some patterns.

In particular, stars outside the nebula would experience way less extinction than you'd expect compared to stars within the nebula. It's pretty easy to approximate the column density of neutral hydrogen based on measurements of extinction at visible wavelengths; the civilization would realize that they were in an overdense region.

More mathematically: In optical light, we can approximate the extinction observed affecting a star as $$A_V\approx C_V\langle n\rangle d$$ where $C_V$ is a constant, $\langle n\rangle$ is the number density of hydrogen, and $d$ is the distance to the star. You can empirically estimate $C_V\langle n\rangle$ by measuring the distances to a bunch of stars and the extinctions. What the civilization would eventually notice is that $C_V\langle n\rangle$ would appear to be much higher for stars within $\sim$11 pc than for stars beyond $\sim$11 pc.

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    $\begingroup$ So in effect the thought process would go from "hey, all the nearest stars seem to have their light scattered or absorbed more than the ones outside this distance" to "there must be some sort of material suffusing the nearby area that blocks their light" to "we live in a nebula which is that cloud of gas"? $\endgroup$
    – controlgroup
    Commented Jun 21 at 23:02
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    $\begingroup$ @controlgroup Yeah, that's basically the idea. $\endgroup$
    – HDE 226868 ♦
    Commented Jun 21 at 23:57
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    $\begingroup$ Also, they can see other nebulae outside their own, so the idea won't be strange once their astronomy has developed a bit. $\endgroup$
    – John Dallman
    Commented Jun 22 at 16:43
  • $\begingroup$ @JohnDallman Yep. I think a good parallel -- though in reverse order -- might be the advent of stellar spectroscopy; Fraunhofer found spectral lines in both solar spectra and the spectra of other stars. $\endgroup$
    – HDE 226868 ♦
    Commented Jun 23 at 13:41
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    $\begingroup$ @controlgroup The nearby stars would not have more scattering/absorption than the faraway stars, but it would be very similar which would be surprising. Plot stars on a graph: x-axis, distance from us; y-axis, extinction. The stars in the nebula, the nearby stars, would lie on a very obvious line (the further, the more scattered the light), but the stars much further away would surprisingly not continue the same pattern; their light is barely any more scattered than the stars at the edge of the nebula. $\endgroup$
    – minseong
    Commented Jun 23 at 14:34

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