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Where are all the nebulae located?

Yes, a simple question that should be obvious but apparently is not to me.

To be more explicit, how far are all these objects? Are they mostly in our galaxy?

I'm not suggesting that the Milky Way galaxy is the only galaxy that has nebulae, but that the ones we have pictures of are only within the Milky Way...if that is indeed the case.

This may also apply to stars themselves. What is the radius in ly of discernibility of (most) stars? Is it beyond the distance to the center of our galaxy (where even in principle individual stars might be discernible but there's too much of a mess to differentiate)?

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  • $\begingroup$ If we know the distance and the position in the night sky and they are in our galaxy, in addition to the earth-centric constellation locations, some quasi-polar galactic coordinates? Distance from center of the galaxy, distance above plane of the galaxy, angle from line through center and our solar system? $\endgroup$ – Mitch Harris Oct 24 '18 at 13:52
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1) The nebulae are spread out over the universe in many different galaxies. Nebulae called extra-galactic nebulae are nebulae outside of our galaxy. They aren't common, but we can see them.

2) Most of the ones we can see, such as the Helix nebula or the Orion nebula, are in our galaxy, but the ones outside of our galaxy are just more harder to see - there is a same dispersion of nebulae in our galaxy than in other galaxies.

3) Credit to @AtmosphericPrisonEscape (thanks!) - many nebulae outside of our galaxy are too dim for us to see. I assume that since the Tarantula Nebula is so massive and prominent, it is able to be visible from such a far distance.

For instance, NGC 2070, or the Tarantula nebula, located in the LMC. (Link: Here). The LMC is rather close to us, and the nebula is around 160,000 light years away.

I really can't find other major nebulae that are outside of our galaxy, but there are a lot of research papers on them, so they are visible - just not as prominent as the ones in our galaxy.

Some research papers with extra galactic nebulae include: Here, Here, Here, and there are many more online.

I don't understand the 2nd part of your question with the coordinates, but I hope this at least helps!

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    $\begingroup$ The wording of this indicates that nebula themselves are "way rarer" outside our galaxy than inside. What you probably really meant, and what's accurate, is that it's difficult for us to see nebula which are as far away as other galaxies (compare nebula size/luminosity vs. galaxy size/luminosity). Thus, we know of very few outside of our galaxy, just because it's hard to see them, not necessarily, because they aren't there. In some galaxies they will be more common than here and in others they will be rarer. However, it's likely that they exist in significant quantity in all galaxies. $\endgroup$ – Makyen Oct 24 '18 at 8:42
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    $\begingroup$ Re the 2nd para (which should probably only be a comment), I feel like we know enough to give location info of these nebulae with respect to our galaxy (angle with respect to our solar system, distance in ly from center, distance in ly above plane). Does that make sense now? $\endgroup$ – Mitch Harris Oct 24 '18 at 13:46
  • $\begingroup$ Oops, I must have phrased that weirdly - yeah, there's a same amount of nebulae dispersed throughout the universe, it's just that we can't see them as well as the ones in our galaxy. I edited my answer to clarify. $\endgroup$ – MystaryPi Oct 24 '18 at 21:06
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    $\begingroup$ Galactic coordinates which would presumably put the Crab Nebula (in Taurus) somewhere around ... looks at pictures ... 180degrees (opposite Sagittarius/center of the galaxy) and a little farther out from the center past the Perseus arm. $\endgroup$ – Mitch Harris Oct 24 '18 at 21:06
  • $\begingroup$ Oh! That makes way more sense. Thanks for the clarification. I can't math at all so I hope someone else can have a chance to answer that... $\endgroup$ – MystaryPi Oct 24 '18 at 21:10
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Firstly you have to specify what you mean by nebulae.
There is a historical term that some of @MystaryPi's links refer to, "extragalactic nebulae". An extragalactic nebula only existed until around the 1930s, as it was then realised, that extragalactic nebulae are not nebulae but galaxies like our milky way.
The classical papers by Hubble and Zwicky that were linked in the other answer were part of establishing this fact. This is why until today, misleadingly, galaxies are sometimes referred to as nebulae.

A planetary nebula (which is an ejected shell of a late-type star) wouldn't be visible in another galaxy neither with the eye nor large telescopes and long-time exposure. They simply posses a much too low surface brightness.

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  • $\begingroup$ There's always a difficulty with definitions of terms. I am talking about nebulae that aren't galaxies (which I think you answered well). By the same token, aren't most stars that are distinctly visible only within our galaxy? Yes, some are discernible in nearby galaxies, but, except for a few exceptions, there's no labeling individual stars outside of or galaxy. Also even outside of our own neighborhood of _this galaxy? (this side of the center, maybe within a distance of 20kly?) $\endgroup$ – Mitch Harris Oct 24 '18 at 13:42
  • $\begingroup$ @MitchHarris: With your naked eye, most stars visible are even only in the local galactic neighbourhood. I've done a quick estimate for the most luminous supergiants with Luminosities of $L \approx 4e5 L_{\odot}$, via naked eye those would be visible to a maximum distance of $d_{max} \approx 8 kpc$, which is our distance to the galactic center. If you want to discern single stars in nearby galaxies, only Hubble-quality angular resolution and sensitivity will help. $\endgroup$ – AtmosphericPrisonEscape Oct 24 '18 at 14:19

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