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I'd like to be able to see from my Earth-like planet, a very bright nebula that can even be seen during the daytime.

I question, however, whether that is possible, because in order for it to be contained in our viewing, it would have to be so far away as we wouldn't see it (since they're hundreds of light-years in size).

Would a nebula close enough for us to see impact our Solar System in any way?

I think planetary nebulae are too short-lived to serve my purpose.

Can I have a sky like this?

enter image description here

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  • $\begingroup$ Does this answer the question? worldbuilding.stackexchange.com/questions/4875/… $\endgroup$
    – Tim B
    Oct 15, 2015 at 18:56
  • $\begingroup$ Thanks @TimB, but it doesn't quite answer the question. I'm more interested in the properties of the nebula (will one that close have gravitational impacts on us, would we be able to see one that size in the night sky, or would it be too far away, because they're so huge, etc.) $\endgroup$
    – Mikey
    Oct 15, 2015 at 19:00
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    $\begingroup$ @TimB I think that question is asking about a planet within the nebula and this one is asking about a planet outside that can see the nebula. Mikey, is that right? $\endgroup$
    – Monica Cellio
    Oct 15, 2015 at 19:04
  • $\begingroup$ @MonicaCellio correct; there is some useful information in the other question's answers, but doesn't quite get to what I'm asking. $\endgroup$
    – Mikey
    Oct 15, 2015 at 19:05
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    $\begingroup$ @Green ~10,000 years. $\endgroup$
    – Mikey
    Oct 15, 2015 at 21:39

2 Answers 2

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I asked How can I safely brighten my secondary star? because I was looking for a bright night, though not specifically a nebula, for my habitable planet. This answer suggested placing a star in a reflection nebula, which could be as close as 20AU to the primary star that the planet is orbiting. Nebulas are usually huge, but there are some small ones, as small as 1AU across.

So, you can have a star inside a tiny nebula (by nebula standards) in your (binary-star) system, visible from a habitable planet orbiting the other star. I do not know whether you could see a nebula that doesn't also contain a star.

This answer owes a lot to prior work by HDE 226868.

(See also: this post on the Worldbuilding blog.)

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  • $\begingroup$ I do not know whether you could see a nebula that doesn't also contain a star. - I'd bet that this is possible. If the central object in a nebula interacted with other objects, it could be ejected, leaving nothing behind. I'm not sure what that would say about the system's stability, though. You'd probably be in trouble. :-) $\endgroup$
    – HDE 226868 ♦
    Oct 15, 2015 at 23:57
  • $\begingroup$ @HDE226868 I was wondering if it would still be bright enough. $\endgroup$
    – Monica Cellio
    Oct 16, 2015 at 2:40
  • $\begingroup$ Most likely not, although perhaps the remaining gas could emit some radiation. I can't give you a definite answer on that, though. $\endgroup$
    – HDE 226868 ♦
    Oct 16, 2015 at 14:57
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Best bet: Closer nebula with a strong power source such as a black hole x-ray stream or a near quasar.

Assuming an earth like earth, it will take a nebula with an apparent magnitude of -4 or larger to be seen with the naked human eye in the daytime. For comparison, our sun has a magnitude of -26 compared to Venus at -4. SN 1054 had a magnitude of -6!

Brightness

The farther away the nebula, the more energetic the power source to make it glow and the larger the nebula. For distant nebula (hundreds or thousands of light years), supernovas are plenty energetic enough to provide illumination though they only shine for a few brief weeks or months (SN 1054 shown for two years). Perhaps a nearer nebula (at tens of light years range) illuminated by the x-ray jet of a nearby black hole or a stellar nursery of supergiant stars would do the trick.

Size

For comparison, the Moon is about 1/2 a degree wide and we can see it quite well. Using an apparent size calculator we can get an idea of how large something will appear from a given distance. Since the units don't matter, only the ratios, we can plug in a size of 10 (light years) and a distance of 100 (light years) and get an apparent size of 5.7248 Degrees or about 11 times the width of the moon.

Other references

A similar concept to your nebula is the Eye of Terror in the Warhammer 40K universe. The Eye of Terror is some 20,000 light year across.

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  • $\begingroup$ Would a nebula that is 100s of light years across look that small (my bad photoshop img) if it were only tens of light years away, though? I would imagine it would engulf the sky if it were so close. $\endgroup$
    – Mikey
    Oct 15, 2015 at 21:43
  • $\begingroup$ Apparent Size Calculator is really handy. Since the units don't matter, you can just plug in ratios of nebula size to distance. $\endgroup$
    – Green
    Oct 15, 2015 at 21:48
  • $\begingroup$ thanks, then I will see if it's possible that a nebula of 100s of light years can be x size in the sky at y distance, but still have z brightness. $\endgroup$
    – Mikey
    Oct 15, 2015 at 21:49
  • $\begingroup$ I know that brightness will be determined by the inverse-square law but I don't have the math or physics to calculate whether a quasar illuminated nebula will be bright enough to see. Sorry. I think if you got the distance and size right, the brightness could be handwaved a bit. (just my opinion though). $\endgroup$
    – Green
    Oct 15, 2015 at 21:55
  • $\begingroup$ A couple of notes here (sorry if it seems overtly critical!). Quasars are massive and form due to supermassive black holes, and are extremely large and powerful relative to nebulae, meaning that they would be poor sources of illumination. Also, jets form quasars typically point out of the galactic plane, so you'd need some weird alignment for this to work. The other thing, as Monica Cellio's answer referencing an answer of mine mentioned, is that the formula for apparent size of a nebula might be changed depending on the type of nebula. $\endgroup$
    – HDE 226868 ♦
    Oct 15, 2015 at 23:53

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