1. Naked eye planetary nebula?

If I were on a planet one light year from a young planetary nebula, would I be able to see it with my naked eye?

2. You betcha!

The Orion Nebula is 1,350 light years away, and I can see it with my naked half-century old eyes.

(It doesn't hurt that it's 24 light years across).

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Taking Hen 1357 as an example (https://en.m.wikipedia.org/wiki/Stingray_Nebula) an absolute magnitude of -3 suggests yes, easily.

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Wiki to the Rescue

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Originally Posted by John Mendenhall
Not there, not expressly.
And Orion Nebula is not a planetary one.

Selecting the brightest planetary nebulae, from https://en.wikipedia.org/wiki/List_of_planetary_nebulae - takes some rearranging, AND looking up data about specifics, missing in the table
And then standardizing to the distance, not of 1 but to 80 ly (not a random number!)

Bug +7,1; 3400ly; 3'; 1888; -1; 127'
Dumbbell +7,5; 1360 ly; 8'x6'; 1764; +1,4; 136'x102'
Helix +7,6; 680 ly; 25'; 1824; +3,0; 212'
Skull +8,0; 1600 ly; 4'; 1785; +1,5; 80'
Saturn +8,0; 3000ly; 41''x35''; 1782; +0,1; 25'x22'
NGC 6572 +8,1; 2500ly; 6''; 1825; +0,6; 3'
NGC3918 +8,5; 4900ly; 10''x8''; 1834; -0,4; 10'x8'
Ghost of Jupiter +8,6; 1400ly; 25''; 1785; +2,4; 7'
Blue Snowball +8,6; 5600ly; 37''; 1784; -0,6; 43'
Blinking Planetary +8,8; 2000ly; 27''x24''; +1,8; 11'x10'
Ring +9,0; 2300ly; 1,5'x1'; 1779; +1,7; 43'x29'

Of the other brighter nebulae:
Orion (diffuse) +4; 1300ly; 65'x60'
Carina (diffuse) +1; 7000ly;
Praesepe (open) +3,7; 600ly; 95'
Omega Centauri (globular) +3,9; 16000ly; 36'
47 Tucanae (globular) +4,1; 13000ly; 31'
Andromeda (galaxy) +3,4; 2500000ly; 190'x60'
Small Magellanic Cloud (galaxy) +2,7; 200000ly; 320'x185'
Large Magellanic Cloud (galaxy) +0,9; 160000ly; 650'x550'
Last edited by chornedsnorkack; 2018-Oct-05 at 03:53 PM.

6. The surface brightness is the key for this question. Any planetary or other emission nebula that is easily seen in a telescope would be easily visible to the unaided eye from up close.

7. I don't think any but the very recently formed planetary nebulae would look visually interesting to the eye. Using a non-planetary examples, you can see M31 naked eye, but all you see is the little blob in the center, and none of the structure, and that's 10-100 times as bright as the brightest plantaries. If you were 10 times closer to the Ring Nebula in Lyra (M57), it would still only be 3rd magnitude, but it would be spread over part of the sky bigger than a full Moon. That would be the sort of thing that would take some convincing that you'd actually noticed it, but would look GREAT in a long exposure with your camera.

8. I took the data provided by rhymeswithorange here, for the 100 brightest planetary nebulae. They're open to considerable debate (see the comments in the rest of the thread on the forum in my link), but hopefully provide some sort of ballpark. I assumed they were all circular patches to calculate angular area.
Crunching to SI units, I get a wide range of luminances from 1 lx/sr to 0.0001 lx/sr. I imagine this relates the age of the nebula, but I haven't checked.
The highest luminance nebulae in the dataset therefore have surface brightnesses less than a thousandth of the lunar disc - comparable to looking at a sheet of white paper under street lights at night. So they'd stand out pretty well, but not stunningly bright.
The lowest luminance nebulae in the dataset have surface brightnesses comparable to that of the night sky itself, under excellent seeing conditions. So we're talking about something at the level of zodiacal light. If these are old, diffuse objects then picking out an edge might be problematic, and they'd tend to fade into the background.

ETA: For those more used to thinking in magnitudes/square asec, the range of surface brightness above is approximately 22 to 12 visual mags per square asec.

ETA2: Another popular measure of surface brightness is S10 (vis), which is tenth magnitudes per square degree. We've got 180 to 1.5 million on that scale. The lower end is sitting about the level of the gegenschein.

Grant Hutchison
Last edited by grant hutchison; 2018-Oct-05 at 03:38 PM.

9. Originally Posted by chornedsnorkack
And Orion Nebula is not a planetary one.
It isn't?
Oops.

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Originally Posted by grant hutchison
I took the data provided by rhymeswithorange here, for the 100 brightest planetary nebulae. They're open to considerable debate (see the comments in the rest of the thread on the forum in my link), but hopefully provide some sort of ballpark.
The upper left part of that table matches to what I compiled - my source was Wikipedia.
Combining the comparable data: first mine from Wikipedia, then from rhymeswithorange
We both missed, for common reasons, the 2 brightest:
1) Sh2-216
2) Hewett 1
Then to ours:
3) Bug +7,1/+12; 180''/45''
4) Dumbbell +7,5/+7,5; 480''x360''/330''
5) Helix +7,6/+7; 1500''/900''
6) Skull +8,0/+8,5; 240''/225''
7) Saturn +8,0/+8,0; 41''x35''/28''
8) Blue Raquetball +8,1/+8,5; 6''/14''
9) Blue Planetary +8,5/+8,0; 10''x8''/16''
10) Ghost of Jupiter +8,6/+9,5; 25''/40''
11) Blue Snowball +8,6/+9,0; 37''/20''
13) Ring +9,0/+9,0; 90''x60''/70''
On to the ones I missed or excluded:
14) Eight-Burst Planetary +8,5; 45''
15) Cat´s Eye +8,5; 20''
16) Turtle +9,0; 16''
17) Magic Carpet +9,0; 14''
Last edited by chornedsnorkack; 2018-Oct-05 at 04:49 PM.

11. I wonder if there's a listing by the general range of surface brightness given for each of these. If you use their apparent magnitude and divide by the area to get average surface brightness, I don't think it will be that useful since planetary nebulae have pockets of brighter regions. The Eskimo, for instance, doesn't have a high average surface brightness but it's the first object I recall seeing that had enough surface brightness in its ring to appear distinctly blue, more so to my son.

12. An important technical point is that for an extended object, a telescope will never make the surface brightness greater than what we would get by moving enough closer to see the same angular size as with the scope from Earth. For me the Ring Nebula stands out boldly in a dark sky with a telescope, so it would do likewise from up close with the naked eye.

13. Originally Posted by Hornblower
An important technical point is that for an extended object, a telescope will never make the surface brightness greater than what we would get by moving enough closer to see the same angular size as with the scope from Earth. For me the Ring Nebula stands out boldly in a dark sky with a telescope, so it would do likewise from up close with the naked eye.
I think this is one of the coolest application of the inverse square law. It's not intuitive since looking through a monster scope one would expect to see greater surface brightness for extending objects; it's not only unlikely but impossible.

[Due to how the retinex works, however, there is a slight apparent gain in surface brightness as the object is magnified optimally.]

14. Originally Posted by George
I think this is one of the coolest application of the inverse square law. It's not intuitive since looking through a monster scope one would expect to see greater surface brightness for extending objects; it's not only unlikely but impossible.

[Due to how the retinex works, however, there is a slight apparent gain in surface brightness as the object is magnified optimally.]
My bold. The key point is that we get the same gain by moving closer by the same factor as the telescope's magnification. With the scope of our dreams, with richest-field magnification and no light loss in the optics, the view will be the same. In practice the close-up naked eye view will be a bit brighter.

15. Originally Posted by Hornblower
My bold. The key point is that we get the same gain by moving closer by the same factor as the telescope's magnification. With the scope of our dreams, with richest-field magnification and no light loss in the optics, the view will be the same. In practice the close-up naked eye view will be a bit brighter.
I have read more than once, but still rarely, that there is a gain effect if you maintain the flux density while increasing the area on the retina. Perhaps there is an efficiency improvement when more rods and cones become active. [I think I first saw this in a excerpt from Visual Astronomy (Roger N. Clark), but this was years ago.] Is this wrong?

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Looking just at the big mismatches - wondering how to resolve them:
Originally Posted by chornedsnorkack
5) Helix +7,6/+7; 1500''/900''
7) Saturn +8,0/+8,0; 41''x35''/28''
8) Blue Raquetball +8,1/+8,5; 6''/14''
9) Blue Planetary +8,5/+8,0; 10''x8''/16''
10) Ghost of Jupiter +8,6/+9,5; 25''/40''
11) Blue Snowball +8,6/+9,0; 37''/20''
And now to the nebulae magnified to comparable size:
Large Magellanic Cloud 1x; 160 000 ly; 39000''x33000''; +0,9
Small Magellanic Cloud 2x; 100 000 ly; 38000''x22000''; +1,2
Andromeda Nebula 3,5x; 700 000 ly; 40000''x12600''; +0,7
47 Tucanae 20x; 650 ly; 37000''; -2,4
Omega Centauri 17x; 940 ly; 37000''; -2,3
Praesepe 7x; 85 ly; 40000''; -0,5
Orion 10x; 130 ly; 39000''x36000''; -1

Dumbbell 80x; 17 ly; 38000''x29000''; -2
Skull 160x; 10 ly; 36000''; -3
Ring 400x; 5,75 ly; 36000''x24000''; -4
Last edited by chornedsnorkack; 2018-Oct-05 at 07:54 PM.

17. Originally Posted by chornedsnorkack
Looking just at the big mismatches - wondering how to resolve them:
What is mismatched? What are you wishing to resolve?

18. Originally Posted by George
I have read more than once, but still rarely, that there is a gain effect if you maintain the flux density while increasing the area on the retina. Perhaps there is an efficiency improvement when more rods and cones become active. [I think I first saw this in a excerpt from Visual Astronomy (Roger N. Clark), but this was years ago.] Is this wrong?
No, it's right. With some faint fuzzies you can gain in spite of decreasing the flux by increasing the magnification while keeping the same aperture. With more of the retina in play, the brain acknowledges things it ignores if they are smaller. I have seen it with faint galaxies and nebulae.

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Originally Posted by Hornblower
What is mismatched? What are you wishing to resolve?
Different data for angular size and magnitude between Wikipedia and rhymeswithorange.

For Helix, fortunately Wikipedia has details: outer-most ring 25', outer torus 22'x12', inner disc 19'x8'.
Rhymeswithorange´s 15' may exclude outer-most ring.

Then as enlarged to the same standard:
Helix 25x; 28 ly; 37500''; +0,6

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