# Thread: Looks of a close planetary nebula?

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## Looks of a close planetary nebula?

No planetary nebula in sky is bright enough to be seen by naked eye (the brightest is Butterfly, at +7,1).

But suppose that a planetary nebula were located really near Sun - a few lightyears or less.

Say, Ring Nebula. Magnitude +9, contains a star magnitude +15,75, at roughly 5000 lightyears distance.

Now imagine the Ring from distance of 5 lightyears. 15 magnitudes brighter. At -6, the Ring would outshine the Milky Way and cast light on moonless night. But if its radius is just 1,3 lightyears, it would still be being viewed from outside. The central star, at +0.75, would be a prominent star but dimmer than Toliman at the same distance.

How much of the structure of the nebula would be visible for an observer at a few radii?

And what would the nebula look like for an observer actually inside the nebula? Say, the Sun at 1 lightyear from the nebula? The central star should be -2,75, outshining all stars and planets save Venus, but how does the nebula look like inside?

2. You need to remember that the surface brightness of an object does not change with distance. The object appears bigger, but the same amount of light is spread over the larger area so surface brightness (magnitudes per arcsecond) does not change.

Having said that, our eyes can detect largish patches of very faint light provided they have a reasonably clear boundary set against a dark sky.

The mean surface brightness of M57 is about 18. Probably the brighter parts of the ring are around 14 or 15. The surface brightness of a reasonably dark, moonless sky is more than 20 so we would be able to see the ring quite clearly against a dark sky. Sadly our eyes do not see colour when the light is faint so we would only see it as a pale white ring.

At 1000th of the distance, the visual diameter of M57 would be about 64 degrees - i.e. it would occupy a large patch of sky.

The brighter areas of the milky way have a surface brightness of 6-10. So M57 would not outshine the bits of the MW we normally see with the naked eye.

If we were located inside the nebula it may be quite difficult to see anything as it would occupy the whole sky and contribute to diffuse "sky-glow".

3. ^
Pretty much what he said; though it would indeed be much brighter, our eyes aren't sensitive to the wavelengths that make M 57 stunning in photographs. Probably the best we could expect would be a ghostly greenish glow, from oxygen ion emission.

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What is the unit of surface brightness? Magnitude per what?

5. As has been posted earlier, in general, though surface brightness would not increase we would see more details over the larger closer nebula than through a telescope from very far away. We experience this when we're fortunate enough to view famous nebulae as well as our galaxy, the Milky Way from a remote location with clear, dark skies. Also, some nebulae, with young stars forming inside them, might appear brighter overall than others if we were in a different position and closer to the bright light sources within.

In some cases, such as the spectacular Orion Nebula, we might be able to faintly see some of the colors though surface brightness would not increase, because we would be closer to the bright forming stars inside and the excited gasses nearby.

There are photographs of nebulae wherein the colors do not represent the chemistry of different gasses but are shown as the human eye would see them even if fainter than a camera's time exposure which can collect the light over time. Mostly though we'd see more detail but at the same level of brightness.

A curious effect might also occur with some planetary nebulae. The nebula would mostly vanish to the naked eye if we were to travel very close and even into it. If we were hypothetically in the forward lounge of a Starship, and entered a nebula, it is so vast and tenuous compared to our tiny Starship that it would disappear and only its distant regions could still be seen. Ironically, it might be better to view such a nebula from a greater distance.

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Surface brightness does not change with distance, but it does change with direction.

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Originally Posted by chornedsnorkack
What is the unit of surface brightness? Magnitude per what?
Magnitude / arcsecond^2

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Originally Posted by jniemann
Magnitude / arcsecond^2
That is not very interesting, because eye cannot resolve an arcsecond.

An arcminute is another matter. But since a square arcminute is 3600 arcseconds, it is nearly 9 magnitudes brighter. If a square arcsecond of Milky Way is +6, a square arcminute is -3, and is a point source of light outshining Mars and Jupiter.

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As the distance decreases, the apparent brightness increases according to an inverse square law.

However, at the same time, the angular area of the object increases in the same manner, so the surface brightness stays the same.

The end result is that the object looks bigger, and its overall brightness is indeed increased, but its total brightness is spread out over the larger area.

10. Ring nebulae are enormously energetic events.
If one were within a few light years, would we be here?

John

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Originally Posted by JohnD
Ring nebulae are enormously energetic events.
If one were within a few light years, would we be here?
Just how energetic?

At magnitude -6, it would be a hundred million times dimmer than Sun - dimmer than full moon.

12. I just looked at M57 with my Celestron 8. It was clearly visible, though subdued by the severe light pollution here in greater Washington. If we were close enough for it to fill half the sky, and in good viewing conditions, it would be stupendous.

13. Originally Posted by TonyE
You need to remember that the surface brightness of an object does not change with distance. The object appears bigger, but the same amount of light is spread over the larger area so surface brightness (magnitudes per arcsecond) does not change.

Having said that, our eyes can detect largish patches of very faint light provided they have a reasonably clear boundary set against a dark sky.

The mean surface brightness of M57 is about 18. Probably the brighter parts of the ring are around 14 or 15. The surface brightness of a reasonably dark, moonless sky is more than 20 so we would be able to see the ring quite clearly against a dark sky. Sadly our eyes do not see colour when the light is faint so we would only see it as a pale white ring.

At 1000th of the distance, the visual diameter of M57 would be about 64 degrees - i.e. it would occupy a large patch of sky.

The brighter areas of the milky way have a surface brightness of 6-10. So M57 would not outshine the bits of the MW we normally see with the naked eye.

If we were located inside the nebula it may be quite difficult to see anything as it would occupy the whole sky and contribute to diffuse "sky-glow".
Addendum to my previous post. I think you are mistaken about the surface brightness of the Milky Way. I could not see any of it, not even the Scutum star cloud. Thus I would conclude that the surface brightness of M57 is much greater. Maybe there are some patches in Sagittarius that are brighter, but they are much lower here and harder hit by the light pollution. M57 was near the zenith, and Scutum was well up in the south.

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