Nebula appearance?

[animaal]

Long exposure images of nebula tend to look very colourful, with intricate shapes. TV programmes show spaceships flying past nebula that appear similar to the long exposure images taken by the Hubble telescope.

How would the Orion Nebula, for example, appear if we were much closer? As we get closer, it should appear brighter (some variation of an inverse-square law, probably). If we could stand so close that it filled a significant proportion of our field of vision, would it appear bright and solid to the naked eye? Or is the gas so diffuse that we'd still only see a vague greyish patch?

[acey]

The inverse square law of luminosity is not the only thing to think about. The other is that the apparent area of an extended object is inversely proportional to the square of your distance from it. If you halve your distance to the Orion Nebula then you receive four times as much light, but it's from an apparent area four times larger. Hence the surface brightness is unaltered.

This is a crucial observation - it tells us that all those DSOs we see in the sky do not increase in surface brightness as we fly towards them. M31, if it was close enough to fill the sky from horizon to horizon, would be about as bright as our Milky Way, i.e. easily swamped by light pollution. Similarly, if you were flying right next to the Orion Nebula, you'd want to make sure all the lights in your spaceship were off, otherwise your eyes might not adapt sufficiently for the nebula to be visible.

As to colour, you need the target to have a surface brightness of roughly 18 mag/arcsec2 at least, if you're to have any chance of stimulating the cone cells in your eye. I should think there are parts of the Orion Nebula that are as bright as that, and if you were flying close then they'd appear very large. So yes, I think you'd manage to see some colour - but only as long as you're in a completely darkened space-ship and you give yourself half an hour of dark adaptation.

I don't know if it would look solid. Does it look solid from Earth? I think it looks like clouds, and a close-up view would look like clouds too - faint ones. If you were flying right through it then the surface brightness would be pretty uniform in every direction, so you wouldn't see anything except a general glow.

One thing's for sure: you 'll never see DSOs looking like Hubble images, even if you invest in super-large aperture or inter-stellar transport.

[animaal]

Thanks, that's a very clear explanation. I can imagine the conversations our descendants will have;

"Very disppointing holiday - the nebula looked great in the brochures, but far less impressive when you acatually get there."

[Knight of Clear Skies]

On the other hand, there is also a young open cluster at the heart of the Orion Nebula, which would become brighter and more spectacular the closer you got, so the trip wouldn't be entirely wasted. (Technically, the surface brightness of stars doesn't increase with distance either, but because they are so far away we perceive them as point sources of varying brightness.)

[ollypenrice]

When you see clouds from the ground they have visual definition and structure. When you fly into them in an aircraft you just see vapour streaming past... We could rephrase an old saying; 'I can't see the nebula for the gas...'

Olly