what colour are nebulae?

[troypiggo]

Ok, I know it's a broad question. Let me clarify...

I'm only new to astrophotography. In fact I've only had one session. I chose M42 (http://farm4.static.flickr.com/3484/3195922587_2ef3828fa6_o.jpg), and shot it with my 30D. It's my only DSLR (for now ;) ) so I can't modify it. M42 came out with blues and reds in it. I know it's not the best shot, I overexposed the Trapezium part, will take shorter frames next time. Still learning.

Anyway, been looking around at other shots of M42 and notice that the colours of it vary widely. Some show it mostly reds (http://en.wikipedia.org/wiki/File:Orion_Nebula_-_Hubble_2006_mosaic_18000.jpg), some more blue etc (http://en.wikipedia.org/wiki/Orion_Nebula#Gallery). So which one is correct?

Should i not worry so much about this when processing my images?

[pjphilli]

Hi Troy
Good question but I think it is a bit like the "how long is a piece of string?".
I have noticed a large range of colours for M42 depending on what camera is being used. For instance, the colours I get from my Meade DSI2 camera tend to be brighter than the colours I have seen from Canon DLSRs.
I think that the main thing is to take images of length that will not cause too much burn out of bright stars and then take lots of images (experience will tell) so that when you stack them and stretch the image to expose the nebulosity, the dimmer nebulosity does not get noisy. The colours will then be how your colour camera interpreted the nebulosity. The tricky part comes if you may decide to "enhance" the colours in the "colour enhance" section of your adjustment program, then you can make it any colour you like although most adjust until they get colours that appear to be right from what they have seen in other images. It partly is a matter of taste, some like subdued colours others like bright colours etc. The colour of the nebulosity is often caused by exitation of gas such as red when hydrogen is involved. Overall the amateur processing of nebulosity images is as much an art as a science.
"Don't worry - be happy!"
Cheers Peter

[RobF]

Same question, phrased different way perhaps.....I often wonder just what these objects would look like 10 or 100 (or 1000?) times closer. Surely the human eye would start to pick up some colour then?

[bojan]

No... The surface brightness would be the same, only the apparent (angular) size would be 10- 100 bigger..

[bloodhound31]

If you were in it, you probably wouldn't see it too well at all.

Think of it like walking in a fog with a flashlight, the first few feet looks clear, even though you are "in" the fog itself. Only once you have looked through a distance of it, does it start to become more opaque.

Now imagine that fog is 1500 light-years across and really thin. Now take away the flashlight.

Get an idea?

Baz.

[Terry B]

It depends what you mean by correct colour. It also depends on the nebula.
If the nebula is a reflection nebula then it will be the colour of the stars that are illuminating it. If it is an emission nebula it will be the colour of the particular emission lines that it is emitting.
If it is a planetary nebula it is mostly an emission nebula so the same thing applies.
How you interpret the particular frequency of light being emitted is up to you and your eyes. Many people try to balance the picture so that the colour balance is similar to our sun. This results in the red images you see of the Ha nebulas.
Many digital CCD cameras are blocked in the deep red and infrared. This makes the light of Hydrogen alpha (which is a deep red) poorly recorded by the sensor. This is why the red colour of a Ha emitting nebula (like the orion neb) is only poorly recorded. If you use a CCD that can see into the IR then the Ha is recorded much better. The orion neb has both reflection and emission components hence the variation in images that you see.

[troypiggo]

Thanks for the detailed explanation, Terry. All made sense to me.

[Geoff45]

If we set Hydrogen beta (Blue-green) to a value of 100, then the major emissions of M42 are:
O II (deep violet) 119
H beta (blue-green) 100
O III (blue-green) 412
H alpha (deep red) 363
N II (deeper red) 77
(Source: Covington, DSLR Astrophotography)
So you can see that there are big contributions from both the blues and the reds, so expect to see both.
Geoff

[troypiggo]

Excellent, thanks Geoff.