As 4AoSers know, I have a fascination for the Great Nebula M42 in Orion. Until now I've only been able to take pretty pictures of it:
But now as a spectroscopist, I can work out what it's made of and explain its colour!
If you have read The ABC of Spectroscopy you will understand that the light from stars will excite the electrons in gas clouds in space, and this causes the electrons to jump up to a higher "orbit" in the atoms. When they fall back down to a lower state they emit photons of light. These photons will be emitted at very specific frequencies, and by measuring these frequencies we can identify the gas. These lines, which appear as bright coloured lines against a black background, are called emission lines.
(Wikipedia)
Great, so lets take a spectrum of M42 and make our own emission lines!
What are we looking at here? On the left of centre is the Trapezium, the 4 stars in the middle of M42, and the main ones exciting the gas in the nebula (only 3 are clearly visible here) Then there are 2 other stars above and one below.
On the right you can see the various specra associated with each star. You can also see 2 clouds, one blue and one red. This already tells us that M42 is made up of 2 separate gases. We've already discovered something profound about M42, which we couldn't know simply by looking at normal images.
Here's the photo superimposed on the spectra, rotated and resized to the same scale.
Now we hit something of a brick wall.
To analyse lines in spectra you need nice sharp emission lines, and to do this the original object needs to be a point source, or a line. With big blobs of stuff, the information is "smeared out" and lost.
This is why serious spectroscopy is done with a slit; a very fine gap only a few micron wide to isolate a "thin slice" of light.
I'm using a slitless set up because I can't afford the serious kit at the moment. This is ok for stars because they are point sources anyway, but for extended objects there's not much you can do.
But then I had a thought. What if I could use Photoshop to artificially isolate a small part of the spectrum?
I selected and boosted a small square in the brightest part of the original image, then selected the same 2 squares in the blue and the red clouds of the "smeared out" spectrum:
I then snipped out this thin slice and put it into the RSpec software. And found.....
....the blue and the red square fit almost perfectly with the emission lines for ionised oxygen (OIII forbidden line 5007 A) and Hydrogen (H-Alpha 6560 A)
I've searched around on the Net, but I can't find any other examples of this technique. I've posted the results up on one of the Spectroscopy forums and I'll let you know what they say. In the meantime, I'm very happy to have identified 2 chemical elements in M42. Exciting times!
Oh, and the colour? M42 is basically a huge cloud of Hydrogen. Hydrogen gives off light at 4 visible frequencies when it gets excited. The main one, as we've seen, is at a frequency of 6560 A, which to our eyes is red:
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