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If you've read Parts A,B and C, on spectroscopy (or you are already a boffin) you will know that all hot bodies radiate a spectrum of wavelengths, and that when this light passes through a gas, the atoms in the gas can absorb some wavelengths leaving dark lines in the spectra. The wavelengths of these dark lines - called absorption lines - can tell us the chemical composition of the atoms because each atom will absorb only very specific wavelengths, whether in the laboratory or in deep space.
So what is happening here?
(13 second exposure, 1600 ISO)
The quality is rubbish because I only had a very short time to take some shots, and within an hour the clouds had rolled in again.
At the bottom left you can see a star, and on the right is its spectrum. It looks pretty similar to the colours you would see in a rainbow because, like the Sun, the star is a hot body, radiating light in a continuum of wavelengths. If the quality was better, you would be able to see the dark lines in the spectrum where elecrons had absorbed specific frequencies of light. Above the star and slightly to the right you can see a much fainter star, and in the right corner you can see its spectrum. But there's something very odd about it: at the blue/green intersection of the spectrum is a large blob.
What you are seeing is the Planetary Nebula NGC 2392, also known as the Eskimo Nebula. Here's my previous photo of it, at much higher magnification:
It's a star which burned up all its hydrogen and collapsed into a white dwarf, sending out a shell of gas travelling at huge speed into space. It's now about 0.3 of a light year in diameter, and lies at a distance of some 2870 light years.
The faint star on the left is the star at the centre of the nebula, called HD59088 ,and its spectrum in on the right. And the blob?
The starlight energises the cloud of gas and excites the electrons in it, and they move up to a higher "orbit." When they drop down again to a lower orbit they emit a photon, in other words, light. And it's at one specific frequency 5007 Angstroms ( 1×10−10 metres, or 0.1 of a nanometre) This is called an emission line.
Here's the funny thing; when this line was first discovered, it did not correspond to any Earthbound elelment, so it was thought to be a new element, and was given the name Nubulium. It wasn't until 1927 that the true identity of this element was discovered. It is..... oxygen! Doubly ionized oxygen actually, which means it has 2 missing electrons, hence ion O2+, also known as [O III].
Why does oxygen never emit light at this frequency on Earth? Because it's forbidden by quantum theory. To emit the "forbidden line" of 5007A, the electrons in the oxygen would have to stay in a specific orbit for some time in a metastable state, and this is impossible on Earth.
What's a metastable state and why can't oxygen be in this state on Earth?
I read this on Wikipedia, but it didn't help my understanding much:
It is when something "is in equilibrium (not changing with time) but is susceptible to fall into lower-energy states with only slight interaction."
As always though, a good image makes it all clear. The ball at 1 is in a metastable state. It won't move if it's left alone. But the slightest "knock" will tip it over the edge (2) and it will fall down to the lowest state (3)
If the electrons can manage to stay in the high state (1) for long enough, when they eventually fall down to the lowest state, the ground state, they will emit a photon of light at 5007 A. But even is the best vacuums boffins can produce on Earth, there are still lots of the oxygen atoms bouncing around and knocking into each other. This means they never have time to emit the 5007 wavelength because the impact of atom against atom, gives the electron a "knock" and it immediately falls to the ground state. In a planetary nebula though, the gas is very rarified, much more so than the best vacuum we can make, and this means the electrons have time to move from 1 - 3 at a rate which allows the forbidden line to appear.
Here's my profile graph of the spectrum, produced with the fantastic spectroscopy software RSpec:
The star (HD59088) is on the left and there is a huge peak showing the 0III line at (almost exactly) the 5007 line. (More about the calibration of this another time)
This next profile is produced using some of the features in RSpec which allow you to remove a lot of the background noise and also smooth out the curves using a pixel averaging process called binning. I boosted levels in Photoshop to bring out the spectrum of the star too, and you can see more of the spectral colours as well as the dominating green/blue of the nebula (bottom right)
It's odd, but capturing this line on a graph and knowing it is produced by light produced by ionised oxygen which has been flying at a speed of 186,000 miles a second for almost 3000 years, just so it can land on my camera's CCD in a garden in Hatfield, and be processed on my laptop, gives me more of a feeling of being connected with something quite magical far out in space, than actually seeing it through the scope.
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