SPECTRA OF NEBULAE
Spectrum of Emission Diffuse Nebulae
Spectrum of emission nebulae is easy to record even with small instruments.
Infact, despite to their low brightness, in their spectrum we find only few lines that
concentrate all the emitted light.
In the picture 1 it is shown the spectrum of the famous Orion nebula (M42), one of my
early spectrograms, when I still use a film as detector.
Picture 1: Spectrum of the central part of M42 that shows the main emission bands and
the faint continuum between 3500 and 5000 ┼. |
Upper right: the spectrum as it looks like after 1 h exposure on Ilford HP5 film with
200 ┼/mm dispersion (horizontal stripes are spectra of two among the trapezium stars).
Although the film is less sensitive than CCD detectors in the visible, it has a good
sensitivity also in the near UV.
The spectrum in an emission nebula is generated by the highly energetic UV radiation
from the hot O and B stars inside the nebula. This UV radiation is able to ionize the
atoms of the surrounding gas. Recombination of hydrogen, nitrogen and oxygen atoms with
free electrons produce emission lines mainly in the visible.
In particular, the recombination process allows to "pump" oxygen and nitrogen into
metastable states from where they decade producing forbidden lines.
Forbidden lines were not reproduced in a laboratory until 1920 when Bowen recognized
that they were emitted from nitrogen, oxygen and neon. Previously astronomers formulated
the ipotesis that these lines come from an element that still was not found on the earth
and called "nebulium".
The faint continuum observed between 3500 and 5000 ┼ is due in part to reflected light from
blue stars and in part to a Lymann emission spitted into two visible photons instead of
only one UV photon.
Goto other nebulae: