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THE SPECTRA OF NEBULAE


Planetary - Diffuse


The spectrum of planetary nebulae
The origin of the spectrum of planetary and diffuse nebulae is the same.
The intense UV radiation of the white dwarf ionize hydrogen atoms and the other gases of the nebula that generate fluorescence when recombining with free electrons.
Thus the same forbidden oxygen and nitrogen lines ca be observed in both diffuse and planetary nebulae but in the latter the intensity of heavier elements is higher due to previous nucleosintesys of the once-existing star.
The central stars are extremely hot, with temperatere that ranges from 25000 to 250000 K and show often a Wolf-Rayet type spectrum rich of carbon or nitrogen. In other cases these stars show some absorbtion bands and then they are classified in the O class ( O Subdwarfs ).

Picture 1: Blue-green spectrum of M57 taken with 900 l/mm f=50 mm spectrograph, 5 minutes exposure with CCD detector.
The slit cross the nebula as it is shown in the picture of M57. Intensities of spectral lines changes from the middle to the edges. Emission from doubly ionized Helium is stronger in the middle (probably due to higher temperature), while oxigen and hydrogen appears mainly at the edges.
Note the presence of lines from the heavy element Argon (Z=18 that is not visible in M42 spectrum) and stronger intensity of Oxygen lines compared to Hydrogen than in M42 spectrum.


The measure of temperature of central stars in planetary nebuale is calculated with the method proposed by Zanstra. All the radiation emitted from the star between the Lyman limit (912 ┼) and the He II Lyman limit (228 ┼) is used for the ionisation of hydrogen atoms in the surrounding nebula. Recombination of free electrons with hydrogen atoms produces Lyman and Balmer lines and continuum. Lyman lines are converted into visible lines by oxygen and nitrogen that produces the well known forbidden lines.
Thus, measuring the total visible emission of the nebula it is possible to know the total UV emission (between 912 and 228 ┼) of the central star. Comparing its UV emission with its brightness in the visible can be computed the temperature assuming a blackbody spectrum emission.

Goto other nebulae:

Emission Nebulae

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28/02/2002