![False color image of the planetary nebula NGC 6778. In blue it is shown the emission coming from the O++ faint recombination lines; this image was taken with the blue tunable filter of the OSIRIS instrument at GTC. In green we see the emission coming fro False color image of the planetary nebula NGC 6778. In blue it is shown the emission coming from the O++ faint recombination lines; this image was taken with the blue tunable filter of the OSIRIS instrument at GTC. In green we see the emission coming fro](/sites/default/files/styles/crop_square_2_2_to_320px/public/images/news/resultados173_189.jpg?itok=148MwdjP)
For more than 70 years we have known that the weak recombination lines of the ions of elements, such as oxygen and carbon, give us values for their abundances that are much larger than those obtained using collisional lines, even though the collisional lines are 1,000 to 100,000 times brighter than the recombination lines. This discrepancy has cast constant doubt about one of the methods that has been mostly used to measure chemical abundances in the Universe. During the past few years the planetary nebulae group at the IAC have discovered that the planetary nebulae with the largest
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