He later found that many nebulous objects such as the Andromeda Nebula (as it was then known) had spectra that were quite similar.
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Physicists showed in the 1920s that in gas at extremely low densities, electrons can populate excited metastable energy levels in atoms and ions which at higher densities are rapidly de-excited by collisions.
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In astronomy, collisional excitation gives rise to spectral lines in the spectra of astronomical objects such as planetary nebulae and H II regions.
The first satellite UV spectrogram of a planetary nebula (IC 2149 in Auriga) was obtained, revealing lines of aluminium and titanium - elements not previously observed in objects of that type.
Caldwell 59 (NGC 3242, the Ghost of Jupiter), a planetary nebula in the constellation Hydra