 The green hue was a puzzle for astronomers in the early part of the 20th century because none of the known spectral lines at that time could explain it . There was some speculation that the lines were caused by a new element , and the name <unk> was coined for this mysterious material . With better understanding of atomic physics , however , it was later determined that the green spectrum was caused by a low @-@ probability electron transition in doubly ionized oxygen , a so @-@ called " forbidden transition " . This radiation was all but impossible to reproduce in the laboratory at the time , because it depended on the quiescent and nearly collision @-@ free environment found in the high vacuum of deep space . 
