Observatory observation
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">observatory observation</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for nova that I've seen before include "It shines" , "Star showing a sudden, brief increase in luminosity" , "Faint star that suddenly flares into brightness and then fades again" , "Star suddenly erupting in brightness" , "Star that becomes much brighter for short time" .)