Observatory observation (4)
I believe the answer is:
nova
'observatory observation' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)