Astronomical sighting (4)
I believe the answer is:
nova
'astronomical sighting' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nova that I've seen before include "sun" , "Suddenly bright star" , "Star that ejects a cloud, thus becoming brighter" , "Suddenly expanding star" , "Star showing a sudden temporary increase in brightness" .)