In astronomy, unable to change a foregone conclusion (7)

I believe the answer is:
nebulae
'in astronomy unable to change a foregone conclusion' is the definition.
I can't judge whether this defines the answer.
'unable to change a foregone conclusion' is the wordplay.
'to change' indicates an anagram.
'conclusion' indicates one should take the final letters.
The final letter of 'foregone' is 'e'.
'unable' with letters rearranged gives 'nebula'.
'nebula'+'e'='NEBULAE'
Can you help me to learn more?
(Other definitions for nebulae that I've seen before include "Interstellar gas clouds" , "Gas or dust clouds in space" , "Stardust" , "where stars are born" , "Bright patches in sky from distant galaxies" .)
