Celestial radio source
I believe the answer is:
pulsar
'celestial radio source' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pulsar that I've seen before include "Rapidly rotating star, emitting electromagnetic waves very quickly" , "Heavenly emitter of radio waves" , "A source of space of radio signals" , "Small and extremely dense star" , "Star producing radio waves" .)