100-year gravitational source linked to inverted star constellation (6)
I believe the answer is:
cygnus
'constellation' is the definition.
(I know that Cygnus is a constellation)
'100-year gravitational source linked to inverted star' is the wordplay.
'100' becomes 'C' (Roman numeral).
'source' suggests taking the first letters (source can mean a beginning or origin).
'linked to' means one lot of letters go next to another.
'inverted' shows that the letters should be reversed in order.
'star' becomes 'sun' (the Sun is a star).
The first letters of 'year gravitational' is 'yg'.
'sun' back-to-front is 'nus'.
'c'+'yg'+'nus'='CYGNUS'
(Other definitions for cygnus that I've seen before include "Prominent northern constellation, the Swan" , "Operator" , "The Swan seen at night" , "Stars" .)