Complex sentence: In 1970, Kalevi Mattila proved the Coalsack is not totally black; it has a very dim glow (10% of the brightness of the surrounding Milky Way) which comes from the reflection of the stars it obscures.
Split into simpler sentences: In 1970, Kalevi Mattila proved the Coalsack is not totally black. It has a very dim glow (10% of the brightness of the surrounding Milky Way), which comes from the reflection of the stars it obscures.