Complex sentence: The Horologium Supercluster (also known as "Horologium-Reticulum Supercluster") is a massive supercluster, which is about 550 million light-years across and has a mass of 10 17 solar masses.
Split into simpler sentences: The Horologium Supercluster (also known as "Horologium-Reticulum Supercluster") is a massive supercluster. It is about 550 million light-years across and has a mass of 10 17 solar masses.