Complex sentence: The Coma Supercluster is the nearest massive cluster of galaxies to our own Virgo Supercluster; it is roughly spherical, about 20 million light-years in diameter and contains more than 3,000 galaxies.
Split into simpler sentences: The Coma Supercluster is the nearest massive cluster of galaxies to our own Virgo Supercluster. It is roughly spherical, about 20 million light-years in diameter and contains more than 3,000 galaxies.