Complex sentence: Astronomers initially thought that the halo was small and light, indicative of a spiral galaxy, but Spitzer found that halo around the Sombrero Galaxy is larger and more massive than previously thought, indicative of a giant elliptical galaxy.
Split into simpler sentences: Astronomers initially thought that the halo was small and light, indicative of a spiral galaxy. But Spitzer found that halo around the Sombrero Galaxy is larger and more massive than previously thought, indicative of a giant elliptical galaxy.