Complex sentence: The objects now called "brown dwarfs" were theorized to exist in the 1960s by Shiv S. Kumar and were originally called black dwarfs, a classification for dark substellar objects floating freely in space that were not massive enough to sustain hydrogen fusion.
Split into simpler sentences: The objects now called "brown dwarfs" were theorized to exist in the 1960s by Shiv S. Kumar. They were originally called black dwarfs, a classification for dark substellar objects floating freely in space that were not massive enough to sustain hydrogen fusion.