Last week, 20,000 screaming fans swamped YouTube celebrities at the worlds largest online video conference in Anaheim, California. Alongside the young celebrity musicians, beauty gurus and pranksters, was an imposing lineup of science rock stars – scientists who have risen to Internet fame through their educational and personality-driven original content.  With tens of millions of views per video and hundreds of millions of views overall, these science YouTubers reach more young people today than Carl Sagan and Neil deGrasse Tyson combined.