On Sunday evening, Fox aired the latest episode of Neil deGrasse Tyson’s Cosmos series. This episode, called “A Sky Full of Ghosts,” explored some more out-of-this-world subjects — the speed of light and how it helps us undertand the Big Bang; the scientific work of Isaac Newton, William Herschel, James Clerk Maxwell; Albert Einstein’s Theory of Relativity; dark stars; black holes; and more. US viewers can watch the entirety of Episode 4 online (above), along with previous episodes in the series below (or on Hulu). For viewers outside the US, we have something perhaps better for you: Carl Sagan’s Original Cosmos Series on YouTube. Plus, we have a bunch of Free Online Astronomy Courses in our collection of 875 Free Online Courses. Enjoy.