The Homunculus Nebula looks alive. Its two lobes of gas, which resemble twin human hearts stuck artery to artery, stretch half a light-year each from its center. And they’re getting bigger all the time, swelling outward at 1.5 million miles per hour. If you could fly through them, on some Magic School Bus, you’d find two stars at their center—two huge stars, whipping around each other once every 5.5 years.
When their orbits bring them close together, as they did in August 2014, the stars