The glowing shell recycles. When a sunlike star runs out of core fuel, it sheds an envelope of hydrogen, helium, carbon, nitrogen, and oxygen; stellar nucleosynthesis has forged much of that cargo before the gas enters the interstellar medium. A massive star can instead eject enriched matter in a supernova.Gravity is the hard editor. Expelled gas does not promptly become a star: it must cool, mix ...More