A giant family of stars arcs across the southeastern sky on September evenings. M15 is a globular cluster — a family of more than a hundred thousand stars packed into a tight ball. The cluster is about 375,000 times brighter than the Sun. That’s a lot of light. On the other hand, quite a few individual stars in the galaxy shine even brighter. That tells us that M15 is dominated by old, faint stars. In fact, M15 is probably at least 12 billion years old — almost three times the age of the Sun. Its most brilliant stars burned out quickly, leaving ultra-dense corpses known as neutron stars. A neutron star forms when the core of a massive star can no longer produce energy. The core collapses, while the outer layers blast into space as a supernova. Today, almost all the stars that are left are the mass of the Sun or smaller. Occasionally, one of these stars reaches the end of its life and puffs up to giant proportions, shining thousands of times brighter. It then sheds its outer layers, exp
