globular cluster

Through a small telescope a globular cluster looks like a fuzzy ball of light, but a larger one resolves it into a breathtaking swarm — hundreds of thousands of stars packed into a tight sphere, blazing thickest at the center and thinning toward the edges. They are some of the most beautiful and most ancient objects in the sky. Where an ordinary open cluster might hold a few hundred young stars loosely strewn in the disk, a globular cluster is a dense, gravitationally bound city of old stars orbiting in the galaxy's halo.

A globular cluster typically contains 100,000 to a million stars within a sphere only about 100 light-years across, so the stars near its core are thousands of times more crowded than in the Sun's neighborhood. The Milky Way hosts roughly 150 known globular clusters, scattered through the halo on plunging orbits far above and below the disk. Their stars are old — often 11 to 13 billion years — and metal-poor, all apparently born together in a single ancient burst, which makes a cluster a near-perfect sample of stars of one age and composition but a range of masses. That uniformity makes them invaluable laboratories for testing how stars age.

Globular clusters matter for two big reasons. First, because all their stars formed at once, the point where the brightest survivors are just leaving the main sequence acts as a clock, letting astronomers measure ages of 12 to 13 billion years — a check that the universe is at least that old. Second, their positions map the halo: in 1918 Harlow Shapley used the distribution of globular clusters to locate the true center of the galaxy and show that the Sun is far from it. A common misconception is to confuse them with open clusters, which are young, sparse, metal-rich, and live in the disk — almost the opposite in every respect.