The Sun was born in a star cluster that fell apart long ago
Almost every lone star in our galaxy, the Sun included, began life packed alongside siblings in a cluster that later dissolved. Some clusters never break up: globular clusters hold up to millions of stars nearly as old as the universe, and in the 1990s they seemed older than the cosmos itself.
A star cluster is a family of stars born together from the same cloud and held by their own gravity. The loose kind, open clusters, usually hold a few hundred members spread across up to 30 light-years, and they sit in the Milky Way's spiral arms. The Pleiades and Hyades in Taurus are the best-known examples. Most are only tens of millions of years old, because passing gas clouds and close encounters between members gradually strip them apart; Messier 67 is a rare survivor billions of years old. That youth explains why they glow with hot blue stars, which burn out fast but usually outlast their cluster.
Clusters start out embedded, wrapped in the dust and gas of a collapsing cloud, as in the Trapezium within the Orion Nebula. Typically under 30 percent of the cloud turns into stars before winds, radiation and supernovae blow the rest away. Losing that mass unbinds most young clusters, scattering stars into the galaxy. Chemical traces in our Solar System hint that a nearby supernova touched it in those crowded early days.
Globular clusters are the opposite: dense balls of ten thousand to several million old red and yellow stars, orbiting in the galactic halo. The Milky Way has about 150; the giant galaxy M87 has more than a thousand. Omega Centauri, the brightest, was catalogued as a single star before telescopes. In 1917 Harlow Shapley used their layout to make the first sound estimate of the Sun's distance from the galaxy's centre. Until the mid-1990s their stars seemed older than the universe, a puzzle settled by sharper distances from the Hipparcos satellite, which put the cosmos at about 13 billion years.
Because a cluster's stars share an age and composition, they are natural laboratories for stellar evolution, and nearby ones anchor the cosmic distance ladder. In 2005 a stranger type, huge and diffuse, turned up in the Andromeda Galaxy.
Source: Star cluster
