Blue straggler

A star in a cluster that is hotter and brighter than the cluster’s main-sequence turnoff predicts. It appears unusually young because it gained mass from a companion or formed through a stellar merger.

Mass transfer: The movement of matter from one astronomical body to another, often through gravitational interaction. A companion can donate hydrogen-rich material, increasing a blue straggler’s mass.
47 Tucanae: A massive globular cluster in the constellation Tucana, among the brightest globular clusters in the sky. Its blue-straggler population has been used to study formation channels in a dense cluster.
Color–magnitude diagram: A plot comparing stars’ brightness with their color or temperature. Blue stragglers appear as a distinct population above and blueward of a cluster’s turnoff.
Binary fraction: The proportion of stars in a population that belong to binary or multiple systems. Comparing it with straggler counts tests how often binary evolution supplies their mass.
Stellar merger: The coalescence of two stars into a single star. A merger can create a more massive, hotter star that resembles a younger cluster member.
M30: A globular cluster in the constellation Capricornus, also catalogued as NGC 7099. Its central blue-straggler concentration helped establish collisional formation as a possible channel.
Main-sequence turnoff: The point in a star cluster’s color–magnitude diagram where stars begin leaving the main sequence. It sets the expected upper mass and apparent age against which stragglers stand out.
Dynamical friction: The gradual loss of orbital energy by a massive object moving through a background of particles. It can concentrate massive binaries toward cluster centers, shaping straggler distributions.
Roche-lobe overflow: Mass transfer that begins when a star expands beyond the region gravitationally bound to it in a binary system. This is one route for a binary companion to feed a blue straggler.
NGC 188: An old open cluster in the constellation Cepheus, notable for its large population of blue stragglers. Its stragglers are commonly associated with binary interaction in an old, relatively sparse cluster.

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