Messier 4

Messier 4 is a globular star cluster in Scorpius, about 7,200 light-years from Earth. It is one of the nearest globular clusters and contains the unusual millisecond pulsar PSR B1620−26.

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Globular cluster: A dense, roughly spherical group of old stars bound together by gravity. Messier 4 is a nearby example of this class of star system.
Gravitational binding energy: The energy required to disperse a gravitationally bound system into separated components. It helps explain why Messier 4 remains a coherent cluster despite stellar encounters.
Charles Messier: A French astronomer who compiled an influential catalogue of nebulae and star clusters. He recorded Messier 4 in his catalogue in 1764.
White dwarf: A compact stellar remnant supported against collapse by electron degeneracy pressure. A white dwarf orbits the cluster’s millisecond pulsar.
Scorpius: A constellation of the zodiac in the southern celestial hemisphere. Messier 4 lies within this constellation’s boundaries.
Two-body relaxation: The gradual redistribution of stellar energies through repeated gravitational encounters. It drives the cluster’s long-term structural evolution.
Messier catalogue: A catalogue of nebulae, star clusters, and galaxies compiled by Charles Messier. Messier 4 is entry number four in this widely used catalogue.
Millisecond pulsar: A pulsar rotating hundreds of times per second, usually spun up by accreting matter from a companion. PSR B1620−26 is a cluster example whose rapid rotation aids precise timing.
Hertzsprung–Russell diagram: A graph relating stars’ luminosities to their surface temperatures or spectral types. Its cluster-wide pattern reveals stellar ages and evolutionary stages.
Blue straggler: A star in a cluster that appears hotter and brighter than expected for its age. Messier 4 contains blue stragglers, which may form through stellar mergers or mass transfer.