Globular star cluster Omega Centauri packs about 10 million stars much older than the Sun into a volume roughly 150 light-years in diameter, according to NASA's Astronomy Picture of the Day for September 25, 2026, credits Javier O. Cadenas Parra. Also known as NGC 5139 and located 15,000 light-years away, it is described as the largest and brightest of the 200 or so known globular clusters that roam the halo of the Milky Way galaxy.

Most star clusters contain stars of the same age and composition, but Omega Centauri shows different stellar populations with a spread of ages and chemical abundances. The source notes it may be the remnant core of a small galaxy merging with the Milky Way. In the telescopic view, the cluster's red giant stars stand out with a yellowish hue.

A two-decade-long exploration of the dense cluster with the Hubble Space Telescope revealed evidence for a massive black hole near its center, the source states. The entry was written and edited by Jerry Bonnell, Cecilia Chirenti, Robert Nemiroff and Keighley Rockcliffe.