How big is Omega Centauri?

Omega Centauri is a globular cluster, a huge ball of stars held together by their own gravity, in the constellation Centaurus. As of 2026, it is the most massive globular cluster known in the Milky Way, with about 10 million stars. It is about 17,700 light-years from Earth, where a light-year is the distance light travels in one year. Its bright main body is about 150 to 187 light-years across.

Size

A globular cluster has no edge. Its stars crowd together at the center and thin out gradually farther away, so its size depends on where the edge is drawn. The size shown above, 1.6 exameters, is 1.6 × 10¹⁸ meters, where 10¹⁸ is a 1 followed by 18 zeros. That is about 170 light-years.

ESO, the European Southern Observatory, describes Omega Centauri as about 150 light-years across. The New General Catalogue of nebulae and star clusters lists it as 36 arcminutes wide, where an arcminute is one-sixtieth of a degree. At the cluster's distance, that is about 187 light-years, so the size shown lies between the two.

Half of the cluster's light comes from within about 25 light-years of its center. Its faintest outer stars spread much farther. A standard mathematical model of the cluster puts its outer edge about 48 arcminutes from the center, which makes the whole cluster about 500 light-years across. NASA gives its diameter as about 450 light-years.

Size comparisons

Seen from Earth, the cluster is about as wide as the full Moon.
The Rosette Nebula, a cloud of glowing gas 130 to 170 light-years across, is about the same width as the cluster's bright main body.
The bright main body is about 35 to 44 times as wide as the distance from the Sun to Proxima Centauri, the nearest star.
If the orbit of Neptune around the Sun were shrunk to the width of a U.S. penny, 19 millimeters (0.75 inches), the bright main body would be about 3 to 3.7 kilometers (1.9 to 2.3 miles) across.

A cluster unlike the others

Most globular clusters hold stars that all formed at about the same time. Omega Centauri holds stars of different ages. This is one reason some astronomers think it may be the core of a small galaxy that lost its outer stars in an encounter with the Milky Way.

In 2024, astronomers reported seven stars near the cluster's center moving faster than the cluster's own gravity could hold. Only a black hole of at least 8,200 times the Sun's mass could keep them in place. Few black holes of this middle size are known, between those left by single stars and the giants at the centers of galaxies.

From star to cluster

Omega Centauri is bright enough to see without a telescope from the Southern Hemisphere and from low northern latitudes. Nearly 2,000 years ago, Ptolemy listed it as a single star. Edmond Halley described it as a nebula in 1677, and in the 1830s John Herschel was the first to recognize it as a globular cluster. The cluster is thought to be around 12 billion years old.