As fascinating as these stunning star clusters are, however, only a single specimen makes it onto our list of the weird: Omega Centauri.

Its moniker hints at its uniqueness. “Omega” is a star name, bestowed more than 400 years ago when Johann Bayer assigned Greek letters to the members of the constellation Centaurus the Centaur. This was pre-telescope, demonstrating that Omega Centauri is easily visible to the naked eye. Indeed, Ptolemy listed it some 2,000 years ago. When Edmond Halley of later comet fame first found it in 1677, it appeared fuzzy in his telescope, so he cataloged it as a nebula.

More than another century passed before John Herschel saw that it was composed not of gas but exclusively of stars — lots of them. Despite its position in Centaurus in the southern sky, forever out of view by those in Europe, it attracted growing attention by astronomers who sailed south, its reputation becoming almost mythical — this was the brightest and richest globular cluster in all the heavens, and the largest, as well. (Later astronomers would discover only a single larger specimen, but not in our galaxy. It lurks instead in Andromeda — a twin named Mayall II.) Omega Centauri seems so different from every other Milky Way star cluster that astronomers wondered if perhaps it did not belong here. Maybe it is an intruder, albeit a handsome one.

Its résumé can be stated quickly. Omega floats at a distance of 15,800 light-years from Earth. Its members are not homogeneous like those of other clusters, but are instead varied and were clearly born at different times. And its stars are oddly legion: While a typical globular cluster has 100,000 to 700,000 members, Omega is off the scale at 5 million. It is so large, in fact, that observers see Omega Centauri as a blob the size of the Moon.

In its center, suns crowd so closely together that they sit just 1/10 of a light-year apart, or only about 6,000 Earth-Sun spans from each other. And while other star clusters have no cohesive spin, Omega rotates, its fastest stars moving at 13 miles (21 kilometers) per second. Why should this sole star cluster be unique? The object of intense study the past 45 years, Omega has appeared more, not less, singular with each new set of data.

Perhaps the most extensive work was done in 1999 by a Korean team of astronomers who obsessively studied 50,000 of Omega’s individual stars. They found that, unlike every other globular, this one indeed possesses multiple population groups. Its stars show a spectrum of metalicities (elements heavier than helium) that indicate formations spanning billions of years, not all at once — almost as if it were a ... galaxy! That lead to a single logical conclusion: Omega Centauri is all that remains of a galaxy that our Milky Way captured and cannibalized.

Drawn in, merged, assimilated, and broken apart, that unnamed, now-deceased galaxy has left only a single calling-card of its former existence — its nucleus. This alone stayed preserved against the Milky Way’s tidal destruction because it was glued and bonded forever by the fierce gravitational epoxy of its tightly huddled core members. So Omega Centauri is more than just huge and gorgeous. It actually came from somewhere else.