STAR CLUSTER
Omega Centauri
— NGC 5139 —
Processed by Nicolas Rolland, Paris. 4.3 hours gathered by Martin Pugh at Heaven's Mirror Observatory, Yass, Australia.
Ptolemy listed this as a star. It stayed a star in the catalogues for fifteen hundred years.
A Greek letter for a cluster
Omega Centauri carries a stellar designation because that is what everyone took it for. Ptolemy recorded it around 150 AD, and in 1603 Johann Bayer gave it the Greek letter that names it today, in a catalogue reserved for stars. Edmond Halley, observing from Saint Helena in 1677, was the first to note that it was not one. The name survived the correction, which is why the largest globular cluster in the galaxy is known by a letter meant for a single point of light.
Ten million stars, a tenth of a light-year apart
It lies about 15,700 light-years away in Centaurus and measures roughly 253 light-years across, which makes it the largest globular in the Milky Way — around ten million stars for a total of some four million solar masses. In the central region they are separated by about a tenth of a light-year. In the Sun's neighbourhood the nearest star is forty times further off than that, and there is nothing between here and there.
The core of something that was eaten
Globular clusters are supposed to be simple: one generation of stars, formed at one moment, chemically alike. This one is not. Its stars belong to several generations with different compositions, its shape is visibly flattened rather than spherical, its stars move faster than a body of this mass should make them move, and an intermediate-mass black hole of around 12,000 solar masses is thought to sit at its centre — an object globulars are not expected to contain. Each of those is an anomaly on its own. Together they point at the same explanation: that Omega Centauri is not a cluster at all but the surviving nucleus of a dwarf galaxy, whose outer parts the Milky Way pulled away and absorbed long ago.
Four hours, and a limit that is not exposure
A Planewave CDK17 at f/6.8 with adaptive optics ahead of the sensor, from Yass in May 2020 — four hours and twenty minutes in ten-minute frames across luminance, red, green and blue. On an object of magnitude 5.3 that is ample: nothing here is faint, and the cluster fills a field of 36 by 27 arcminutes almost exactly. What no amount of exposure changes is the middle. A tenth of a light-year at 15,700 light-years subtends about 1.3 arcseconds, which is the same order as the atmosphere's own blur on a good night — so the innermost stars arrive already merged, and the adaptive optics ahead of the sensor buy back rather more of them than another four hours would. Martin Pugh gathered the data at Heaven's Mirror.
