Omega Centauri: The Milky Way’s Lunch?

Omega Centauri: The Milky Way’s Lunch?

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The sight of countless brilliant stars in an inky black sky exhilarates the human spirit, for reasons no one can explain. Such intensity reaches its epitome in a crowded star cluster, and the best of these are the “globulars”. Some 150 known globular clusters surround the centre of the Milky Way. The stars within them not only array in a spherical formation, but they also whiz around the cluster’s centre in random elliptical orbits. Adding a final layer to this motif, the globulars themselves form a vast ball-shaped pattern around our galactic core like lights on a spherical crystal chandelier, meaning they ignore the Milky Way’s busy flat plane, which is home to its spiral arms and virtually everything else. Instead, the globular clusters arrange themselves in the “galactic halo” visibly deſining it. If we looked at them alone, we would conclude our galaxy is not a pancake but a ball whose component stars are but balls of nuclear ſire.

This is why globulars are strong candidates for being the most beautiful objects in the universe. Unlike the tens of thousands of open clusters, whose mostly young blue members congregate far more loosely, the stars in globular clusters are tightly packed, held firmly in the mutual grip of the collective. These stars are collective members forever. Moreover, virtually all globulars consist of ancient stars that date from within a half-billion years of the Big Bang itself. All the stars they hold formed at the same time. Why is it special, why is it unique? Omega is the name of a star, whose name was bestowed upon it 400yrs ago when Johann Bayer assigned Greek letters to the members of Centaurus the Centaur. This was pre-telescope, demonstrating that Omega Centauri was easily visible to the naked eye. Indeed, Ptolemy listed it some 2000yrs ago. In 1677 Edmund Halley catalogued it as a nebula as it still appeared as just a fuzzy patch in early telescopes. A century later with much improved optics John Herschel was able to see that it was not composed of gas at all, but exclusively of stars, lots of them, in fact lots and lots of them.

Although forever out of reach to Northern Hemisphere observers, it was well known to sailors and astronomers sailing into Sothern latitudes, its reputation becoming almost mythical. This was now recognized as the brightest and richest globular cluster in all the heavens, and the largest as well (there is in fact a larger specimen, not in our galaxy but in our nearest neighbour Andromeda).

Omega Centauri is so different to other globulars that astronomers began to think maybe it was an interloper, that it didn’t really belong here at all. Its resumé can be stated quite quickly. Omega is 15,800 light-years from Earth. Its members are not homogenous like those of other clusters, but are instead varied and were clearly born at different times. While a typical globular cluster contains 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 centre, suns are so closely crowded together that they sit just 1/10th of a light-year apart and that is over 40 times closer than our nearest star Alpha Centauri (ignoring our Sun of course) at 4.35 light-years distant. And while other star clusters have no spin, Omega rotates, its fastest stars moving at 21kms/sec. Why is only this cluster so unique?

After an extensive study of over 50,000 of Omega’s individual stars by a team of Korean astronomers at the turn of the millennium, they found that, unlike every other globular, this one indeed possesses multiple population groups. Its stars show a spectrum of metallicities ie. elements heavier than helium that indicate formations spanning billions of years, not all at once, as if it were, yes, a galaxy!

Our galaxy, is, even now, feasting on a number of smaller dwarf galaxies, such as the Large and Small Magellanic clouds, the Sagittarius dwarf galaxy and Sculptor amongst others. They are trapped in its gravitational influence, and that leads to the logical conclusion that Omega Centauri is all that remains of a galaxy captured and cannibalized by our very own Milky Way.

Drawn in, merged, assimilated, and broken apart, the 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 ſierce gravitational epoxy of its tightly huddled core members. Omega Centauri is more than just big and very beautiful, it actually came from somewhere else, it deſinitely qualifies as a Michelin 3-star lunch for the Milky Way. Still we don’t get it all our own way as in 4 billion years’ time our much bigger galactic neighbour Andromeda will be on our own doorstep looking for dinner!

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