Published in the Summer 2000 issue of Comet Tales
Globular clusters are isolated cities of stars. Many tens of thousands of stars are aggregated together in a spherical grouping, gravitationally bound in a volume up to a few hundred light years in diameter. In the Milky Way, the distribution of globular clusters is concentrated around the galactic centre ie. in the constellations of Sagittarius, Scorpius, and Ophiuchus. These three constellations account for about half of the known Milky Way globulars. Indeed, the overwhelming majority of globular clusters are to be found in the hemisphere centered on Sagittarius, whereas only ~10% are found on the opposite side of Earth. This observation was used by Harlow Shapley in 1917 to determine that the galactic centre lies at a significant distance from us, and not nearby as previously thought.
Measurements of globular clusters show that they move in very eccentric elliptical orbits which take them well outside the Milky Way disk. The orbits form a halo that reaches out to several hundred thousand light years, far beyond the Milky Way itself, and even the Magellanic Clouds.
By studying the spectra of stars, we can determine the chemical composition of the source of the light. Such spectroscopic studies of globular clusters show that they are much lower in the amount of heavy elements (metals) than stars such as the Sun that formed in the disk of galaxies. This tells us that globular clusters are very old. Why? Globular clusters consist of stars which have formed from the material present when the galaxy was just forming (or even perhaps before its formation). Stars in the disk, on the other hand, have seen the benefit of many cycles of stellar evolution in their neighbourhood, with supernovae expelling the heavy elements to be used later in new star-forming areas.
It's worth noting that the globular clusters we see today are the survivors of what would presumably have been a larger population of globulars in the early years of the Milky Way - the unlucky ones having been disrupted as they moved through the Galactic disk, thus spreading their stars throughout the Galactic halo.
Being as bright as 4th magnitude, the location of Omega Centauri (also known as NGC 5139) has been known since ancient times. It was listed in Ptolemy's catalogue as a star, and received its "Omega" designation from Beyer in his catalogue of stars. Edmund Halley (of Halley's Comet fame) was the first to document its non-stellar appearance, listing it as a "luminous spot or patch in Centaurus".
Sir John Herschel wrote : "The noble globular cluster Omega Centauri is beyond all comparison the richest and largest object of its kind in the heavens. The stars are literally innumerable, and as their total light affects the eye hardly more than a star of 4th magnitude, the minuteness of each star may be imagined".
In the recent edition of Hartung, Omega Centauri is described as : "...Its myriad stars are broadly compressed towards the centre. It is powdered with faint stars with 7.5cm [aperture], and with 10.5cm looks like delicate tangled threads of beaded gossamer. Larger apertures show a pronounced lace-like pattern which seems to be made of small crossing curved lines of stars. Dark lanes and streaks are evident with moderate magnification and the star distribution is far from uniform. On a clear night it is a most impressive and beautiful sight."
At a recent conference on the Galactic Halo, astronomers from the University of Virginia and from the Carnegie Institution's Las Campanas Observatory presented findings in which they claimed that Omega Centauri, a globular cluster dear to our hearts, may in fact be the core remains of a formerly larger dwarf galaxy that has undergone considerable stripping by the Milky Way's tidal forces!
Wanna run that one by me again?
Let's take a closer look at the features of Omega Centauri, as outlined
in the paper.
This in itself is not a new idea. In 1991, Raymond White, of the University of Arizona, suggested in a popular article that Omega Centauri's obvious ellipticity, combined with its chemical abundances, could be seen as suggesting a possible origin as a dwarf galaxy. White noted that dwarf ellipticals such as Sculptor, Fornax, Ursa Major, and Leo II were all metal poor and showed features in their colour-magnitude diagrams that were shared by Omega Centauri.
The idea (which isn't the sole possible explanation for Omega Centauri's unusual features) gains further interest when we consider that the globular cluster M54, discovered by Charles Messier in 1778, was found a few years ago to be not part of the Milky Way, but instead part of the Sagittarius Dwarf Elliptical Galaxy (SagDEG), which itself was only discovered this decade (thus, Messier has the credit of being the first to discover an extragalactic globular cluster). At least one team of astronomers have suggested that M54 may itself be the nucleus of SagDEG. This galaxy is the closest to the Milky Way and is currently being tidally disrupted by the Milky Way.
During Summer, Omega Centauri is admittedly only best observed during the hours before dawn. However, as the year progresses, Omega Centauri will become better placed at an earlier hour. It's well worth revisiting this delight of the Southern sky - one whose nature might not necessarily be what you may have thought.
Copyright Greg Bryant