General Meeting Report 13 August 2003
Syllabus item: GLOBULAR CLUSTERS
a talk given by Lisa M Elliott
School of Mathematical Sciences, Monash University.
Globular Clusters are an enigma. Independent wanderers, island universes, they have a mind of their own and don’t seem to follow accepted galactic guide-lines. 150 of these gravitationally bound star systems attached to our Galaxy have now been identified, scattered more or less uniformly across the galaxy. They are striking objects, nearly spherically symmetrical containing an immense number of stars. In a diameter of 60 to 300 light years they pack between 10,000 and several million stars. The colour-magnitude curves indicate that they are very old objects, at times assumed to be as old as the Universe, older than the Galaxy itself. It was the distribution of these clusters in the halo of the galaxy, where they are relatively un-obscured by the dust that blots out distant disk objects, that in 1918 gave Harlow Shapley his then revolutionary idea about the overall extent of our Galaxy and the position of its centre. Shapley again used globular clusters in 1949 as the basis for his estimate of the age of the Galaxy as 10 billion years.
The first globular cluster discovered (reported as a nebula by Abraham Ihle in 1665) was M22 in Sagittarius, followed by Omega Centauri (NGC 5139) by Edmond Halley on his 1677 journey to St. Helena. This "nebula" had been known since ancient times but classified as a star. Charles Messier was the first to resolve stars in a globular cluster, in this case his M4, but still referred to the other 28 of these objects in his catalogue as "round nebulae." By the time William Herschel started his comprehensive deep sky survey in 1782 there were 33 globular clusters known. Herschel himself added 37 new globulars, bringing the number to 70. He resolved virtually all of them into stars, and coined the term "globular cluster" in the discussion adjacent to his catalogue of 1000 deep-sky objects (1789).
Right from the start globular clusters were thought of as (or suspected to be) agglomerations or swarms of stars held together by their mutual gravity, all close together and at about the same distance from us. The proof for this working model came only with the advent of spectroscopy, showing that the stars of these clusters have radial velocities as expected for such swarms, and perfectly match in colour-magnitude or Hertzsprung-Russell diagram, i.e. they represent a population of stars of about same age, all at about the same distance. The H-R diagrams for globular cluster populations typically have short main sequences and prominent horizontal branches, representing very old stars that have evolved past the giant or supergiant phases. (Refer box M3)
Although globular clusters are often thought of as stars of homogeneous composition, careful close analysis shows that this is not the case. While in last decade the number of Extremely Metal Poor Stars (EMPS) for which a detailed chemical composition is available has increased considerably, abundance variations are now observed in almost all elements subject to H burning. Is it from some process in the star? Or some process in the cluster? Recent observation and theory suggests that the blue giants in globular clusters that so long have puzzled astronomers (since there is no sign of star formation in globular clusters) are the result of the merging of two old stars, and not recently formed young stars.
Lisa Elliott’s globular cluster research includes, as part of her PhD work, investigations into spectroscopic anomalies in cluster populations, such as an apparent deficiency in Nitrogen and Oxygen, and a ubiquitous presence of iron. Using a new self-consistent model allows the study of the the effect an external tidal field has on the dynamics and lifetime of globular clusters. It enables a study of the system on time-scales far greater than is possible with current n-body simulations. Preliminary results show that for realistic orbital parameters tidal field can affect the internal dynamics of the cluster. This new discovery will be important in predicting parameters for the survival of globular clusters, which in turn, compared to the current observed system, may have implications for models of the Big Bang. The similarity between the shape of a globular cluster and the central bulge of an average galaxy is unmistakable, and globular clusters may yet prove to be more than just incidental interlopers in our universe.