Hi Outback & All,
I'll make a note of that mate, but I'm no astrophysist and don't pretend that I am one -- I'm just an amateur with a slightly-better-than-most knowledge of such things -- mainly because I work at Sydney Observatory and need to know it to answer questions from the public and also many pesky HSC/Yr 12 physics students.
As for life expectancy of metal-poor stars, I think it would make hardly (if any) difference, except perhaps for the high-mass stars.
Low-mass stars make their energy via the proton-proton reaction -- a quite slow reaction chain. A succinct explanation is set out here:
http://en.wikipedia.org/wiki/Proton-proton_chain
Metals play no part as either fuel or catalyst.
In high-mass stars the dominant reaction is via the CNO cycle -- a much faster and more vigorous reaction chain. A succinct explanation is here:
http://en.wikipedia.org/wiki/CNO_cycle
Where the "metals" play a significant role as a catalyst but are conserved.
The fabled Population III stars that are said to have existed in the early Universe were _extremely_ massive stars that were _extremely_ metal poor because all the baryonic matter in the Universe at that time was basically 1H, 2He and _minute traces_ of 3Li and 4Be.
So how did they "burn" without metals? It is thought via proton-proton but because of this they could be much, much larger than today's most massive stars without tearing themselves apart. Maybe up to 250 solar-masses they were. It was this initial generation of super-stars that seeded the very first metals into the interstellar medium when they exploded as supernovae. From this (so the theory goes), came the Population II stars that are (still) very metal poor and are found in the Hub and Halos of spirals and in the GCs.
These so called Population III stars (I still reckon they should have called them "Population 0" stars) have not been observed directly because they only existed for a brief time in the very earliest epoch of the Universe. If we are going to find them (or at least their spectra), in all probability we have to look at light from the very first generation of stars that is so distant it will probably only be found in the gravitational lenses.
As for the "typical" spectrum of the stars in a GC, it is quite late -- late F or G generally. The Colour-Magnitude (or H-R) diagram for a G.C looks distinctly different to an open or galactic cluster. This is not due to metallicity but because of age. They have very short main-sequences from M upward to a turn-off point to red-giant-hood quite low at about the G mark. There will also be pronounced Red-giant branch where there are a large number of individual stars and most importantly a well-populated AGB (horizontal branch) where the RR Lyrae stars live.
By contrast, even for a large/massive open cluster, you would expect a long main sequence right on up to B or even O type stars, and then perhaps the odd red-giant and nothing at all on the horizontal branch.
Apart from metallicity, this C.M diagram is how a true globular can be easily distinguished from an open cluster -- all true G.Cs are 10-12 Gyr old and have CM diagrams of this sort with _well populated_ red-giant _and_ horizontal branches.
As a matter of interest, there are quite a few "GCs" that are commonly catalogued or mapped as part of the LMC and SMC, but many of them are not "true" globulars. They may be globular in size (ie >10^5 stars, but they are not globular in "population" -- ie extremely old, metal-poor stars. Indeed the only "true" GC in the SMC is NGC 121 -- all the others marked as such in many star maps are either very populous or very old open clusters (mostly both). The LMC has about 7 "true" GCs (from memory).
Paddy wrote:
"I have one question arising from this. If the Milky Way's GCs are so old and were formed with the galaxy itself, what has prevented the gravity of the disc pulling them into the disc?"
The simple answer is "the conservation of angular momentum". Plus, gas is "sticky", stars (or balls of them) aren't.
There is a Wikipedia page on GCs that by and large also looks pretty good (accurate and brief) and sets out a lot of this stuff (and other interesting facts).
It is here:
http://en.wikipedia.org/wiki/Globular_cluster
Interesting ain't they!
Best,
Les D