![]() The same is indicated by the bimodal globular cluster distribution, which separates into a group that is more confined toward the Galactic plane, such as ω Cen, while most are more broadly distributed and follow the halo mass profile. Dynamical evidence from halo stars points to accretion of stars from galactic collisions with the Milky way. ![]() However, despite the uncertainty, it appears that even the most metal-rich globular clusters, like 47 Tuc, barely if at all reach the current abundances of the disk. The lines from these stars are extremely strong and the stars are far brighter than the main sequence so that they also wash out the contribution from these fainter, although more numerous, stars. Specifically, the population of the blue end of the horizontal branch can cause spurious abundance results when photometric indices are used. Furthermore, the metallicity determined from integrated cluster spectra, in which the entire system is treated as if it were a single star, are severely affected by the morphology of the cluster H-R diagram. These tend to be stars on the red giant branch, which may have undergone internal mixing processes and so may not be representative of the entire cluster abundances. The metallicity scale for the globular clusters is not as well established as that for the open systems, since they are intrinsically fainter and only the more evolved stars can in fact be individually analyzed. These are the most metal poor clusters, having values of Z, the metal fractional mass abundance of metals, ranging from 10 −4 Z ⊙ to about 10 −1 Z ⊙. One is distributed approximately spherically around the disk, centered on the galactic center, and concentrated toward the bulge of the galaxy. Their distribution can be separated into two fairly distinct groups, differentiated on the basis of metallicity. This is normally quoted as a differential measure relative to the Sun, as = log(Fe/H) − log(Fe/H) ⊙.Īn important observational fact, still not well understood, is that there are no galatic globular clusters with the characteristics of the Population I stars and that they do not now appear to form in the disk. The usual means of quoting metallicity for these clusters, as with the open galatic clusters, is in terms of the Fe/H ratio. Their metallicities range from the lowest values observed in the oldest open clusters to ∼10 −3 the solar metallicity (which is ∼0.02 by mass). They are distributed in a halo around the disk, although some are present in the disk. The globular clusters are characteristic members of the old population, Population II, of the galaxy. Shore, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 IV.C Metallicities and Space Distribution ![]()
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