<P> The number of globular clusters discovered continued to increase, reaching 83 in 1915, 93 in 1930 and 97 by 1947 . A total of 152 globular clusters have now been discovered in the Milky Way galaxy, out of an estimated total of 180 ± 20 . These additional, undiscovered globular clusters are believed to be hidden behind the gas and dust of the Milky Way . </P> <P> Beginning in 1914, Harlow Shapley began a series of studies of globular clusters, published in about 40 scientific papers . He examined the RR Lyrae variables in the clusters (which he assumed were Cepheid variables) and used their period--luminosity relationship for distance estimates . Later, it was found that RR Lyrae variables are fainter than Cepheid variables, which caused Shapley to overestimate the distances of the clusters . </P> <P> Of the globular clusters within the Milky Way, the majority are found in a halo around the galactic core, and the large majority are located in the celestial sky centered on the core . In 1918, this strongly asymmetrical distribution was used by Shapley to make a determination of the overall dimensions of the galaxy . By assuming a roughly spherical distribution of globular clusters around the galaxy's center, he used the positions of the clusters to estimate the position of the Sun relative to the galactic center . While his distance estimate was in significant error (although within the same order of magnitude as the currently accepted value), it did demonstrate that the dimensions of the galaxy were much greater than had been previously thought . His error was due to interstellar dust in the Milky Way, which absorbs and diminishes the amount of light from distant objects, such as globular clusters, that reaches the Earth, thus making them appear to be more distant than they are . </P> <P> Shapley's measurements also indicated that the Sun is relatively far from the center of the galaxy, also contrary to what had previously been inferred from the apparently nearly even distribution of ordinary stars . In reality, most ordinary stars lie within the galaxy's disk and those stars that lie in the direction of the galactic centre and beyond are thus obscured by gas and dust, whereas globular clusters lie outside the disk and can be seen at much further distances . </P>

In what part of the milky way galaxy do most globular star clusters live