Description

This simulation shows the distribution of globular clusters around the plane of the Milky Way as determined by Harlow Shapley in 1918. The galactic longitudes, latitudes, and distances to the clusters are taken from Shapley's 1918 paper entitled "Studies Based on the Colors and Magnitudes in Stellar Clusters, Seventh Paper: The Distances, Distribution in Space, and Dimensions of 69 Globular Clusters." The Space View frame shows a 3D view of the distribution of the clusters around the galactic plane, along the location of the Sun and Shapley's location for the galactic center (as well as the boundary of what Shapley called the "Big Galaxy") as determined by the cluster distribution. Globular clusters are shown in different colors depending on how their distances were determined: using Cepheid variables (red), brightest stars in the cluster (green), or apparent diameter of the cluster (blue). Other windows show a plot of the cluster locations projected onto the galactic plane, a histogram of the number of clusters versus galactic longitude (showing a peak toward Shapley's proposed galactic center), and a histogram of distances from the galactic plane (showing a dearth of clusters very close to the plane).

The simulation also allows the viewer to adjust Shapley's distances to account for the effects of absorption. Absorption by dust in the galactic plane caused the cluster stars to appear dimmer than they should be from distance alone. This led Shapley to overestimate their distances, with larger errors for clusters near the galactic plane where absorption effects are greatest. The simulation uses a model for absorption proposed by Joel Stebbins in 1933. Stebbins assumed a thin (1 kpc thick), uniform absorbing layer along the galactic plane, with light passing perpendicularly through the plane being dimmed by 0.36 magnitudes. A slider in the simulation allows the user to adjust the dimming produced by the absorbing layer from 0 (Shapley's assumed value) up to 0.36 (the value found by Stebbins). The simulation will also display Stebbins' revised galactic center and boundary, which essentially matches the modern view of the Milky Way. [Note that in 1933 Stebbins did not use Shapley's 1918 globular cluster data. Instead he used 1929 data on 93 globular clusters from Shapley and Helen Sawyer. The 1929 distance estimates were based on a revised Cepheid period-luminosity relation, which gave distances about 11% smaller than Shapley's 1918 estimates.]