Shapley's Globular Clusters
General 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 plot 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.]
Space View Frame
This frame shows the 3D distribution of globular clusters about the
galactic plane.
- Options Menu
- Show Galactic Plane: show a disk representing the galactic
plane.
- Show Shapley's Center: show a point representing
Shapley's 1918 estimate for the location of the galactic
center.
- Show Boundary of Big Galaxy: show a sphere representing
Shapley's 1918 estimate for the size of the galaxy.
- Show Projection Onto Galactic Plane: show a plot of
cluster positions projected onto galactic plane.
- Show Distribution of Longitudes: show plot of number of
clusters versus average galactic longitude.
- Show Distribution of Distances from Plane: show
(symmetrized) histogram of distances from the galactic
plane.
- Include Absorption Effects: show slider that allows the
user to adjust the dimming caused by absorbing dust in the
galactic plane.
- Show Stebbins' Center: show a point representing
Stebbins' 1933 estimate for the location of the galactic
center.
- Show Boundary of Stebbins' Galaxy: show a sphere
representing Stebbins' 1933 estimate for the size of the galaxy.
- Visual Elements
- Red points: globular clusters with distances from Cepheid
variables.
- Green points: globular clusters with distances from
brightest stars.
- Blue points: globular clusters with distances from
apparent diameters.
- Yellow disk: galactic plane.
- Orange point: our Sun.
- Magenta point/sphere: center and boundary of Shapley's
Big Galaxy (1918).
- Pink point/sphere: center and boundary of Stebbins'
galaxy (1933).
- Partially transparent black disk: absorbing layer of
dust in the galactic plane.
- Controls
- Absorption: total absorption (in magnitudes) for light
passing perpendicularly through the layer of absorbing dust
in the galactic plane. Shapley (1918) assumed 0, Stebbins
(1933) found 0.36.
Projection onto Plane Frame
This frame shows a plot of the globular clusters locations projected
onto the galactic plane.
- Visual Elements
- Red points: globular clusters with distances from Cepheid
variables.
- Green points: globular clusters with distances from
brightest stars.
- Blue points: globular clusters with distances from
apparent diameters.
- Orange point: our Sun.
- Magenta point/circle: center and boundary of Shapley's
Big Galaxy (1918).
- Pink point/circle: center and boundary of Stebbins'
galaxy (1933).
- Radial grid lines: galactic longitude with lines spaced
15 degrees apart (0 degrees lies right of the Sun).
- Circular grid lines: distance from the Sun with circles
spaced 10 kpc apart.
Distribution of Longitudes Frame
This frame shows a plot of the number of clusters within each bin of galactic
longitude versus the average longitude for clusters in that
bin (bins with no clusters just use the midpoint of the bin
as the average longitude).
Distribution of Distances Frame
This frame shows a histogram of the distances of the clusters from the
galactic plane (positive distances indicate clusters North
of the plane, negative distances indicate clusters South of
the plane). The histogram has been symmetrized, so in
addition to counting the actual clusters the histogram also
counts the mirror image of the cluster reflected through the
galactic plane.
Todd K. Timberlake (ttimberlake@berry.edu)