Robert Trumpler, (1886 - 1956), was born in Zurich Switzerland. He received his PH.D. in astronomy in 1910.
In 1915, he took a position at Allegheny Observatory, and later in 1918 went to Lick Observatory.
Trumpler studied the brightness of distant open star clusters in order to determine the size of the Milky Way galaxy.
In 1930, he published a catalogue of open clusters titled:
"Preliminary results on the distances, dimensions and space distribution of open star clusters"
His investigation of distances, dimensions, and space distribution of galactic open star clusters was a significant contribution to astronomy,
Trumpler's work contains a table of 37 new open star clusters, now known as the Trumpler catalog.
The amateur astronomer will find many of his 'Tr' objects listed on star atlases and observing guides.
While cataloguing open clusters, Trumpler also devised a classification system according to the number of stars observed within them,
how concentrated these stars are in the center of the cluster and the range of their apparent brightness.
This system, known as the "Trumpler classification", is still used today.
While some of the 'Tr' objects are already listed under other catalog designations such as 'NGC',
the majority are unique objects, not listed in any other prior catalog.
Trumpler clusters also cover a wide range in brightness, and can be found along sections of the Milky-Way,
with several nice ones lying between Scutum, Sagittarius, and Scorpius.
The Saguaro Astronomy Club (SAC), has a great pdf download of the Trumpler Clusters.
Touring the Trumpler Classes
Trumpler classified his list into four categories, which are further broken down by brightness and concentration.
A Roman numeral from I to IV indicates its concentration and detachment from the surrounding star field (from strongly to weakly concentrated)
class I = Detached cluster with strong central condensation.
class II = Detached cluster with little central condensation.
class III = Detached cluster with no noticeable condensation.
class IV = Appears as a star-field
A whole number from 1 to 3 indicating the range in brightness of cluster stars (from small to large range)
1 = Small range in brightness. (most stars look about the same)
2 = Medium range in brightness. (equally bright and faint stars)
3 = Large range in brightness. (large gap between a few very bright stars and remainder of cluster stars mostly faint).
A lower-case 'p', 'm', or 'r' indicates whether the cluster is 'poor', 'medium' or 'rich' in stars.
p = Poor. (star clusters with less than 50 stars)
m = Medium. (moderatey rich star clusters with 50 to 100 stars)
r = Rich. (rich star clusters with more than 100 stars)
Finally, an 'N' is appended if the star cluster lies within nebulosity.
For example the Double Cluster is classified as a "IV3r". The Pleaides is a "II3rN'.
On the left side of the webpage is my personal observation list of the
'Trumpler catalog of star clusters'.
I currently have 23 out of a possible 37 objects visible from my Pittsburgh, PA latitude of around +40.
(14 of the 'Trumpler' objects are not observable from latitude +40).
Hope you enjoyed the visit. Come again soon!
Larry McHenry, Pittsburgh, PA. USA