Trumpler (Tr)

In the world of star clusters, Trumpler is a name that represents both a specific catalog of objects and the most widely used "language" for describing what a cluster actually looks like.

History & Who Made It

The catalog and system were created by Robert Julius Trumpler (1886–1956), a Swiss-American astronomer at the Lick Observatory. In 1930, he published a landmark paper titled Preliminary results on the distances, dimensions and space distribution of open star clusters.

Trumpler's most famous discovery during this research was interstellar extinction—the realization that space is filled with dust that dims and reddens the light of distant stars. To prove this, he had to meticulously categorize and measure the distances of star clusters across the Milky Way.

The Purpose: The "Trumpler Classification"

While Trumpler discovered 37 new clusters, his greatest contribution was a standardized system to describe any open cluster's appearance. It uses a three-part code (e.g., II 3 m) that tells an astronomer exactly what to expect before they look through the eyepiece.

1. Degree of Concentration (Roman Numerals I to IV):

• I: Detached; strong central concentration (stands out clearly).

• II: Detached; little central concentration.

• III: Detached; no noticeable concentration.

• IV: Not well-detached from the surrounding star field (looks like a random patch of stars).

2. Range of Brightness (Arabic Numerals 1 to 3):

• 1. 1: Most stars are nearly the same brightness.

  2. 2: A medium range of brightness.

  3. 3: Composed of both very bright and very faint stars.

3. Number of Stars (Lowercase Letters p, m, r):

• p: Poor (less than 50 stars).

• m: Medium (50 to 100 stars).

• r: Rich (more than 100 stars).

• n: An extra suffix used if the cluster is surrounded by nebulosity (gas clouds).

What Objects and How Many

The Trumpler Catalog (Tr) itself is relatively small, containing 37 objects. However, these were 37 "new" discoveries that had been missed by previous major surveys.

• Trumpler 1 to 37: A list of open clusters, many of which are located in the summer and winter Milky Way.

• The "Trumpler Classification" Usage: This system is used for all thousands of known open clusters in every other catalog (Messier, NGC, IC, etc.).

The Observer's Secret

The Trumpler system is the secret to "seeing" clusters. If a cluster is a Type IV, don't expect a bright ball of stars; look for a subtle change in the density of the star field. If it's a Type I, it will pop out of the background like a diamond on velvet.