Astronomy - February 2014

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motions of stars in the Pleiades. A few
months later, Schlesinger invited Trumpler
to come to Allegheny as an assistant, with
the opportunity to work on the Pleiades.
Trumpler accepted the position, but when
World War I began, the Swiss militia
drafted him, made him an officer, and sta-
tioned him in the Alps.
Schlesinger wrote that he would not be
able to hold the position open indefinitely,
and Trumpler presented his case to his
superiors. They released him from duty
with permission to travel to the United
States. In May 1915, he arrived at the
Allegheny Observatory. Once there, he
began to investigate star clusters to learn
more about their characteristics.

Star cluster research
His work in this field expanded when, in
1918, he began work at Lick Observatory in
California. After a dozen years of research,
Trumpler produced his greatest contribu-
tion to astronomy: “Preliminary Results on
the Distances, Dimensions, and Space Dis-
tribution of Open Clusters” (Lick Observa-
tory Bulletin, vol. 14, 1930).
By making a detailed investigation of
the apparent sizes and distances of these
objects, Trumpler demonstrated that the
seemingly empty space in our galaxy con-
tains a tenuous haze that dims the stars
seen through it.
This paper included data on 334 clus-
ters. Trumpler showed that astronomers
were overestimating distances to them
because interstellar material, previously
thought to be nonexistent, was dimming
the starlight by an average of 0.67 magni-
tude for every kiloparsec (3,260 light-years)
of distance. Indeed, this “interstellar
extinction” inf luences our view of the
entire universe.

The classification scheme
In his famous paper, Trumpler titled a sec-
tion “Classification of Clusters According
to Appearance.” In it, he first divided all
clusters into four main groups:
I. Detached clusters with strong central
concentration.
II. Detached clusters with little central
concentration.
III. Detached clusters with no noticeable
concentration, in which the stars are more
or less thinly but nearly uniformly scat-
tered.
IV. Clusters not well detached but pass-
ing gradually into the environs, appearing
like a star field condensation (that is,
ungrouped into a cluster).

NGC 7686 in Andromeda has a Trumpler classification of III2p. Through a 4-inch telescope, you’ll spot

about two dozen stars ranging from magnitude 7.5 to 11.

Stock 12 in Cassiopeia carries the Trumpler designation IV2p. That means it doesn’t stand out well from

the background, falls in the medium range of star brightnesses, and contains fewer than 50 stars.

NGC 129 lies in the constellation Cassiopeia. At magnitude 6.5, it’s an object that sharp-eyed observers

can detect from a dark site. In the Trumpler system, this open cluster has a classification of II2m.