skyandtelescope.com • NOVEMBER 2019 61
Tr u m pler
Classifi cation
All open clusters can be graded using the
classifi cation scheme devised by Swiss-
American astronomer Robert Trumpler and
further described by Swedish astronomer Gösta
Lyngå. The scheme is based on three scaled
parameters: concentration, range of brightness,
and richness. The fi rst parameter describes how
well the cluster stands out, or is “detached,”
from the rest of the star fi eld and whether there
is a concentration of stars towards the center.
The breakdown of the categories is as follows:
CONCENTRATION
I Detached clusters,
strong central concentration
II Detached clusters,
light central concentration
III Detached clusters,
no central concentration
IV Clusters that are not well detached
from the surrounding star fi eld
RANGE OF BRIGHTNESS
1 Most cluster stars are of similar
apparent brightness
2 Medium range in brightness
3 Both bright and faint stars are present
in the cluster
RICHNESS
p Poor: fewer than 50 stars
m Medium rich: 50–100 stars
r Rich: more than 100 stars
Clusters can be characterized by the
combination of these three parameters. For
example, both components of the Double
Cluster, NGC 869 and NGC 884, are classifi ed
as I3r. Although none of the King clusters is
embedded in nebulosity, you may see an “n”
added to other Trumpler descriptions.
For each of the clusters represented, the
visual magnitude (Mv) of the cluster is listed.
When that wasn’t available, the magnitude
stated is that of the brightest star in the cluster,
indicated by an asterisk (Mv*). In addition, the
size (in arcminutes, S), number of stars (No.),
and Trumpler classifi cation (from Archinal and
Hynes, Tr) are noted, as is the magnifi cation
(Mag.) for the visual observations.
tKING 11 Staying
in Cepheus, look
for this small and
very sparse cluster
almost exactly
halfway between
8th-magnitude
HD 223434 and
9th-magnitude
HD 223103. Note
that north is to the
lower left.
Mv* = 17 | S = 6′ |
No. = 50 | Tr = I2m |
Mag. = 337×tKING 18 This
is an elongated
open cluster with
no concentration
toward its center
and milkiness in
the background. I’d
classify it as III3m
(as opposed to the
catalog description
of II2p). Ninth-mag-
nitude HD 216426
may guide you to
the cluster some 5′
north-northeast of
the star.
Mv* = 12 | S = 5′ |
No. = 20 | Tr = II2p |
Mag. = 272×uKING 19 The stars in this cluster are nicely
resolved. The brighter members form the vertices
of a triangle — it could almost be an asterism.
Mv = 9.2 | S = 5′ | No. = 52 | Tr = III2p | Mag. = 66×
and 256×
HD 223103HD 223434NHD 2164265 ′5 ′5 ′