Creating a Colour-Magnitude Diagram of Open Cluster M37 With ...

Creating a Colour-Magnitude Diagram of Open Cluster M37
With CCD Photometry from the BGO
- Daniel Majaess, Spring 2004
Abstract
When astronomers examine an open cluster the
colour-magnitude diagram proves to be an essential
tool in understanding the composition, age, distance,
and evolution of the constituent stars—the amount
of information gained is simply unprecedented. Using
the 0.4m reflecting telescope at the BGO observatory,
CCD photometry was taken of M37 which
produced a colour-magnitude diagram that bears a
striking similarity to more academic results obtained
from WEDBA (the database for galactic open clusters
(BDA)).
Introduction
An open cluster is a grouping of stars—generally population
I—which have evolved from the same molecular
cloud. Being of a high metallicity, the cluster’s
progenitors are areas of abundant gas and are thus
found in the plane of the Milky Way. Although open
clusters are often confused with their globular counterparts,
they’re stark differences in member numbers,
age, formation, composition, and location as
the table bellow summarizes. A great way of studying
these clusters is by constructing a colour-magnitude
diagram. As Figure 1 demonstrates one can observe
the main sequence where core hydrogen fusion occurs,
the turn-off point when that fusion begins to
cease, and the cluster’s red giant population is easily
identifiable.
Cluster Location Age Population #
Open Plane Young 500
Globular Halo Old 10 4 − 10 5
Apparatus
The cluster was observed at the BGO observatory
which is equipped with a 0.4m reflecting telescope
and an SBIG ST8 CCD camera. The telescope was
controlled using The Earth Centered Universe and
the photometric data was analyzed by another commercially
available software entitled MaximDL.
Figure 1: Colour-Magnitude Diagram
Procedure
M37 was chosen due to its central population density,
its high altitude during observing—lower altitude
photometry suffers from increased air mass and
light pollution—but mainly because 120 of its members
are brighter than 12.5 magnitude. After exposing
the cluster, MaximDL was used since it has a
photometry tool specifically designed for this type of
analysis. To calculate the apparent magnitudes for
stars in the images, the program requires the input
of known magnitudes for some reference stars from
which the relative brightness for the others are then
deduced. These reddening corrected reference stars
were chosen from the Webda catalogue, which provides
a wealth of data on various open clusters. Figure
2 is a screenshot of MaximDL’s photometry tool
and Figure 3 is a map of the cluster’s central quadrant.
Data for 116 stars were extracted, however
determining membership in the cluster was not addressed
and so stars were taken from throughout the
field.
I

Figure 2: Photometry Tool, MaximDL
Figure 4: CMD M37, BGO Observatory
Figure 3: Cluster Map of M37
Results & Discussion
Figure 4 is our colour magnitude diagram (CMD) for
116 possible members of M37, a diagram from the
WEDBA database is also provided (Figure 5). One
immediately notices the stark similarities between the
two diagrams: from the main sequence, to the turnoff,
to the group of red giants, there is an obvious consistency.
However, transformation coefficients were not
calculated which would have normalized the uniqueness
of our camera, filter set, and telescope to the
standards. Furthermore the CCD chip is known to be
rather sensitive through the blue band so accounting
for these effects may explain the ±0.2mag translational
discrepancy in the (B-V) axis.
Figure 5: CMD M37, Webda
II