8.5 Target-Specific Problems
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8.5.1 Imaging Open Clusters
Open clusters, also known as galactic clusters, are sites of the most recent star formation in the galaxy, and as the spiral arms are delineated by star formation, they
are always found within the confines of the Milky Way. Hence, we do not find open
clusters in, for example, Ursa Major, but we find many in Cassiopeia. Open clusters
form from collapsing molecular clouds that have become dense enough and cold
enough that gravity overcomes the thermal motion, turbulence, and magnetic fields
that can prevent collapse. However, do not be fooled by science fiction shows: the
density of these clouds is still lower than any vacuum ever achieved on Earth!
As the cloud collapses, local, smaller regions become superdense, and these further collapse into stars. Over time, radiation pressure, stellar winds, and supernovae
from the more massive stars formed within the cluster strip the cluster of the gas and
dust that was not incorporated into the stars. It is this dust and gas that makes up
the bulk of the mass of the cluster, not, as you might expect, the stars. In fact, star
formation is quite inefficient, with only about 1–3% of the mass of the initial cloud
becoming stars. With the stripping of this gas, the cluster becomes gravitationally
unbound and begins to break up, with the stars becoming members of the galactic
disk. Depending on the mass of the cluster, this can take anywhere from tens of
millions to a billion years.
As a point of interest, open clusters appear to be mass segregated, with more
massive stars found near the centre of the cluster and lower-mass stars near the edge.
Why this happens is a matter of debate. Models suggest that over time, gravitational
interaction will segregate the cluster. However, large-scale star formation models
suggest that the most massive stars tend to form in the densest part of the cloud, which
is typically at the centre. Perhaps it is more likely that both processes contribute to the
overall distribution of stars within the cluster. Nonetheless, open clusters are some
of the less challenging objects for the proto-astrophysicist to image.
The principal problem undergraduates encounter when exposing an open cluster is
overexposing and therefore saturating the brighter stars, especially when an exposure
time is not adjusted between bands. Overexposure, or in fact entering the nonlinear
region of the CCD, that occurs before saturation can render the image effectively
useless for science, which is not immediately obvious when an image is displayed.
It is important to understand a number of effects. Firstly, the size of the star on the
image is not representative of its brightness, as the processes that cause the light to
be spread over multiple pixels, diffraction and atmospheric seeing, affect all stars
in the field identically, and the star’s apparent size is therefore independent of the
brightness of the source. Visual detection of saturation is problematic, as computer
monitors are unable to display the full pixel range in grey levels, and even if they
could, the human eye would be unable to distinguish between the grey levels over
the entire range. To resolve this problem, you will have to perform an image stretch
(also known as scaling). This has nothing to do with changing the shape or size of
the image; rather, it is how the image is displayed. In general, stretching does little to
115
8.5.1 Imaging Open Clusters
Open clusters, also known as galactic clusters, are sites of the most recent star formation in the galaxy, and as the spiral arms are delineated by star formation, they
are always found within the confines of the Milky Way. Hence, we do not find open
clusters in, for example, Ursa Major, but we find many in Cassiopeia. Open clusters
form from collapsing molecular clouds that have become dense enough and cold
enough that gravity overcomes the thermal motion, turbulence, and magnetic fields
that can prevent collapse. However, do not be fooled by science fiction shows: the
density of these clouds is still lower than any vacuum ever achieved on Earth!
As the cloud collapses, local, smaller regions become superdense, and these further collapse into stars. Over time, radiation pressure, stellar winds, and supernovae
from the more massive stars formed within the cluster strip the cluster of the gas and
dust that was not incorporated into the stars. It is this dust and gas that makes up
the bulk of the mass of the cluster, not, as you might expect, the stars. In fact, star
formation is quite inefficient, with only about 1–3% of the mass of the initial cloud
becoming stars. With the stripping of this gas, the cluster becomes gravitationally
unbound and begins to break up, with the stars becoming members of the galactic
disk. Depending on the mass of the cluster, this can take anywhere from tens of
millions to a billion years.
As a point of interest, open clusters appear to be mass segregated, with more
massive stars found near the centre of the cluster and lower-mass stars near the edge.
Why this happens is a matter of debate. Models suggest that over time, gravitational
interaction will segregate the cluster. However, large-scale star formation models
suggest that the most massive stars tend to form in the densest part of the cloud, which
is typically at the centre. Perhaps it is more likely that both processes contribute to the
overall distribution of stars within the cluster. Nonetheless, open clusters are some
of the less challenging objects for the proto-astrophysicist to image.
The principal problem undergraduates encounter when exposing an open cluster is
overexposing and therefore saturating the brighter stars, especially when an exposure
time is not adjusted between bands. Overexposure, or in fact entering the nonlinear
region of the CCD, that occurs before saturation can render the image effectively
useless for science, which is not immediately obvious when an image is displayed.
It is important to understand a number of effects. Firstly, the size of the star on the
image is not representative of its brightness, as the processes that cause the light to
be spread over multiple pixels, diffraction and atmospheric seeing, affect all stars
in the field identically, and the star’s apparent size is therefore independent of the
brightness of the source. Visual detection of saturation is problematic, as computer
monitors are unable to display the full pixel range in grey levels, and even if they
could, the human eye would be unable to distinguish between the grey levels over
the entire range. To resolve this problem, you will have to perform an image stretch
(also known as scaling). This has nothing to do with changing the shape or size of
the image; rather, it is how the image is displayed. In general, stretching does little to