Some of you may recognize this, I’ve published it on the Zone before. It was my first published article, back in 1995, and was accepted by a national popular SETI journal, unfortunately, now defunct. I’ll reprise it here so VRB can read it, and for those of you who may have forgotten it.

In any form of SETI research, logic suggests that the
probability of success will be increased by observing, or
transmitting towards, clusters of potential targets. In
this way, the number of possible transmitting or receiving
sites is concentrated in the field of view or primary
reception/transmission lobe of the instrument. For example,
rather than observing hundreds of solar-type stars scattered
throughout the celestial sphere, would it not be more
productive to examine these stars simultaneously by
gathering them all together in one small area of sky?

It is fortunate that nature has provided us with
astronomical objects which meet this criterion. They are, in
increasing order of size and distance, open star clusters,
globular clusters, and galaxies. Let us examine each of these
object types separately and evaluate the SETI potential of each.

Galaxies are colossal objects, thousands of lights years
across and containing billions of stars. They would be ideal
SETI targets except for the fact that they lie at almost
unimaginable distances from us. As an example, the nearest major
galaxy to ours, Andromeda (Messier 31), is 2.2 million light
years away. It is difficult to conceive of any extraterrestrial
civilizations’ activities which would be detectable, or even
recognizable, as artifacts at such a distance.

Globular clusters, of which several hundred are currently
known, are spherical balls of up to one million stars,
distributed in random elliptical orbits around the galactic
nucleus. Unfortunately, globulars are very old. They appear
to have formed when the Milky Way itself first formed and when
the interstellar medium was empty of the chemical elements
essential to life.

The early Universe was deficient in what stellar astronomers
call “metals”; that is, all elements except hydrogen and helium.
It is likely that any planets formed around stars in globular
clusters will be gas giants like Jupiter, made up primarily of
hydrogen.
After the initial burst of star formation, the primeval
Milky Way’s interstellar medium collapsed into the flat disk
of the galactic plane. Then began the enrichment of the medium
by “metals” from the first generation of stars and the supernovae
and stellar winds from those stars. These metals, produced by
thermonuclear reactions in the earliest stars, gradually enriched
the interstellar medium, making rocky planets possible. The
enriched gas and dust gravitationally collected in the galactic
disk and new stars and planets were born in thick clouds of
diffuse material that formed there.

About 4.6 billion years ago, our solar system formed in a
thick, metal-enriched, dust and gas cloud. Today, we can see
this occurring throughout our galaxy and even in other such
“star islands”. The process appears to be similar to rain
drops forming in a thunderstorm. Stars condense from a cloud,
their nuclear fires ignite, and the resulting stellar winds
blow away the residual gas and dust, leaving our third type
of object, the open cluster.

Open clusters — they may also be referred to by the misleading
and obsolete term of “galactic cluster” — are what is left of a
stellar nursery after the primeval stellar medium is blown away.
Thousands of these objects are known to exist, though most are too
far from Earth to be detected. They have been found at every stage
of their evolution, from the Orion Nebula (Messier 42), where new
protostars are still condensing from the roiling clouds of matter,
to loosely-bound associations where only indirect methods suggest
a common origin to the stars.

It now appears that all stars in the galactic disk were
formed this way, and all were members of clusters which have
since dispersed. It is existing open clusters which I believe
are an ideal place to concentrate our SETI efforts. They
contain from several hundred to several thousand stars and
are typically several thousand light years from Earth. They
all have a common origin in the galactic plane which guarantees
us that, unlike the globulars, the individual stars are at least
capable of having rocky, life-supporting planets. Those clusters
are also well distributed throughout the sky, although for obvious
reasons, they are found most frequently along the plane of the
Milky Way.

From the observational point of view, this means that when
the cluster is centered in the field of view from a telescope,we are sighting along a tunnel already crowded with a large
number of disk stars, lying between us and the cluster as well
as beyond it. To summarize, observing an open cluster allows
us to simultaneously observe a large number of stars of common
origin, common age, suitable chemistry, and at a reasonable
distance. At great distances, the clusters are lost in the
dust haze of the galactic disk.

The compact nature of these clusters (individual member
stars are less than one light year apart) not only increases
the probability that we may stumble across stars of interest
to SETI, it also suggests that any civilizations evolving there
may have colonized other star systems in the cluster, further
increasing our chances of eavesdropping on inter-cluster
communications.

All star clusters have long been intensely studied by
astronomers. During the course of these investigations,
additional information has come to light which places some
restrictions on the utility of clusters as SETI targets. It
was found early on that the color-brightness distribution among
cluster stars gave a means of determining their ages. This is
how we know that globulars are too old and metal-poor to be
suitable abodes of life.

It was also learned that most open clusters are too young
for life of any sort to have evolved. Actual observations and
theoretical studies have confirmed that, unlike their massive
globular cousins, open clusters are gravitationally unstable
and tend to be torn apart by galactic tidal forces. This is
why most disk stars, although cluster-born, are solo wanderers
like Earth’s Sun. Age studies indicate that relatively few
open clusters survive past one billion years as discrete objects.
Instead, they tend to evaporate, gradually donating their stars
to join the billions already orbiting the galactic center.

But all is not lost. Some open clusters do survive to a
ripe old age. A cursory examination of a good contemporary
source* shows 750 open clusters considered accessible to
advanced amateur telescopes. Of these, roughly one-half have
their ages listed. A search through the current astronomical
literature would probably double this list.

Using an admittedly arbitrary cut-off age of one billion
years to allow for experimental error and accelerated evolutionon some worlds, a total of 32 open clusters survive as possible
targets, with an age range from 1 to 6.3 billion years. These
clusters range in distance from 1,300 to 17,000 light years from
Earth. Details on these 32 candidates are listed in Table 1.

These distances are much greater than those normally
associated with SETI research. It would require the activities
of a very highly advanced civilization to be detectable over
such a long way, especially since that civilization, no matter
how advanced, would have no particular reason to be aiming a
beam in our direction.

If a cluster civilization has SETI ambitions, it may be
conducting active and passive programs, concentrating on the
galactic plane and the Milky Way’s center in particular.
Consequently, suitable open clusters of galactic longitude near
180 degrees and low galactic latitude would be particularly good
targets for us. From our position between the cluster and the
densely populated galactic core, we would be in an excellent
position to fulfill both of our expectations. At any rate, the
substantial increase in potential targets may more than make
up for the greater distances involved.

* Sky Catalogue 2000.0, Volume 2, Double Stars,
Variable Stars, and Nonstellar Objects. Edited by Alan
Hirshfeld and Roger W. Sinnott. Cambridge University Press
and Sky Publishing Corporation, 1985. Pages 274-284.

TABLE 1: List of Open Clusters as Potential SETI Targets

Object, Galactic Coordinates, Age, Distance, Equatorial Coordinates (2000.0)

Name, long. lat, (x10^9 yr), (Parsecs), RA DEC

NGC 188 123 22 5.0 1550 0h 44.4m 85d 20′
NGC 559 127 1 1.3 900 1 29.5 63 18
IC 166 130 0 1.6 3300 1 52.5 61 50
NGC 752 137 -23 1.1 400 1 57.8 37 41
NGC 1245 147 -9 1.1 2300 3 14.7 47 15
Berkeley 19 177 -4 3.0 4000 5 24.1 29 36
King 8 176 3 1.3 4150 5 49.4 33 38
NGC 2141 198 -6 4.0 4400 6 03.1 10 26
NGC 2158 187 2 3.2 4900 6 07.5 24 06
NGC 2204 226 -16 3.0 4450 6 15.7 -18 39
NGC 2243 240 -18 3.9 4600 6 29.8 -31 17 NGC2360 230 -1 1.3 1630 7 17.8 -15 37
Melotte 66 260 -14 6.3 2500 7 26.3 -47 44
NGC 2420 198 20 4.0 2500 7 38.5 21 34
NGC 2506 231 10 4.0 2200 8 00.2 -10 47
NGC 2527 246 2 1.0 600 8 05.3 -28 10
NGC 2682 216 32 3.2 800 8 50.4 11 49
NGC 2818 262 9 1.0 3200 9 16.0 -36 37
NGC 3680 287 17 1.8 800 11 25.7 -43 15
Ruprecht 97 297 0 1.0 4000 11 57.3 -62 39

NGC 6208 334 -6 1.0 1000 16 49.5 -53 49
IC 4651 340 -8 2.4 780 17 24.7 -49 57
NGC 6791 70 11 6.3 5100 19 20.7 37 51
NGC 6802 55 1 1.7 990 19 30.6 20 16
NGC 6819 74 8 3.5 2200 19 41.3 40 11

NGC 6939 96 12 1.8 1250 20 31.4 60 38
NGC 6940 70 -7 1.1 800 20 34.6 28 18
NGC 7039 88 -2 1.0 700 21 11.2 45 39
IC 1369 90 0 1.3 1500 21 12.1 47 44
NGC 7082 91 -3 1.6 1400 21 29.4 47 05

NGC 7142 105 9 4.0 1000 21 45.9 65 48
NGC 7789 116 -5 1.6 1900 23 57.0 56 44