(originally run February 9,2001)
The more and farther we look out into space, the more we keep finding. Every new advance in our instruments and our techniques shows us more and more galaxies in the most distant reaches of the universe, and there does not seem to be any kind of end in sight.
One of the more recent programs geared toward this endeavor is the Massive Cluster Survey (MACS), an effort initiated by Harald Ebeling at the University of Hawaii. Ebeling and his colleagues used archived data from the recently-deactivated ROSAT x-ray satellite to locate regions of what appear to be large collections of very hot gas. When these were examined with some of the world's largest telescopes -- for example, one of the two 10-meter Keck telescopes on top of Mauna Kea in Hawaii -- they were found to be distant, large, populous clusters of galaxies.
We see numerous such clusters of galaxies here in our own part of the universe, the nearest one of these being in the springtime constellation of Virgo. Such clusters may contain anywhere from several hundred to several thousand galaxies as large as our own Milky Way (and probably contain several times that number of much smaller galaxies). The galaxy clusters found during the MACS program are similar to these but are much farther away; their typical distance from us is five billion light-years or more. In other words, the light that we now see from these objects left on its journey to us before our solar system began forming.
These galaxies, however, are not the limit of what we've detected. Five years ago the Hubble Space Telescope took a 10-day-long exposure of a tiny nondescript region of sky near the handle of the Big Dipper. This so-called "Hubble Deep Field" revealed some three thousand individual galaxies in a region of sky no more than one-one-hundred-fiftieth the area of the full moon and extending as far out into the universe as it is possible for us to see. A similar "Hubble Deep Field South" exposed three years later of a region of sky in the southern hemisphere revealed similar numbers.
If Hubble were to perform this same experiment over the entire sky and do so continuously it would take over one million years to complete the task. If the number of galaxies that we see in the northern and southern Hubble Deep Fields are typical of what we would see elsewhere -- and there is no reason to suspect otherwise -- then the number of galaxies in the universe detectable with Hubble is in the neighborhood of 100 billion.
Astronomers are sometimes guilty of throwing around numbers of this size as if they are everyday quantities, and thus it might help to try to put this number into perspective. One way to do so is to recall that one billion seconds is equivalent to 31.7 years. Thus, if one were to count all these galaxies at the rate of one per second and never did anything else, it would take well over three thousand years to do so.
Each of these galaxies is more or less about the same size of our own Milky Way. We've determined over the course of the past century that our galaxy contains between one and four hundred billion stars. If we assume that the average number of stars in each of the galaxies is 100 billion, then the total number of stars in the universe is approximately 10 sextillion, i.e., 10,000,000,000,000,000,000,000. If one were to count all these at the rate of one per second it would take approximately 20,000 times the age of the universe to get them all counted.
Somewhere within this universe of a hundred billion galaxies is one utterly unremarkable galaxy that is essentially indistinguishable from any of the others. Within this unremarkable galaxy we will find perhaps a few hundred billion stars, and out in the backwaters of this galaxy we will find one entirely unremarkable star that has no reason to have any attention called to it. Surrounding this unremarkable star are a few objects that we might call "planets" as well as many thousand much smaller objects that we might call "asteroids" or "comets."
The third of these planets out from this unremarkable star is actually one of the smaller of these objects. Among the many life forms that inhabit the surface of this tiny insignificant dust grain of a world that we have christened "Earth" is a species that, every once in a while, pauses for a little while to stare out into the rest of the universe and wonder about what might be out there.
But most of the time, however, our thoughts aren't so lofty. In the words of the late Carl Sagan, "Think of the rivers of blood spilled by all those generals and emperors so that, in glory and triumph, they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this [dot] on the scarcely distinguishable inhabitants of some other corner, how frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds."
Perhaps the next time we start to feel we're important, we should take a moment to remember just how large is this universe within which we live.
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