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f course, old red stars become visible at longer wavelengths even without dust, since that is where they radiate most of their energy and light. However, the situation becomes greatly skewed in the presence of cosmic fog.
A simple analogy may be in order: a ship approaches a harbor at night. The captain does not see the harbor in much detail. His view is dominated by the brilliant light of the harbor lighthouse, just as our view of the spirals is dominated in visible light by the brilliant hot stars and the regions of star formation. Now imagine that the night is foggy. The captain sees even less of the harbor, just as our view of galaxies is affected by the fog of cosmic dust. So the Captain's view of the harbor is misleading because of two critical issues: once by the brilliance of the optical light beaming from the lighthouse, and secondly by the fog enveloping the actual lighthouse. How strikingly does our full moon mask the presence of fainter stars (Figure 129).
Astronomer Ben Gascoigne recalls the great obscuring effects of dust when he first observed a class of variable stars in the Small Magellanic Cloud, using the 30-inch Reynolds reflecting telescope:
There was one major surprise, obvious from the first night of observing. All eleven ofour Small Magellanic Cloud Cepheids were strikingly blue, definitely bluer than the Cepheids in the galaxy ... it was hard to believe that the [Milky Way]galaxy could contain enough dust to redden its Cepheids to the extent we had found. The Dust Penetrated Universe
Of course cosmic dust has absolutely nothing to do with dark matter that dominates the mass of galaxies: the dust mass in a typical spiral galaxy represents an insignificant fraction of the total mass of the galaxy. But herein is an absolutely crucial point: a substance's effects should never be judged by its mass alone. How much, after all, does a fog weigh? Almost nothing; yet it is capable of blinding pilots, diverting flight paths, shutting down airports.
Think for a moment of an erupting volcano. Particles of ash may effectively obscure the entire volcano. And so it is, with galaxies. The effects of dust or smoke particles in Shrouds of the Night are enormously effective. The penetrating power of the near-infrared allowed us to see, for the first time, a majestic new view of the structure of spiral galaxies.
Instead of volcanic masks, think of the Asaro mudmen wearing their historic masks (Figures 2 and 3). In optical light, we can photograph their masked faces, dancing with the majestic Eastern Highlands as a backdrop. Let us now suppose that we were to ask them to remove their masks: a very different image emerges: the effects of the mask are gone, and rich facial structure is revealed. We see their true eyes, their actual noses and facial structure: how impossible to predict, a priori, what their faces would look like before we ask them to remove their masks! A rich duality in structure - that of the mask and that behind the mask - emerges.
The Australian Aborigines capture this kind of duality of structure in their x-ray paintings. While their paintings of a fish and a turtle have the unmistakable outline of these creatures, they also unveil their hidden skeletal backbones and organs, as if seen with x-rays ... these arresting x-ray paintings, one of which is seen in Figure 130, are drawn as if we could penetrate the very inner morphologies - much as we do with our galaxies in the infrared.
Just as Röntgen a century before had opened up a new era to penetrate the skeletal frame encased in a mask - our skin - so too, can our infrared images allow us to unveil the actual backbones of galaxies.
When galaxies are imaged photographically in the visual domain of the electromagnetic spectrum, the focus is obviously on the young and brilliant spiral arm tracers - the young Population I component - the icing or frosting on the cake, as it were.
The young Population I component - the mask - is relatively very light: only about five percent of the total luminous mass. The grand photographic surveys of earlier epochs give us a very incomplete picture of what spiral galaxies actually look like. Behind the fog lies the older Population I component of stars, where ninety-five percent of the galaxy's total luminous mass is distributed.
It soon became apparent that it was impossible to predict what the dust penetrated image of a galaxy might look like: each image behind the Shroud was a grand revelation, in its own Shrouds of the Night right. The words of Benjamin Franklin (1706-1790) ring in our ears:
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