It is not difficult to appreciate how (a) the idea, that a spiral galaxy’s arms are concentrations of stars, is inconsistent with (b) the relativistic tenet that no physical object can move with a speed v>c. For owing to the enormity of galactic radii, the orbital periods of the outermost stars must be greater than those of the more centrally located ones. Consequently the arms (assuming they are concentrations of stars) would relatively quickly evolve into tightly wound spirals.  But such is not observed to be the case in galaxies whose ages far exceed the tight-spiral-evolution-time.

Astronomers now believe that the galactic arms are the visible manifestations of compression waves whose tangential speeds are proportional to their distance from the galactic center. As a result all parts of a given arm revolve at a common angular velocity, and the arms do not degrade into tightly wound coils.

One theoretical consequence of such compression waves is that the stars in a visible arm (whose thickness presumably approximates a compression wavelength) are compressed more closely together as the wave passes through. And they are "rarefied" between compression maxima. Among other things, times of compression may see a marked increase in the birthrate of new stars.

A given body is in thermal equilibrium with its surroundings when its temperature remains constant. In such cases the rate of radiant energy absorption must equal the rate at which thermal radiation is emitted. And the emission rate is monotonically related to the temperature: the higher the object’s temperature, the higher its rate of thermal radiant energy emission. For a given object in equilibrium with its environment, then, absorbed energy is also monotonically related to the body’s temperature (or vice versa).

Let us suppose that a planet is adrift among a galaxy’s stars, but is not held close to any particular star. The planet is constantly bathed in starlight from every direction. And of course the rate of radiant energy absorption is higher when it comes from nearby stars. If the stars in a given arm are compressed closer to the planet, then the planet’s temperature will rise. Such would be the case when a compression wave passes through the arm’s stars, dust, etc. Subsequently, when the arm lies in a rarefaction zone, the stars would disperse to greater distances, and the planet’s surface would cool.

Of course the heating and cooling frequency may be very small (or the period very large), given the estimated magnitudes of compression wavelengths and the modest phase/group velocities. Indeed the rates of compression and rarefaction may be such that, to a denizen on the planet’s surface, the stars in an arm (and more generally in the galaxy) might seem not to move relative to one another at all, particularly if records have been kept only for a few thousand years.

If we consider planet Earth, by far most of our incident radiant energy comes from the Sun. But a relatively small part does come from the stars in our parent arm. And this part of the total incident radiation bathes the planet over its entire surface constantly, theoretically at alternating greater and lesser intensities as compression waves pass through. Above and beyond the effects of solar insolation, then, the planet’s temperature might be expected to rise and fall slightly owing to stellar "astralation."

Planet Earth has experienced many cool epochs interlaced with warmer periods. Indications are that the Earth’s mean temperature is presently edging upward and we are heading into warmer times. Polar ice caps are melting, the oceans appear to be warming, tropical storms are becoming more intense. There is great human concern over this trend, and understandably so. The specter of great port cities around the globe being awash (or at risk behind improvised dikes) looms, not to mention undesirable effects on global climate, changes in ocean plant and animal life, and indeed even changes in the concentrations of atmospheric gases!

Despite the fact that the planet is historically characterized by alternating cool and warm periods, belief is widespread that the present warming trend is caused by human activities, particularly by the emission of carbon dioxide into the atmosphere. The contention is that CO₂ is a "greenhouse" gas that nudges the equilibrium temperature upward. But trapped heat from the Sun might not be the only factor in rising temperatures. For each and every automobile loses most of the energy from burned fuel as waste heat. Only a small fraction of the fuel’s energy is converted to desirable mechanical energy. As a result, every moving or idling automobile is like a bonfire. And at any moment there are a billion-plus of these figurative bonfires blazing away around the planet, adding heat to its atmosphere 24/7. Similar remarks apply to other power generation facilities, particularly fossil fuel and nuclear power plants. "Green" energy resources … hydroelectric, wind and photovoltaic … seem to produce the least heat as a byproduct.

Now the good news is that we can (a) change the methods we use to produce power, and (b) decrease our utilization of power. Regarding the latter, we should consider going on a worldwide energy diet. If nothing else, the amounts of heat and CO₂ released daily into the atmosphere would decrease.

With regard to CO₂, there are some who reject the whole idea that CO₂ is the culprit it is widely touted to be. But no one can deny that significant quantities of heat are added daily to the atmosphere by human-instigated combustion. (Proponents of Hydrogen as the cure-all fuel of the future: Take Note.)

Of course no matter what we do, if astralation is increasing then Earth’s equilibrium temperature will almost certainly also increase True enough, the radiant power intercepted from a single star may not seem like much (although a circle of 4000-mile diameter is absorbing that light). But the light from many billions of stars is being absorbed all the while … even when the Sun shines! Prudence would seem to dictate that we not exacerbate the problem by continuing (and even increasing) our current energy glut.

There are of course other problems associated with our dependence on motor vehicles. In recent history, obesity, diabetes and a host of other maladies have become epidemic in the so-called developed world with its motor-vehicle-addicted citizenry. Up until now China and much of Asia has fared better, with its citizens getting around on bicycles, etc. But rest assured that as China adds hundreds of millions of motor vehicles to the planet’s brew of heat producers and exercise reducers, the Chinese will also fall prey to the maladies we in the west presently struggle with. And untold billions of additional BTUs of heat energy will be added to the planet’s atmosphere every year.

How much of the present global heating trend can be attributed to increases in astralation, and how much to human activities? The former we have no control over; the latter we do … perhaps. The question is, do we realistically have the will and discipline to change? Historically our forebears survived on much more modest quantities of combustion-produced energy. And they were arguably healthier in the bargain. People in olden days largely stayed put. A 100 mile trip by horse and carriage was not casually undertaken.

Is it probable that we can and will go back to some of the old ways? In this writer’s opinion, probably not. To the extent our alteration of the atmosphere’s gases and its heat content contribute little to global warming, there seems little motivation to do so (other than the fact that we’re less healthy as individuals every time we hop into the car to travel more than a hundred yards). But then again, the deleterious effects of this added heat might, over the long term, be catastrophic. The problem warrants more rigorous study.

There are so many problems, aside from or related to global warming, that a brief article like this can scarcely list them let alone discuss them. Exponential population growth, global desertification through soil mismanagement, AIDS … we are without doubt our own worst enemy. On a personal note, this writer is overweight and would without question benefit from more exercise. But he is reluctant to take to city streets on a bicycle or (heaven forbid) get rid of the family car altogether. And this despite his knowledge that nature will exact a toll for his sins of omission and commission. Swept along by time, he finds himself acting more or less wisely at times, and lapsing into foolishness at others.

Is what the average human cannot accomplish likely to be accomplished by his/her entire species? It is a sobering question. The geologic record suggests that no species escapes extinction without adapting and overcoming the forces that would destroy it. And as Pogo might say, "We have met the forces, and they is us."

Can we overcome our sloth and imprudent impulses, and live up to the name we have so grandly given ourselves: Homo sapiens … wise man? On the one hand it seems that our very survival as a species demands it. On the other, I wouldn’t bet my Toyota on it!