Until I have been filled with light!
Darkness leaves when there is sufficient light.
May I who am truly dead still behold
the brilliance of the Sun!"
-Gilgamesh, The Epic of Gilgamesh, Tablet X
All life on Earth depends on the light and heat of the Sun, without it, we could not survive. It is natural that our nearest star is an object of study for science. The study of the Sun not only tells us about the Sun itself, but it informs and been informed by our study of our home, the Earth. At one point, scientists felt that they had determined the likely age of the Sun. But geologists found evidence that the Earth was older than the age of the Sun.
A Problem of Timing
How could the Earth be older than the Sun? Especially when much of the geological evidence was the action of water and wind, both effects driven by the power of the Sun?
Lord Kelvin brought his knowledge of physical science to the debate. The mass of the Sun was easy enough to determine. He calculated the possible age of the Sun based on the possible heat that he knew that mass could generate, and obtained a range of ages for the Sun, from 100 million to 40 million years. He felt the lower number was more likely, and later he gave even lower numbers for the likely age, down to about 10 million years. The science was simple, the calculations were brief and consistent with all known science.
What Heats the Sun?
Central to Lord Kelvin's work was the way in which the Sun generates heat. In order to assess different ideas, some idea was needed of just how much heat the Sun does give off in the first place. A first estimate of this was possible after Herschel's experiments in 1838.
He used an apparatus that sounds laughably amateur today, yet with it he managed to get the first measurement of the energy output of the Sun. He put a carefully measured amount of water amounting to about a cupful into a vessel very nearly like a tin cup. He put this inside a double walled tin vessel to exclude the heat radiated from everything else in the area, but had a three inch hole that would let through a ray of sunlight. He put a thermometer in the cup, and stirred the water to keep it all at as consistent a temperature as he could, then measured the temperature change as the Sun's light struck the cup over a measured time period.
Using his data and some brain power, he produced figures that amounted to the Sun producing about 800 million calories of energy per second for every square meter of visible surface area. Later observations with better equipment brought this figure up even higher, to about 1.2 billion calories per square meter per second. What was the Sun doing to create all this energy?
Let's look at the course that was taken to figure out the answer to this question. Even to early scientists, the question was intractable. Analogy with known means of incandescence and heating was tried first. The first attempts as scientific study of the Sun were attempted, but the results were vague:
"Two opinions, or theories, have been entertained in order to account for the production of heat and light by the sun; one supposes that the sun is an intensely-heated mass, which throws off its light and heat like an intensely-heated mass of iron: the other, based on the ground that heat is occasioned by the vibrations of an etherial fluid occupying all spaces, supposes that the sun may produce the phenomena of light and heat without waste of its temperature or substance, as a bell may constantly produce the phenomena of sound.
Whatever may be the true theory, a series of experiments, made some years since by Arago, the eminent French astronomer...by examining the light which it affords...was found to be in the unpolarized or ordinary condition of light...from which proceeds this light proceeds must be in the gaseous state, or, in other words, in a state of flame. From other experiments and observations, Arago was led to the conclusion that the sun was a solid, opaque, non-luminous body, invested with an ocean of flame."
-Wells's Natural History, David A. Wells, 1861
The concept of the Sun as a solid bovY seems strange today, but remember that the mass and size of the Sun were both well known. This means that the density of the Sun could easily be calculated, and that density was higher than that of any known gas or liquid.
"Conditions concerning the surface of the sun many opinions are held. That it is hot beyond all estimate is indubitable. Whether solid or gaseous we are not sure. Opinions differ: some incline to the first theory, others to the second; some deem the sun composed of solid particles, floating in a gas so condensed by pressure and attraction as to shine like a solid."
-Recreations in Astronomy by Henry White Warren, D.D., 1879
Ongoing study paid off near the end of the 19th century with this picture of the Sun:
"The received opinion as to the constitution of the sun is that the central mass, or nucleus, is probably gaseous, under enormous pressure, and at an enormous temperature.
The photosphere is probably a sheet of luminous clouds, constituted mechanically like the terrestrial clouds, that is, of small, solid, or liquid particles, very likely of carbon, floating in gas."
-Lessons in Astronomy, Charles A. Young, PhD., LL.D., 1896
As to the source of its heat, there were several concepts advanced. One being that the Sun began as a white-hot mass. In the time since its creation, it had thrown off some heat to cool to its present temperature and color. As time passed, it would continue to throw off heat as it cooled. Under this model, it would have radiated more heat when it was hotter. As it cooled, it would emit more heat. This tied the geological record to the brief life of the Sun. Geological activity that appeared to take a very long time, with the present levels of heat received from the Sun, would actually have occurred at a far faster rate early in the history of the solar system. When the Sun and Earth were hotter, things moved faster. A geological formation that would have taken hundreds of millions of years to form at present rates might have actually been created in some small fraction of that time in a hotter, more energetic, time.
Another idea about the Sun's heat was based on the new knowledge of meteors. The number of small masses in space was obviously numerous. They could count how many struck the atmosphere of the Earth. The Sun, as a far more massive object than the Earth, would clearly attract far more meteors. By counting the numbers of meteors falling to the Earth, 19th century scientists were able to calculate that the Earth was heated in some small degree by the infall of these meteors. Perhaps the Sun, with its much larger number of meteoric strikes, was heated enough to account for all the heat it produced? Then the heat of the Sun would last for as long as the meteoric material in the solar system.
Combustion was also a possibility, particularly if the Sun were made of some unknown material that burns with especially high energy. The picture of the Sun's atmosphere as a sea of fire seemed to support this idea. Perhaps the energetic material of the Sun not only burned, but produced flammable gases when it burned, which would themselves burn at higher altitudes. To some, the corona that was visible around the Sun during eclipses appeared to be towering flames of hydrogen gas.
Young states that some earlier ideas have been discarded:
"Maintenance of the Solar Heat.--We cannot here discuss the subject fully, but must content ourselves with saying first, negatively, that this maintenance cannot be accounted for on the supposition that the sun is a hot body, solid or liquid, simply cooling; nor by combustion; nor (adequately) by the fall of meteors on the sun's surface, though this cause undoubtably operates to a limited extent. Second, we can say positively that the solar radiation can be accounted for on the hypothesis first proposed by Helmholtz, that the sun is mainly gaseous, and shrinking slowly but continuously. While we cannot see any such shrinkage, because it is too slow, it is a matter of demonstration that if the sun's diameter should contract by about 300 feet a year, heat enough would be generated to keep up its radiation without any lowering of its temperature. If the shrinkage were more than about 300 feet, the sun would be hotter at the end of the year than it was at the beginning.
We can only say that while no other theory meets the conditions of the problem, this appears to do so perfectly, and therefore has probability in its favor."
-Lessons in Astronomy, Charles A. Young, PhD., LL.D., 1896
The Collapsing Sun
And so we have it. The only method that could account for the heat of the Sun. Other methods may contribute. These other factors, along with the exact make-up of the Sun's gases resulted in estimates for the Sun's shrinkage from about 140 feet per year to the about 300 quoted by Young. Lord Kelvin also considered this the most likely cause of the Sun's heat, and as new data became available he revised his estimates of the likely age and lifetime of the Sun. Each estimate came in shorter than the last. His estimates for the age of the Sun dropped from 40 to 100 million years down to 10-12 million years as more was learned.
To common society, the expansion of the history of the universe from a few thousand years to several million was the opening of a vast, broad area of time almost unimaginable. Even the shortest of Lord Kelvin's estimates seemed more than vast enough. To the geologists, however, studying the processes of the formation of the Earth, the few million years that Lord Kelvin granted were not even close to enough to explain what they saw. But there was no disputing the science. The rate at which the Earth lost heat could be measured as well. Assuming that the Earth began as a molten mass, its own life could not have been more than a few million years to reach its current temperature.
Yet the formations the geologists saw in the Earth appeared to take far longer to form. One response to the dilemma was a branch of Catastrophism. Not a Catastrophism married to an attempt to constrain all of time to the strictest views of Creationists, but a form of the theory that sought mechanisms to produce structures that would seem to take millions of years in far less time.
How Long Have We Got, Doctor?
With the Sun collapsing into itself, and about ten million years behind it, how much longer can it last? What will the future bring?
Returning to Doctor Young, we find:
"Age and Duration of the Sun.--Of course if this theory is correct, the sun's heat must ultimately come to an end; and looking backward it must have had a beginning. If the sun keeps up its present rate of radiation, it must, on this hypothesis, shrink to about half its diameter in some 5,000000 years at the longest. It will then be about eight times as dense as now, and can hardly continue to be mainly gaseous, so that the temperature must begin to fall quite sensibly. It is not, therefore, likely, in the opinion of Professor Newcomb, that the sun will continue to give heat sufficient to support the present conditions upon the earth for much more than 10,000000 years, if so long."
During this time between Herschel measuring the rate at which the Sun heated water in a tin cup and Doctor Young's writings, work had continued on nailing down just how much heat the Sun produced. This, and the larger picture of the Sun's operations and age was assessed in another book by Doctor Young, The Sun, published in 1897:
"We have spoken, a few pages back, of Professor Langley's experimental comparison between the brilliance of the solar surface and that of the metal in a Bessemer converter. At the same time he made measurements of the heat by means of a thermopile, and found the heat radiation of the solar surface to be more than eighty-seven times as intense as that from the surface of the molten metal...
Thus, in the composition of a body's radiation, we get some clew to its temperature. Hitherto all such tests concur in putting the sun's temperature high above that of any known terrestrial flame.
And now we come to questions like these: How is such a heat maintained? How long has it lasted already? How long will it continue? Are there any signs of either increase or dimunition?--questions to which, in the present state of science, only somewhat vague and unsatisfactory replies are possible.
As to the progressive changes in the amount of solar heat it can be said, however, that there is no evidence of anything of the sort, however, that there is no evidence of anything of the sort since the beginning of authentic records. There have been no changes in the distribution of plants and animals in the last two thousand years, as must have occurred if there had been, within this period, any appreciable alteration in the heat received from the sun...
What then, maintains the fire? It is quite certain, in the first place, that it is not a case of mere combustion. As has been said, only a few pages back, it has been shown that, even if the sun were made of solid coal, burning in pure oxygen, it could only last about six thousand years: it would have been nearly one third consumed since the beginning of the Christian era. Nor can its heat lie simply in the cooling of its incandescent mass. Huge as it is, its temperature must have fallen more than perceptably within an thousand years if this were the case.
Many different theories have been proposed, two of which now chiefly occupy the field. One of the finds the chief source of the solar heat is in the impact of meteoric matter, the other is the slow contraction of the sun. As to the first, it is quite certain that a part of the solar heat is produced in this way; but the question is whether the supply of meteoric matter is sufficient to account for any great proportion of the whole. As to the second, on the other hand, there is no question as to the adequacy of the hypothesis to account for the whole supply of solar heat; but there is yet no direct evidence that the sun is really shrinking...
We do not know enough about the amount of solid matter and liquid matter at present in the sun, or the nature of this matter, to calculate the future duration of the sun with great exactness, though an approximate estimate can be made...it is hardly likely that the sun can continue to give sufficient heat to support life on earth (such life as we now are acquainted with, at least) for ten million years from the present time.
...we are inexorably shut up to the conclusion that the total life of the solar system, from its birth to its death, is included in some such space of time as thirty million years...
At the same time, it is obviously impossible to assert that there has been no catastrophe in the past--no collision with some wandering star, endued, as Croll has supposed, like some of those we know of now in the heavens, with a velocity far surpassing that to be acquired by a fall even from infinity, producing a shock which might in a few hours, or a few moments even, restore the wasted energy of ages. Neither is is wholly safe to assume that there might not be ways, of which we yet have no conception, by which the energy apparently lost in space may be returned, at least in part, and so the evil day of the sun's extinction may be long postponed."
A New Hope: Selective Sunlight
But this does not close out other possibilities, newly proposed. All theories so far have presumed that sunlight is radiated in all directions of space around the Sun equally. But what if this isn't actually the case? What if the Sun only emits its energy in certain directions, one of which happens to be toward the Earth?
"In 1882 Dr. C.W. Siemens, of London, proposed a new theory of the solar energy much in this line, and the scientific eminence of its author secured it most respectful consideration and discussion. Although it was soon abandoned as untenable...
'The fundamental conditions' of Dr. Siemen's theory are the following, in his own words;
'1. That aqueous vapor and carbon compounds are present in stellar and interplanetary space.
'2. That these gaseous compounds are capable of being dissociated (decomposed into their elements) by radiant solar energy while in a state of extreme attenuation.
'3. That these dissociated vapors are capable of being compressed into the solar atmosphere by a process of interchange with an equal amount of reassociated vapors, the interchange being affected by the centrufugal action of the sun itself.'
Moreover--and this is the point of the theory upon which he puts special emphasis--he teaches that these compound gases resulting from the combustion intercept the solar heat not received by the planets (heat which, from the human point of view, would otherwise be wasted), and utilize it in their own decomposition; thus the solar fire is made to prepare its own fuel from the ashes of its own furnace, and an explanation is found for its enduring constancy."
-The Sun, Dr. C. A. Young, 1897
So we have one of several theories that somehow, the Sun's light does not spread equally through space. The light and heat that would be "wasted" on interstellar space is somehow preserved. Dr. Young rejects this specific theory, yet notes that there is nothing in it which is absurd. If the precepts that the theory are based on are correct (that space is filled with composite vapors, that light and heat can decompose them again into their elements) then the theory not only may be, but must be correct.
In this case, the total amount of energy actually expended by the sun becomes far less, as only the light and heat reaching the planets themselves is lost. The problem is, that an interplanetary vapor would have an effect on the motions of the planets. An effect that should have been detectable even in Dr. Young's time. And that effect is not seen. A number of other difficulties arise as well. If some gas in the solar system absorbed the light that doesn't strike the planets, and assuming our solar system is no different from that of any other star, how is it that we're able to see those other stars? Why isn't their light either absorbed at the source, by their own local vapor, or absorbed upon reaching the vapors in our system?
As Dr. Young states:
"And yet one almost regrets that the theory can not be accepted, for it would remove some very serious difficulties that now embarrass the problem of the evolution of our planetary system. The accepted contraction theory of Helmholtz certainly appears to allow too little time for the sun's lifetime of radiant activity to be consistent with a reasonable explanation of the process by which the present stat of things has come about."
-The Sun, Dr. C. A. Young, 1897
The Turn of the Century
So we have an inadequate theory, but one which is fully supported by science, unlike any other.
In the last year of the 19th century, Dr. Simon Newcomb sums up the situation like this:
"The view now commonly held is that the heat of the sun is kept up by the constant contraction of its mass through the gravitation of its particles toward the center. The theory of energy teaches us that heat is produced when a body falls toward a center without having its velocity increased. For example, the temperature of the water of Niagra Falls must be about one-quarter of a degree higher after it strikes the bottom than it is before it goes over the falls...
If this view is correct, a time must come when the sun can contract no more. Then a solid crust will form over its surface, this crust will gradually grow dark and cold. But the period necessary for this is many millions of years, so we need not trouble ourselves about it."
-Elements of Astronomy, Dr. Simon Newcomb, 1900
The Dying Sun
Or is Dr. Newcomb merely whistling in the dark? The study of the Sun with instruments such as bolometers and spectroheliographs has only just started at the time of his writing. By 1923, a more complete picture of the surface of the Sun, including not only the visible but the invisible, had emerged.
"It is a portentious vitality for our sinking sun. And that our sun has passed the prime of its life follows from all that we have said. It contains some forty of our chemical elements. A star in the prime of life has very few of them, and it has hardly any absorbing layer of cooler vapors. The face of our sun is covered with a veil of such vapors. The spots are mighty oceans of them. The vitality of the sun has sunk to such a point that it cannot entirely keep itself ablaze. The dark vapors will gradually increase. Our yellow star will become a red star. The chemical combinations in the spots will increase until large areas shine with a dull red glow. It will be too cold for life on the planets. When? Not for many millions of years--the general tendency now is to say, not for tens or hundreds of millions of years. But here we are on less firm ground, and we must wait until we know more about the evolution of stars."
-The Wonders of the Stars, Joseph McCabe, 1923
Something else new has appeared by this time as well. In the same book where the Sun is described as a dying star, we find the following about its sources of energy:
"There is no question of combustion of the stars, as in our fires. It would be impossible to sustain the fires of the stars for hundreds (perhaps thousands) of millions of years, as they are sustained, by combustion. The gradual contraction of the mass of the sun is one great source of heat. Some astronomers think it is an all-sufficient source, but the general belief now is that the heat given off as the lighter atoms combine to form the heavier is a most important source of the sun's heat."
-The Wonders of the Stars, Joseph McCabe, 1923
And here we have it. A new source of energy, one that was previously unknown to Lord Kelvin in his calculations. The "heat given off as the lighter atoms combine to form the heavier" is what we call fusion power today. Note, also, what the above account gives for the lifetime of a star. No longer ten or possibly tens of millions of years, but now hundreds or perhaps thousands of millions of years--billions to us today. Though Joseph McCabe sees the sun as a dying ember, the science of his time allows for a Sun that has seen a life of perhaps billions of years. Long enough for the Earth to form as the geologists see it, without any special trickery to get here.
By 1932 the picture of what powers the Sun has become pretty well complete. In Simon Newcomb's Astronomy for Everybody, revised by Robert H. Baker, PhD., the section on "The Source of the Sun's Heat" reviews the ideas of the last century. It states the well-characterized quantity of energy that the Sun emits, given as 70,000 horsepower per square yard of surface area. The concept of the Sun as a cooling body is discarded, as is the concept of the Sun as a combustable mass. The infall of meteors is recognized as an ancillary source of energy, but as far too small to account for the amount of energy radiated by the Sun.
Then comes the contraction theory. The strong scientific basis for it is described, as well as its agreement with other current scientific theories. The discussion concludes:
"The contraction theory pictured a gloomy prospect, the end of the world of living beings within a brief interval--brief astronomically, at least. But in recent years, the contraction theory has met with disapproval, along with the hypothesis of Laplace. In shrinking to its present size from dimensions as large as we please, the sun has gained enough heat to keep it shining at the present rate for scarcely twenty million years. It has certainly been shining at this rate for a vastly longer time. Thus the contraction theory fails to account for the maintenance of the sun's radiation in the past. We have no greater confidence in its prediction for the future. There is, in fact, no certain evidence that the sun is contracting progressively at all."
Simon Newcomb's Astronomy for Everybody, rev. by Robert H. Baker, PhD., 1932
And so, the "collapsing Sun" has been laid aside. An entirely new, almost magical source of energy unknown to scientists of the 18th century has appeared. Atomic energy:
"With the discovery of radioactivity, astronomers inquired as to whether the sun's long-continued radiation might not be kept up by the disintegration of radium and similar elements in the interior. Appropriate calculations soon gave the negative answer. A way is left open, however, if we wish to imagine that the sun contains radioactive elements more complex than the heaviest element, uranium, found on the earth. It should be added that we have no knowledge of such super-radioactive elements."
But wait! What? Radioactivity doesn't explain the sun? Such radioactivity had explained the heat loss of the Earth. The Earth was now known to not be a cold mass after its long life because most of its heat comes not from being a cooling mass but from the heating caused by radioactive elements within it.
In fact, McCabe's answer from 1923 was the correct one, which was missed somehow in Baker's revision of Simon Newcomb's landmark book. The combination of elements into higher elements, or atomic fusion, turned out to be the primary source of power within our Sun. The fusion occurs deep within the Sun, in an area around the deepest core of the Sun. The heat slowly filters out through the various layers of the Sun. This heat "puffs up" the Sun, preventing its contraction. In time, the Sun will burn through its present fuel of hydrogen, and will change to a reaction fusing helium. This will cause the Sun to heat up even more, and its outer atmosphere will puff up even more to create a "red giant" with a greater surface area to radiate the heat.
As a red giant, the Sun will expand beyond the Earth's orbit, placing the Earth within the outer reaches of the Sun's atmosphere and destroying it. But rather than a few million or tens of millions of years, our planet has a future of about five billion years before this occurs.
One hundred years ago, people looked up and saw our Sun as a dying star, collapsing in upon itself. The past was limited to a few million years, and the future as well. Their sky was ruled by a shuddering, poxed old star in its dotage.
But already, at that time, the discoveries were underway to change that view. An entirely new source of energy was found, as investigations were made into why the accepted view of the Sun did not entirely account for the details of what was known. These investigations gave us a whole new branch of physics, and a whole new span of time in which to exist.