Monday, August 30, 2010

The Collapsing Sun

"Let my eyes look upon the Sun
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 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 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.

New Life

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.

Saturday, August 14, 2010

What is a Galaxy, Anyway?

Though we can't see it directly, the sky is full of galaxies. A few can be seen directly, like the Andromeda Galaxy, and some others under excellent viewing conditions. A pair of binoculars will show more of them, and in fact is one of the best ways of viewing Andromeda. Andromeda covers such a large area of sky that few telescopes will show all of it at once--it takes the wide field of view of a pair of binoculars to see it all.

In pictures taken by space telescopes, we can see fields filled with galaxies. The galaxies are scattered across the image like stars. These images cover a ridiculously small area of the total sky above us. If the number of galaxies in these pictures is multiplied by the number of pictures that it would take to cover the entire sky, we learn that there are more galaxies in the sky than there are stars that we can see with our eyes, even on the darkest of nights.

Our view of a galaxy has not always been the view we have now, though. The word "galaxy" has had, historically, a rather slippery sense of meaning.

The Galaxy as a Place of Myth and Legend

Before the telescope was turned on the heavens by Galileo in 1609, there was only one Galaxy. The Galaxy. Galaxy referred to the Milky Way. Known for as long as anyone has looked upward, it's a part of the sky that appears to be a stream of milky light. Exactly what it was couldn't be known, of course. Myths gave it various forms, such as milk from a great godlike cow, or a path of the gods.

Galaxy comes from the Greek galax, the word for milky. Galaxy is a combining form. To our ears, it falsely suggests the sound of lacto, or the Latin for milk, but it's actually unrelated. To the Romans, the Milky Way was Via Lactea. Milky Way is a literal translation of that.

When Galileo turned his telescope on it, he was the first to see it as stars rather than as a milky cloudiness. The stars in it are so close together and so numerous that there's no way our eyes can make it out as stars without the resolving power of the telescope. The telescope not only magnifies the view, but improves the contrast we see, making it easier to see the individual stars by magnifying the spaces between them and allowing us to pick them out from the general glare.

The Galaxy as Nothing Special

From that time on, the Milky Way, or the Galaxy, became a name for an area of the sky with most of the stars in it. In the book Elements of Astronomy by Simon Newcomb, published in 1900, the terms "galaxy" and "Milky Way" do not even appear in the index. The Milky Way receives only one short mention in the book, describing Galileo's discovery. The 1886 book Recreations in Astronomy by H.W. Warren has "Milky Way" in the index with two citations, but "galaxy is absent. The greater of the two passages cited reads:
Every one has noticed the Milky Way. It seems like two irregular streams of compacted stars. It is not supposed they are necessarily nearer together than the stars in the sparse regions about the pole. But the 18,000,000 stars belonging to our system are arranged within a space represented by a flattened disk. If one hundred lights, three inches apart, are arranged on a hoop ten feet in diameter, they would be in a circle. Add a thousand or two more the same distance apart, filling up the centre, and extending a few inches on each side of the inner plane of the hoop: an eye in the centre, looking out toward the edge, would see a milky way of lights; looking out toward the sides or poles, would see comparatively few. It would seem as if this oblate spheroidal arrangement was the result of a revolution of all the suns composing the system.

Lessons in Astronomy by Chales A. Young, from 1896, contains one entry in the index under "Milky Way, the", and one under "Galaxy, the", both giving the same citation. The description given is as follows:
The Galaxy, or Milky Way.--This is a luminous belt of irregular width and outline, which surrounds the heavens nearly in a great circle. It is very different in brightness in different parts, and is marked here and there by dark bars and patches, which at night look like overlying clouds. For about a third of its length (between Cygnus and Scorpio) it is divided into two roughly parallel streams. The telescope shows it to be made up almost entirely of small stars from the eighth magnitude down; it contains, also, numerous star clusters, but very few true nebulae.

The galaxy intersects the ecliptic at two opposite points not far from the solstices, at an angle of nearly 60 degrees, the north of the "galactic pole" being, according to Herschel, in the constellation of Coma Berenices. As Herschel remarks,--
"The 'galactic plane' is to the sidereal universe much what the plane of the ecliptic is to the solar system,--a plane of ultimate reference, and the ground plane of the stellar system."

It further describes the distribution of the stars in the heavens as being non-uniform, but that with what is known can give some information about the structure of the Stellar Universe:
The great majority of the stars we see are included within a space having, roughly, the form of a rather thin, flat disc, like a watch, with a diameter eight or ten times as great as its thickness, our sun being not very far from its centre...As to the Milky Way itself, it is not certain whether the stars which compose it form a sort of thin, flat, continuous sheet, or whether they are arranged in a sort of ring with a comparitively empty space in the middle, where the sun is situated, not far from its centre.

Following this comes a section asking whether the stars form any sort of system, mentioning that it is unlnown whether gravitation operates between the stars and whether there might be some sort of orbital motion that defines the overall form of the Stellar Universe, with a concept called Maedler's Hypothesis being somewhat more likely, that there is motion about a common center of gravity. It goes further in naming Alcyone, in the Pleiades, as a star described as closest to that center and therefore the possible "central sun", but discards the idea as having no proof to sustain it.

Over all, the subject is treated lightly, though in great thoroughness for the books of the time. After two pages on the Stellar Universe, the book returns to the serious discussion of the solar system.

The Revolution

In these same books, in sections which are practically appendices to the important main matter of the nature and operation of stars and the solar system, there are brief mentions of nebulae, "clouds" in space. Among these are some unusual ones taking the form of spirals. The spirals go unmentioned in Newcomb's book. In Warren's book, they are only listed as one of many possible shapes of nebula:
"Nebulae are of all conceivable shapes--circular, annular, oval, lenticular, conical, spiral, snake-like, looped, and nameless."

Young's book takes advantage of the very latest work in photography and spectroscopy. It has a more extensive section on nebulas, stating that photography reveals features which, for example, reveal a regular annular structure in the Andromeda nebula. Conclusions formed by the new information are absent, however. There is only the excitement that comes with new information to be studied.

The 1923 book The Wonders of the Stars by Joseph McCabe is an exuberant popular book on astronomy that leaps into the subject of the spiral nebulas, however:
Some astronomers think that spiral nebulae may be stages in the formation of solar systems like ours. It is estimated that if two stars approached within a few million miles of each other they would raise such tides (analogous to the ocean-tides raised on the earth by the moon) of metal that a vast quantity would be shot out into space. We are imagining, remember, a volcanic outpour streaming thousands of millions of miles into space. If two such mighty streams were shot out opposite sides of a star, its gravitational power would cause them to wind round the central mass, and it is said the resulting structure would be like the spiral nebula in the photograph. The arms and the star would continue to whirl around, like a gigantic Catherine-wheel. The next stage would be that the matter contained in the spiral arms would begin to gather round the denser centres, and ultimately it would be all (except, perhaps, for a little residual matter, to make meteors) collected in a large number of smaller globes circulating around the star.

Even at this stage, however, questions appear. The rotation of the spiral nebulas has been observed, and questions about their size and distance from these measurements can not be answered, but at the very least some boundaries can be established. As mentioned in the McCabe book, what is known about the spirals would seem to make them too large for solar systems, as they were then understood.

The Island Universes

About the time of McCabe's book, astronomers were beginning to establish distances to these spiral nebulas. In Young's book, it was an open question whether they were part of the same system as the stars around us. In McCabe's book, he notes that some feel they are outside the system of stars but discounts the idea himself.

The controversy about the size of the Milky Way and its relationship to the spiral and other nebulas which appeared to be at exteme distances was known as "The Great Debate".

This work found that the distances to the spiral nebulas, and many others that were not of spiral form, were greater than those to any of the stars known in our "Stellar Universe." This placed them outside that universe and made them ambiguous objects, compared to those nebulas that had been established as being within the Stellar Universe.

The name "Island Universe" arose for these objects, though it was an abuse of the term "universe." They appeared to be islands of stars and nebulas that fell outside the main system of which we are part. The falsehood of our own system being the primary among them fell away quickly as measurements revealed the sizes of these "islands." Their extent was as great as that as the circle of stars about us in the Milky Way.

In 1932 Robert H. Baker revised the popular book Astronomy for Everybody written in 1902 by Simon Newcomb. He updated it with the vast quantity of new understanding that had arisen. It includes a new chapter titled "Galaxies."
In the description of the Milky Way we have noticed some of the star clouds, in particular, the great Sagittarius cloud whose center is 50,000 light-years away, and the somewhat smaller and nearer Scutum cloud. According to the view recently set forth by Shapley, these and other star clouds are galaxies, that is to say, vast assemblages of stars and nebulae. They average 10,000 light-years in diameter. Some are considerably smaller, while the largest are three or four times greater in diameter.

The galaxy of which our sun is a member is known as the local system...These star clouds are grouped nearly in one plane in the supergalaxy which we call the galactic system. For the past century and a half, astronomers have been trying to determine precisely the form and extent of this system whose principal feature, as we see it in projection in our skies, is the Milky Way.

At this point there was still uncertainty about where to draw the lines. Were the various star clouds that we now call the Milky Way Galaxy all part of one galaxy, or several galaxies in an interacting system? Were the external galaxies part of this system, or were they entirely independent?

Over time the consensus became that the external nebulas, the Island Universes, were full independent galaxies in their own right, and that the various star clouds around us in the near distance were different components of a single spiral structure like that seen in Andromeda and Triangulum.

The word galaxy became a general term for the "Island Universes" or assemblages of star clusters independent of our own. In spite of the fact that the word itself means "Milky Way", it was extended to include those structures in space like our own Milky Way, as well as describing the Milky Way itself.

The Spiral Path

The galaxy began as a thing in the sky. Unique, mysterious, and unexplained. It was an object of legend, where gods tread, that led from one storied place to another, with a story behind its unique existence. Then, with the telescope, it was seen to be nothing more than an aggregate of common objects. No longer was it a mystery. It was just a bunch of stars. Like pebbles in the aggregate of a path, their number was nothing of greatness, just a result of their proximity.

Then, with a closer look at the star clouds within the Milky Way, and anomalous objects outside it, it was found to be one of a number of immense associations of stars in our universe. As we dream of and seek other civilizations like our own, the galaxy, as its own mini-universe, has become a place of legend again. Where so many stars come together, we infer many planets. Where so many planets lie, there lie the greatest chances for other life like our own. New places for others with consciousness to arise and to experience their own epic sagas.

Once again, "galaxy" has become a place of myth and legend. In 1977 we were treated to the line "A long time ago, in a galaxy far, far away..." Since then, the land of galaxies has only extended further and become more mysterious and colorful, thanks to the amazing images provided by our astronomers.

'Galaxy' has gone from naming a thing, to an area, to a span of stars that fills a small universe (by present standards) to a thing that is scattered in unimaginable plurality through a far, far larger universe. A universe that is once again large enough for stories of strange peoples and strange things that reflect our view of the vast unknown.

Tuesday, August 10, 2010

Which Star is That? Don't Fret!

In the evening I really enjoy watching the stars appear. It's sort of a personal challenge with me, which I find relaxing (believe it or not.) I like to see how early I can pick out  a star. Even better, I like to know what star it is.

It's hard to tell, if you don't already know the star from a prior evening's viewing. I recognize stars by their relationships to each other, and if it's the first star I see, there aren't any other stars. That leaves me with guessing on the basis of how high it is in the sky, the time of year, and so on.

Stars change their positions in the sky, which is why I use the relationships between them to identify them. Tonight Vega will appear up there, in a month's time it'll be over there. If I don't get a chance to look at it in between, I might think I'm seeing Arcturus when it first appears.

Location Matters

On top of that, when I leave my most frequented observing locations, the sky appears to change. The parts of the sky I can see, and the parts that are blocked by local scenery change. I might not know which way is north as well, or as accurately. This happens most often when I'm at public star parties in new locations. There I am, supposedly one of the expert astronomers. Someone walks up to me as the first stars are appearing and I'm still trying to get my bearings.

"What star is that?" they'll ask.

And I won't know. One of the very brightest stars in the sky, and I can't tell which it is.

What kind of an astronomer am I anyway?

Get It Wrong

Fortunately I can usually make a guess. And I'm honest about it. I'll say, "I don't know, but from where it is and how bright it is I think it's..." whichever. I guess, and then in the next half hour I'll find out if I'm right or wrong.

When I'm by myself, I guess, too. I try to figure out what star I'm seeing. I try to put together a few facts, then make a guess and check back on myself later when I can see the star's neighbors. The only way for me to get better is to be willing to get it wrong, and make a guess. Later, when I find out whether I was right or wrong I can re-assess the reasons I used for guessing as I did. Then I learn something.

Like, yes, Arcturus really can appear that low in the sky, during this time of year. Or, yes, we're far enough along in the year for Altair to start to show. Sometimes it sticks, sometimes it doesn't. But over time, it adds up and I get better at it.

Start at Dark

At the outset, I had to learn the stars when I could see them among the other stars. I didn't learn the Winter Triangle and Summer Triangle at twilight, I learned them when it was fully dark. I also had to learn to remember which was which, and which constellation went with each. The same goes for the other bright stars.

As I often tell people when they're impressed by what I know, I wasn't born knowing this stuff. I had to learn it piece by piece, just like anyone else. The first pieces were the basics. The next step was challenging myself to do more. Like pick out the first stars and know what they are--maybe.

The next step in expanding on the ability to tell one star from another comes with trying to pick out the stars earlier when you're already familiar with the sky. You've been out under the dark sky within the past few days. You pretty well remember what you saw in the early part of the night. Now you see what you can see earlier. The stars will be in slightly different positions, shifted to the east from where you remember them being. But not due east, since the stars travel in circles about the pole.

Then as you see stars, guess to yourself. "I think that's Spica, because that other star over it is probably Arcturus." Then check up on yourself later, when more stars are out.

Optical Illusions

As it is, I now press myself to try to find stars when the sky is still bright. When doing this, I remind myself that the definition of "detectable" is that you can find it 50% of the time. And that's about how it works out. I'll scan the sky, and it's hard not to have my eyes either glaze over or focus on some near object while looking at the apparently empty sky. Then I'll see a little pinprick of light standing out, barely. Aha! A star!

Then I'll move my head. And it'll disappear. It can take me several minutes of looking to find it again. If I've got a scope, I'll try to keep it in view by staring at it as I bend toward the viewfinder. And it'll disappear as I start to crouch. It's funny, but it often seems it's harder to see things that are just detectable if my eyes aren't level with each other.

But sometimes, I can manage it. What helps the most in re-finding a star at this stage is locating the star with respect to something on the horizon. It's one hand span above that tree top. If I put my hand just like so it'll be just over the tip of my pinky. And so on.

It really does feel like an optical illusion. You can practically feel it slipping in and out of view sometimes. It's like one of those pictures where you have to really concentrate to see the picture hidden within the picture.

I know it doesn't sound relaxing. Bit it is, if you don't get wound up over it and just sort of let it happen. After all, in a few minutes the sky will be a little darker. So things will be a little easier to see. Can't catch it now? Wait a few minutes. Check some other part of the sky in the meanwhile. Carry on a conversation while you scan.

Most of all, don't be afraid of being wrong. Make guesses. As they say, you miss every shot you don't take. Take a shot, and see what happens. If nothing else, you'll have to dredge up some star names from memory, even if you aren't sure which stars they apply to, yet.

Relax to See More

The most important reason to stay relaxed? Your eyes are far more sensitive to small differences in contrast when you're relaxed. If you take it easy, and enjoy the view, you'll see more.