Science in Christian Perspective
In a scientific discussion of the subject, "The Origin of the Universe," one attempts to systernatically reconstruct past cosmic events in terms of our present scientific knowledge. Any such discussion is launched from a philosophical or theological starting point which embraces certain presuppositions about the laws of nature and primordial matter, the so-called ylem.
Given the fundamental building blocks, it is of great scientific interest to attempt to explain the origin of the elements. It is this particular aspect of the "origin problem" which I shall emphasize. At present more than one hundred elements are known and on the average each element has about ten isotopes. This means that approximately one thousand different nuclear species have been identified. The present cosmic abundance of the elements is related to both the way the elements were formed and their age. In attacking the problem of the synthesis or formation of the elements, therefore, one must study the relative cosmic abundance of the ele-
*Presented at Wheaton College Science Symposium on "Origins and Christian Thought Today," February 17, 1961.
**Dr. Huizenga is a Nuclear Chemist at Argonne National Laboratory and has a number of publications in his field.
Before embarking on a discussion of the cosmic abundances, formation and age of the elements I would like to outline briefly the magnitude of the known universe and our relative position in it. Only a few hundred years ago man thought he was living in the center of the universe. The earth in the Ptolemaic theory was stationary and the sun and other heavenly bodies were thought to be rotating about it. Copernicus dethroned the earth and gave the sun the central position in the universe. Digges and others made slight alterations in the Copernican theory, which at the time were thought to be rather minor. These new ideas, however, have led to important changes in man's view of the universe. The solar system does not occupy a central position in our galaxy, the Milky Way, but instead is located out on the edge of our galaxy which has a disklike shape. The Milky Way is 80,000 light years in diameter and about 800 light years thick. A light year, which is the distance light travels in one year, is equivalent to approximately six trillion miles. A typical galaxy has a radius of 15,000 light years and contains about one hundred million stars which are approximately the size of our sun. In the average region of space the average distance between galaxies is about three million light years, and one considers two galaxies in collision when their centers are no more than 30,000 light years apart. Approximately one hundred million galaxies lie within the range of present telescopes.
One is interested in the abundances of the elements in various regions of the universe, although the determination of such abundances in faraway regions of the universe is a very difficult task. It is, therefore, only natural that our earliest information on abundance came from analyses of the crust of the earth, the ocean, and the atmosphere. Already in 1889 Frank W. Clarke read a paper at the Philosophical Society in Washington entitled "The Relative Abundance of the Chemical Elements." In this classical work an attempt was made to determine the abundances of the elements primarily from the earth's crust. Certain elements have preferentially diffused to the surface layer of the earth and the interior composition of the earth is undoubtedly much different than that of the surface.
As time passed it became more and more evident that meteorites were better objects than terrestrial rocks for the study of the average abundance of the chemical elements in nature. The paper by Goldschmidt on this subject is classical and still referred to in present literature. Meteorites are objects from outer space which strike the earth. Their origin, however, is thought to be from within the solar system. They are remnants of larger bodies in our solar system which have suffered collision. The advantage of analyzing meteorites lies in the fact that some of the meteorites are from the inside of the original body. These original bodies are probably much like the earth which is made up of core, mantle, and crust. As I mentioned before, samples of the earth have come, to date, only from the crust. Meteorite samples supposedly come also from the core and mantle of the larger body. We expect the stony meteorites to be comparable to the material in the mantle of the earth. It will be interesting to check this postulate. The project Mohole is an effort to drill through the earth's crust to obtain a sample of the earth's mantle. The elemental abundances obtained from meteorite analyses are thought to be representative of the solar system.
A third method of obtaining information on the abundance of the elements is to spectroscopically analyze the light from stars. This method has been extensively used to obtain the abundance of the elements in the sun. The logic of the method goes something like this. Nuclear reactions deep inside the sun produce light of all wave lengths, i.e., a continuum. As this light passes through the outer layers of the sun certain characteristic wave lengths of the light are absorbed by the elements in the sun. The particular wave lengths which are strongly absorbed by each element are known from laboratory experiments. The astrophysicist with his powerful spectroscope, a machine which analyzes the light, can determine the abundance of the elements in the sun by analyzing the residual light of the sun which reaches the earth. In a recent publication, the solar abundances were given for about 65 elements. If meteorites and the sun are both part of the solar system, they should have approximately the same elemental composition. Present indications are that this is the case except for light gases like hydrogen which will be chiefly present in the sun for energetic reasons.
The importance of the spectroscopic method lies in the fact that elemental abundances can be determined for stars other than our sun. One can go beyond our solar system to distant stars in our galaxy, the Milky Way. In addition, one can obtain information on elemental abundance in stars completely outside our galaxy. The method also allows one to measure isotopic ratios in certain cases. Isotopes are members of the same element with different mass. The mass change causes a small shift (isotopic shift) in the absorption wave length.
The fourth and last method which I will mention for determining elemental abundance is the rather new technique falling in the area of radio~ astronomy. The earth receives radio waves from space as well as light waves. As I mentioned previously, when light from the incandescent gas in the interior of a star passes through the cooler outer layers of the star, the atoms of the cooler gas identify themselves by absorbing their characteristic wave lengths. These appear as dark lines against the bright background spectrum of the star. "Dark" lines can also be observed in the radio spectrum. One such line which radio-astronorners presently work with is the radio wave length absorbed by the cold hydrogen in interstellar space. One of the applications of this method at present is its use in telling us the hydrogen concentration in space between the stars and between the galaxies. The radio waves absorbed by hydrogen are 21 centimeters long. Waves of this length carry just the amount of energy needed to flip the single spinning electron of the hydrogen atom from a state in which its magnetic axis is opposed to that of the nucleus of the atom to a state in which the two axes are parallel.
Hydrogen is the most abundant element. In the solar system, 92.80/,, of the atoms are hydrogen, 7.1 % of the atoms are helium. In terms of mass, hydrogen represents 75% of the mass of the solar system and helium 23%. This leaves about 2% for the mass of all the other elements in the solar system. These results come from meteorite analyses and spectroscopic measurements of the light from the sun. The distribution of this 2% of solar matter among the other 99 elements is, however, a point of great interest and one which sheds light on the formation of the elements.
One of the most important discoveries in astrophysics is the recent observation that some stars have different elemental compositions. The variation in the elemental abundances of cosmic matter is such that young stars have larger concentrations of heavy elements than old stars.
For our purpose let us consider two extreme stellar populations, Population I and Population 11. Population 11 stars are very old, have small heavy element contents and are distributed in a large spherical or ellipsoidal system in the galaxy. Population I stars, on the other hand, have enhanced heavy element content, are young and are found in a flat disk of the galaxy. Why is it that young stars have more heavy elements? Let us look at the life history of a star. The first phase of the life of a star is an accumulation of galactic dust which is contracting due to gravitational energy. As contraction continues the core of the star becomes very hot and hydrogen burning is the main source of energy. These stars are on the so-called main sequence of stars and our sun is a typical example. After the hydrogen in the core is depleted, further contraction occurs, and helium burning begins. The star at this point is very hot and is known as a red giant. At this point rather drastic things happen to the star. It begins ejecting matter and may even suffer a major instability and explode. These exploding stars are called novae or supernovae. The difference between a nova and a supernova lies in the size of the explosion. A supernova ejects more than 90% of its matter back into intergalactic space. The energy liberated in one of these events is enormous compared to a hydrogen bomb. The remnant of the supernova is a white dwarf and this is considered the final state of the star. White dwarfs are often referred to as the graveyard of stars.
With such an efficient mechanism available for feeding matter back into space, it is of interest to examine these gigantic explosions in more detail. Each of these explosions can be represented as a giant furnace in which the elements of nature are synthesized. History records three of these events which occurred in our galaxy. The earliest event was recorded by the Chinese in 1054 A.D. To my knowledge there is no record of this tremendous event in European history. The Oriental people were interested in astrology and kept a very careful record of the light intensity of this unusual star. All at once it became one of the brightest stars in the sky, and then it began to fade. The remnant of this explosion is known as Crab Nebula. Other such events happened in 1572 and 1604, the era of Kepler.
Excellent photographs of the appearance and fading of supernovae have been recorded. For a period of time following the explosion, a supernova is brighter than all the stars in its galaxy which in an average galaxy is about 100 million stars the size of our sun. The decay of supernovae have also been studied on a quantitative basis. The decay is unusual in that it follows the decay law of radioactive atoms.
With this background, let me ask again why the young stars have more heavy elements? Looking back into time, the old stars were formed at a time when few supernovae had occurred. With the passage of time more and more of these giant explosions have occurred. The observed frequency of Type I supernova is about one per galaxy per 300 years. Young stars were formed from intergalactic dust which contained the debris of all past supernovae in the galaxy. From these considerations it follows that young stars are expected to have more heavy elements.
Our most detailed information on elemental abundances comes from measurements of material in the solar system. The experimental abundances of the elements in the solar system decrease in an over-all way with atomic weight; however, there are a number of local fluctuations. One of the tasks of the astrophysicist is to explain this structure in the curve obtained when the elemental abundances are plotted against the atomic weight. It is beyond the scope of this lecture to discuss in any detail the nuclear processes involved in element synthesis. I would, however, like to mention the neutron capture process. During the last ten years it has become particularly evident that element synthesis by neutron capture can follow at least two different paths which lead to rather different elemental abundances. One of these general paths is followed when moderate numbers of neutrons are available for long periods of time. This is approximately the situation in a nuclear reactor or in some of our stars such as red giants. Another rather different path of element synthesis is followed, however, when a large burst of neutrons is available for only a very short period of time. An example of a man-made device which produces a neutron burst of sizable magnitude is a hydrogen bomb. On the astronomical scale, it is thought that certain types of supernovae release enormous numbers of neutrons during a time scale of about a second.
Our experimental information on abundances of heavy elements can best be interpreted if both types of neutron sources contributed to element synthesis. The experimental elemental abundances should reflect the composition of the dust of our galaxy at the time the solar system was formed.
Except for minor changes due to radioactive decay and a small number of trace nuclear reactions, no significant changes in the elemental composition are expected to have occurred since the solar system was formed.
One piece of evidence for neutron fluxes of long time duration is the presence of technetium in red giant stars. The element technetium is not found naturally on earth since the longest-lived isotopes have half-lives of only a few million years, a period of time too short for them to still be present in our old solar system. The most reasonable explanation for technetium in the red giants is that the technetium is being currently produced by neutron reactions on the stable elements.
The most dramatic evidence for element forniation from neutron bursts of short time duration comes from study of supernovae events. The observed decay curves of the energy of supernovae have the familiar characteristics of a radioactive decay curve, with a good possibility that such exotic elements as Californium are produced.
Any theory of element formation which attempts to explain the abundances of the nuclides in the universe has to account for the differences in abundances in various stars. In my opinion, such a theory will have as one of its important aspects the synthesis of elements in the interior of hot stars and during the more spectacular supernova events. The assumption often credited to Gamow, that all the elements were produced in a single catastrophic event of short time duration many billions of years ago is an oversimplified one. On the other hand the evidence is such that no positive conclusion can be reached on whether the heavy elements of our solar system were produced over a long period of time or during a single event prior to the formation of the solar system.
The age of the elements, rocks, earth, solar system, the stars in our galaxy, the Milky Way and other galaxies and the universe is in most cases a complex property of the system. A simple sweeping answer to the age question can without ample clarification lead to erroneous conclusions. The age of rocks on the surface of the earth are known to range up to approximately three billion years. This simply means that certain deposits have not been disturbed for long periods of time, and in terms of the age of the earth is only a lower bounds, Experimentation on meteorites has been interpreted in terms of the parent meteorite bodies crystallizing about 4.6 billion years ago. This age is commonly associated with the age of the solar system. The experimental age is related to the signal one uses as the initial time marker. This is usually the time at which the body is able to retain specified daughters of various radioactive nuclides. On the basis of retention of the gaseous element xenon, arguments have been advanced to show that the earth is about 200 million years younger than meteorites. Similar reasoning leads to a comparable interval of time between the solidification (retention of xenon) of meteorites and the last element synthesis. The last element synthesis is specified because the possibility exists that the elements of the solar system were not formed in a single "event" but over a long period of time in many "events."
The quesiion can be raised whether the initial Ilylem" was contained in a single, primeval, mammoth atom which exploded giving rise to an expanding universe. This subject is usually treated by also considering the opposing view of the steady state universe which advocates continuous creation of new matter. The two theories make very different predictions and lend themselves to experimental verification. The steady state theory predicts that the density of stars will remain constant in any given volume. The stars are receding from each other in the steady state theory, however, with the continuous creation of local matter the star density remains constant. The expanding universe leads to a reduced star density. The observed density of stars in distant galaxies should be greater than nearby galaxies on this view since the light from distant stars represents the situation as it existed billions of years ago.
The two theories also, predict a different red shift. E. P. Hubble at Mount Wilson Observatory found the first evidence for the physical expansion of the universe. He correlated the distance of galaxies with the amount of shift in light toward the red end of the spectrum, and found the extent of the shift was in direct proportion to the galaxy's distance from us. The experimental information on the red shift of the light spectrum of near and distant galaxies is not good enough at this time to definitely favor either theory.
just as the stars are sources of light, they are also sources of radio waves. The study of these radio signals is a currently exciting research field which offers tremendous potentialities in solving some, of the unanswered questions of our universe. A recent press release indicates that a group of British astronomers have finally proved the expanding universe theory. This is a good note on which to close insofar that such a statement points out the fallacy of converting scientific results into popular statements. There is a considerable gap to jump between the recent experimental measurements of the intensities of radio signals from outer space and the conclusion stated in the press release. If the ability of the stars to transmit radio waves is not constant with time, then the press statement is untrue. Although our knowledge of the universe has increased very rapidly in the last few years, many puzzles are still to be solved and I'm sure the next few years will bring many surprises.