We shall not cease from
exploration, and the end of all our exploring will be to arrive where we started and know
the place for the first time.
-- T. S. Eliot
Chapter 1: The Question of Life, the Universe, and Everything ...................
Where do we come from? Where, when, and how, did it all start? What's the point of it all (if any)? These questions, simple but fundamental, have endured since the onset of human consciousness and continue to occupy philosophers and thinkers of all persuasions. Way back in the dark ages the popular belief was that the Earth was in the centre of the Universe, and that it was flat (with waterfalls and dragons around the edges) and for all we knew the moon might have been made of green cheese. We know better today. During the Renaissance, the combined efforts of Tycho Brahe, Copernicus, Kepler, and Galileo helped to knock the Earth (and with it man) from centerstage to the more humble place of a planet orbiting around the sun. This paradigm shift, however, was not without pain. The revolutionary change in thinking that it accompanied caused intellectuals to be at odds with the powers that be (princes, kings, and of course the all powerful Catholic Church). Some, like for example Giordano Bruno, got burned at the stake for their aberrations of thought, while others, like Martin Luther, succeeded to cause enough upheaval to change the existing order permanently. I should add here that Luther, a highly religious man, opposed the heliocentric world view. His main quarrel was with the worldly appetites that in his view had corrupted the Catholic Church. He did most likely, however, benefit in his efforts from the intellectual foment of the times.
To confine our attention to terrestrial
matters would be to limit the human spirit.
When thinking about science, sometimes deceptively simple thought experiments can be quite illuminating. With regard to the Universe, Olbers' Paradox (or why it should not be dark at night) is one of these.
Over the centuries, whenever new observations and facts suggested that the accepted view of the world was in need of revision, conflicts arose. Either because the new ideas sat uneasy with the establishment, or because spiritual leaders felt that they had to defend religious dogma. These conflicts are far from over (even in our relatively enlightened times), and are likely to persist for a long time to come. Think for example of the recent (8/12/99) decision to remove all mention of evolution from the state school curriculum in Kansas, or of the fatwa issued against Salman Rushdie by Ayatollah Khomeini in February of 1989. Nonetheless, men have walked on the moon 30 years ago now, we are making great strides to catalog and unearth the mysteries of the human genetic code, and it it is going to get ever more exiting from here on out. In the end the facts will not be hostage to ideology, but rather will speak for themselves. We will boldly go where no man has gone before, and the journey has just begun.
Some say the world will end in fire,
Although the main objective of this class is to learn about how the Earth works, it is beneficial to understand how the Earth fits into the "cosmic" scheme of things, and thus we will start with an overview of our current thought about the origin of the Universe and the Solar System.
The Origin of the Universe
| Something mysteriously formed,
Born before heaven and earth.
In the silence and the void,
Standing alone and unchanging,
Ever present and in motion.
Perhaps it is the mother of tenthousand things.
I do not know its name.
Call it Tao.
For lack of a better word, I call it great.
--From the Tao Te Ching by Lao Tsu, an early Chinese philosopher of the 6th century B.C.
If we look at creation myths over a broad range of cultures, there is the recurring theme that initially all was dark, that light appeared, and that after that things got gradually organized into stars, planets, oceans, land, plants, beasts, etc. (useful references would for example be Joseph Campbell's book "Primitive Mythology", and Sir James George Frazer's "Golden Bough"). In essence the universe (paraphrased as the "tenthousand things" by Lao Tsu), appeared magically out of nowhere. In the King James version of the bible, for example, "And God said, Let there be light : and there was light". Apache and Navaho creation myths state that "In the beginning nothing was here where the world now stands: no earth - nothing but Darkness, Water, and Cyclone". In the Icelandic Eddas, Yamir, the first man was born from the yawning void of the beginning, and he was then cut up to form everything there is in the world today. A survey of the mythologies of many cultures across the world yields comparable creation myths. That the scientific theory, popularly known as the "Big Bang", seems to bear similarity to these myths has encouraged some to claim the theory as proof of divine origin for the universe. As yet, however, we are only at the beginning of a long intellectual journey (the book "A Brief History of Time", by Stephen Hawkin makes for interesting reading in this context). So far the scientists have been, and will be for a long time, busy trying to describe what the universe is. The question of why there is a universe, why it bothers to exist, whether it needs creative input, is an entirely different matter. Perhaps some day we will indeed have enough knowledge and understanding to produce a unified theory of the universe and everything in it. If it is indeed compelling it will over time not only be understood by a few specialists, but all of us will be able to participate in the essential discussion of why we and the universe exist. To arrive at this point will be the ultimate triumph of human reason. May be then it will be our turn to say "Let there be light".
Let us now outline the
current view of the history of our universe
(there might be others, universes that is)
(useful readings on this topic can be found in "Life in the Universe", a special edition of Scientific American from 1994, as well as in the January 2001 issue of Scientific American; direct link to Scientific American)
This view of a universe that was initially very hot and dense, and then cooled and
expanded, is in agreement with the observational evidence we have been able accumulate to
date (it's popular name is the Big
Bang theory). There are many details to be answered still, but the theory and its supporting
evidence represent one of the great achievements of 20th-century science. We have
come a long way from the waterfalls and dragons. Among the questions of interest are
for example whether the universe will keep expanding infinitely, or whether it has enough
mass to eventually collapse back into itself (the "Big Crunch"). If we
take for example all the observable mass (stars, planets, galaxies etc.) it would
seem that it should expand indefinitely, but if there is enough "invisible"
matter hidden in interstellar gas clouds (sometimes referred to as the "missing mass")
it might eventually slow its expansion and collapse. Another complex of questions concerns
to earliest history of the Big Bang and how it might have influenced the world we
experience today. For example, at stage 3. (see above) particles and antiparticles
would have been constantly annihilated and created (by collision), and if there had been a
perfect balance between matter and antimatter the universe would be very different.
Instead of consisting of matter and energy, it would consist of pure energy
(electromagnetic radiation). A little bit more antimatter than matter would have
lead to an anti-universe (they may be out there). Scientist estimate that there was
initially a 1 part in 1010 excess of matter over antimatter, and it is
this tiny excess that constitutes the mass of our universe. A tiny difference, but
one of the key conditions that determined the future development of the universe.
All the rest (the 1010 excess) of our initial universe was transformed
into radiation/heat, and spread out evenly throughout the universe. Because the
universe has expanded so tremendously this "background" heat has dropped to very
low temperature levels (2.726 Kelvin's) and is the cause for the so called
cosmic background radiation. This radiation had been predicted from
theoretical considerations, and was found to actually exist in the 1960's. It is
one piece of physical evidence for the Big Bang.
Read about a new theory, the "Big Splat" that has been proposed in April 2001.
Another piece of evidence is the measurable fact that the universe is indeed expanding. That the universe is expanding was discovered by Edwin Hubble. The first clue that something like expansion was happening came from the observation of the spectral redshift. Hot bodies such as stars and galaxies send out a continuous light spectrum. This spectrum is a composite of the specific wavelengths emitted by the various chemical elements in the periodic table, and because the latter are not uniform in abundance, there are maxima and minima in the spectrum that can be attributed to emission and absorbtion of light by chemical elements. This phenomenon can be conveniently studied in the lab and is also used to determine the chemical composition of materials. When we combine a spectrometer with a telescope we can then determine the composition of a star or of any other light source out there. The redshift means that emission and absorbtion maxima within the spectrum of a distant astronomical object seem to shift towards longer wave length, meaning towards red (red light = long waves; blue light = short waves). Hubble discovered that the farther a galaxy is away, the more pronounced the redshift becomes. To give a comparison to your own life experience, remember waiting next to a railroad track as the train comes, passes, and moves away. As the train approaches its horn makes a high pitched sound, and as it moves away the pitch of the sound drops very noticeably. What happened is that as the train approached the sound waves were "compressed" (shorter wave length = higher frequency/pitch) and as it moved away the sound waves were "stretched" (longer wave length = lower frequency/pitch). This phenomenon is called the Doppler Effect, and can also be used to explain the redshift (lower frequency light) of galaxies that move away from us. The faster they move away, the more the spectral maxima and minima shift to lower wave lengths and "redder" light. Hubble's Law states that the velocity of a receding galaxy equals its distance multiplied by a constant (the Hubble Constant). This is a very important number for astronomers. The main problem with its precise determination is to get good measurements of the distance of galaxies. The other unknown, the amount of the redshift is easy enough to measure with a spectrometer. Judging from our current determinations of the Hubble constant, some of the most distant galaxies we can observe may be moving away from us at close to 90% of the speed of light. The Hubble Constant is critical for calculations of the age of the universe, although there is an alternative method to do so. As of late, both methods seem to agree that the universe is approximately 12-14 billion years old.
Minor imbalances between matter and antimatter, so important for the outcome of the Big Bang, could be due to random fluctuations or inhomogeneities during stage 2. (see above), and could have other side effects as well. Scientists wonder for example whether the values of fundamental constants of the universe were set at random during this early stage. Under fundamental constants we understand quantities that are fundamental to the laws we know, but which we can not account for from basic principles. They usually show up as coefficients in the laws of physics, such a the gravitational constant, the fine structure constant. Planck's number, Bolzmann's constant, the mass ratio between electron and proton, the mass of subatomic particles, the speed of light, the number p, and the base of natural logarithms. A slight change in any of these constants could conceivably have prevented life as we know it from ever existing. That these number seem so finely adjusted that they permit life in our universe can suggest to some that there was a divine purpose in "creating" the universe as it is, but on the other hand it may simply be an expression of the "anthropic principle".
The "anthropic principle" is a theory according to which due to aforesaid inhomogeneities, there may be either many different universes or many different regions of a single universe, that each have their own set of constants and possibly even their own set of laws of science. In most of these universes (or regions), conditions would not be right for the development of complicated organisms (such as we find living on earth), and only in universes that are pretty much like ours would intelligent beings develop and inquire why the universe is the way we see it. According to the "anthropic principle" there is a simple reason why we are here to inquire into these matters: If the universe (ours) had been any different, we simply would not be here to ask the question! Now, all this may seem like a cop-out, or at least an elaborate display of circular reasoning, but there is a sizeable number of cosmologists (including Stephen Hawking) that presently favor it as the most sensible approach to the dilemma.
Just so that we don't feel too complacent, most recently acquired data even suggest that the universal expansion is accelerating rather than slowing down. Some mysterious, repulsive "dark energy" seems to fuel the acceleration, overpowering the tendency of the expansion to decelerate. What this "dark energy" is, is everyone's guess at the moment.
The beginning of the universe
Is the mother of all things.
Knowing the mother, one also knows the sons.
Knowing the sons, yet remaining in touch with the mother,
Brings freedom from the fear of death.
Lao TzU, The Tao Te Ching
Galaxies formed at stage 7 in our history of the universe. They are vast islands of stars, gas, and dust that populate the universe by the billions. Galactic size and structure range from subtle ellipticals to grand pinwheel spirals with the mass of at least 100 billion stars. Instead of randomly scattered throughout the universe, galaxies tend to be clumped together in clusters. Nonetheless, detailed surveys have revealed that galaxies are quite evenly distributed through the universe. Our own galaxy is known as the Milky Way, because we see it as a broad ribbon of abundant stars in the summer sky (a possible view from far away, and a view from a vantage point far above).
Going with our earlier Big Bang scenario, the universe was for all intents and purposes a fairly homogeneous entity in the beginning. How then, was it possible that matter separated out into galaxies, stars, and planets as we see them today? If it had been perfectly homogeneous at the beginning, how did that happen? To reconcile the apparent uniformity of the early universe with the lumpy distribution of galaxies in the present universe is a challenge that scientists are currently working on. If there were for example slight irregularities in the density of the early universe, the denser regions would have had more of a gravitational pull upon further expansion and may have been able to collect matter into gas clouds and galaxies. Because these density irregularities also imply slight temperature variations, they should be reflected in the cosmic background radiation. Astronomers known now that the latter shows slight irregularities, and various theories for galaxy formation also predict that there should be small scale, but systematic (not random) fluctuations of the cosmic background radiation associated with galaxy formation. To test these theories will requires very sophisticated measurements. Several promising Experiments to that end have been undertaken recently or will be undertaken in the near future.
Our most sophisticated and powerful telescope, the Hubble Space Telescope, has pushed the limits of space observation to a distance of about 11 billion light years. That means that light that reaches the telescope today had to travel 11 billion years to get there. That means a telescope is also a way to observe the universe as it was in the past, and at 11 billion years we are actually getting close to the point where theory predicts that the first galaxies appeared (possibly as close as 90% of the way back to the Big Bang). A picture taken during a long exposure survey (10 days, to capture even very faint light sources) shows numerous galaxies even that far away.
Astronomers think that so called globular clusters (although much smaller than typical galaxies) may be the most ancient accumulations of stars, possibly being some type of primordial galaxies. Thus, they study them to learn more about the life history of stars. The Andromeda Galaxy, M31, is the nearest major galaxy in our vicinity, and located 2.3 million light years away. As a whole, galaxies are amazing and beautiful. The next page contains a collection of images with brief comment. Han Solo says....
Useful web Links: Scientific American; NASA; Space.com (all three sites can be searched for items mentioned above); National Geographic
Stars: How they form and what they are made of
Once we come to accept that galaxies formed as a result of slight irregularities in the matter distribution (density) of the early universe, and grew and accreted due to gravitational forces. Gravity pulled the denser regions together into a cosmic tapestry of voids and matter concentrations (clouds of gas initially), and it is only a small step to assume that even in these there were irregularities that caused further gravitational differentiation into denser and less dense regions. Lets assume now that we have one of these gas clouds, possibly many times the size of the present solar system, and that due to the intense forces of the Big Bang, these gas could have maintained at least some degree of turbulence and rotational momentum.
What follows here is a brief outline of the current theory of how
scientists think that stars and also our solar system evolved from that point forward.
1.Because matter is not evenly distributed within this gas (and dust) cloud, some areas start contracting under gravity. As these grow denser over time, they exert more gravitational pull, pull in more matter, etc. Once started, this process leads to a gravitational collapse. Even if there are no significant original variations in matter distribution, a triggering disturbance might come from the shock wave sent out by a nearby supernova.
2.As the cloud collapses, it heats up and compresses in the center. It heats enough for the dust to vaporize. The initial collapse is supposed to take less than 100,000 years. Because of the gravitational contraction, the shrinking gas cloud starts to rotate faster and faster (just like a figure skater that pulls in his/her arms during a pirouette).
3.The center compresses enough to become a protostar and the rest of the gas orbits/flows around it. Most of that gas flows inward and adds to the mass of the forming star. The gas is rotating, and the centrifugal force from that prevents some of the gas from reaching the forming star. Instead, it forms an "accretion disk" around the star. The disk radiates away its energy and cools off.
4.The gas cools off enough for heavier elements (heavier than say hydrogen and helium) to condense into tiny particles in the "accretion disk".
5.The dust particles collide with each other and form into larger particles (gravitational contraction at a small scale), and this goes on until the particles get to the size of boulders or small asteroids.
6.Once the larger of these particles/bodies get big enough to have a nontrivial gravity, their growth accelerates. Their gravity (even if it's very small) gives them an edge over smaller particles; it pulls in more, smaller particles, and very quickly, the large objects have accumulated all of the solid matter close to their own orbit. How big they get depends on their distance from the star and the density and composition of the protoplanetary nebula. In the solar system, the theories say that this is large asteroid to lunar size in the inner solar system, and one to fifteen times the Earth's size in the outer solar system. The accretion of these "planetesimals" is believed to have taken taken a few hundred thousand to about twenty million years, with the outermost taking the longest to form.
7.How big were those protoplanets and how quickly did they form? At about this time (step 6.), about 1 million years after the nebula cooled, the star's interior would have reached pressures and temperatures high enough to allow fusion of hydrogen and deuterium into helium. The star heats up, and once it is hot enough, radiation and gases will emanate from it (the so called solar wind) and gradually sweep away the gases left in the solar nebula and the interplanetary space. If a protoplanet was large enough (and far enough from the sun to escape excessive heating), its gravity would pull in the nebular gas, and it would become a gas giant. If it was smaller, it depended whether is was far away from the sun, or relatively close. If it was close, the heat and solar wind would prevent accumulation of gases, and vaporize most of the gases accreted earlier. As a result we would get a body made mostly of heavier elements (like the earth or mars). If it was far away we would get a mixture of solids and condensed gases, but it would tend to be small because of lesser density in the outer gas disk (we would have a comet).
8.At this point, the our solar system is composed of a central star that is orbited by solid, protoplanetary bodies and gas giants. As time passes the planets may loose further volatiles (gases) as they equilibrate with the heat put out by the central star.
9.Eventually, after tens to may be a hundred million years, we would have a solar system like ours, with planets in stable orbits. These planets may undergo further differentiation, and their surfaces may be heavily modified by collisions with other bodies (asteroids, comets).
In its basic outlines, this "solar nebula" theory has been around for decades, but in recent years observations made with the Hubble Space Telescope have finally provided observations that show that the theory was in essence correct (it's cool when a plan comes together). Pictures taken from the Eagle Nebula (M16) show huge pillar-like dust clouds in which new stars are developing. Other pictures from the Orion Nebula (M42), show new stars that begin to shine through the covers of a protoplanetary disk (proplyds). Scientists have also developed a variety of methods to develop evidence for the existence of planets around other stars than our sun, and have in recent years compiled a growing list of extrasolar planets.
Much what we know about the processes that occur within stars has come from the study of nuclear physics, mostly in connection with research into nuclear weapons. As pointed out earlier, hydrogen and helium were the chemical elements that emerged from the Big Bang. Learning about nuclear physics, scientists realized that all the more complex elements, such as carbon, oxygen, etc. can be produced by fusing together these light elements under the right pressures and temperatures.
The nuclear fusion "food chain
At the extreme temperatures inside a star (15*106 Kelvin's), hydrogen atoms (protons) are stripped of their electrons, and undergo frequent collisions (density is very high). Although the hydrogen nuclei repel each other (same electric charge), under the pressures inside the core of a star they will go through a series of nuclear reactions and fuse into helium nuclei (2 protons, 2 neutrons). In the process 2 positrons, 2 neutrinos and energy will be emitted as well (in the animation protons are red, neutrons are yellow, neutrinos are green, and positrons are blue). After a long period of hydrogen fusion, more and more helium accumulates in the core, and the star eventually changes.
Red Giants and White Dwarfs: Because matter is getting more densely packed, the stars core shrinks and also heats up. The increased radiation pressure causes the outer layers of the star to expand (about 50 times for a star the size of the sun) and it transforms into what is known as a red giant. Because the central furnace gets ever hotter and denser, nuclear reactions that would have been impossible before can now become reality. Helium, previously the end product of the nuclear fire, now becomes fuel. Two heliums lead to a short-lived form of beryllium, and the latter forms a carbon nucleus when it collides (in time) with yet another helium nucleus. Add another helium nucleus to carbon -- and we get oxygen. The outcome of this process depends on the initial size of the star. If its the size of the sun, it will swell up into a cool but luminous red giant, become unstable, and cast off its outer layers, form a gas cloud around itself (a planetary nebula). It and may also release some of the heavier elements synthesized within. Once the fuel runs out completely, the core will contract as much as it can and the remainder will be a so called white dwarf.
Supernovas, Neutron Stars, and Black Holes: Stars more massive than the sun have a more interesting fusion history. They are rarer, and also more short lived. To support the stars outer layers, the pressures and temperatures in the core have to be higher, and as a result much more energy is produced (and emitted). A star 20 times as heavy as the sun will burn 20,000 times a bright, and goes through its hydrogen reserves fairly rapidly. While the sun may need 10 billion years to get to the red giant stage, it may take that star just 10 million years (1000 times faster). Under the more intense conditions inside this star we can generate neon and magnesium by fusing carbon, fuse oxygen to make silicon and sulfur, and keep fusing our way up to iron. Once the "iron stage" has been reached, no further fusion occurs. While so far every fusion step provided more energy for the operation of the star, to make even heavier elements now requires energy input. Their production will consume energy rather than creating it.
Once the core of a star has converted to iron, there is no more radiation pressure to counterbalance gravity. The core collapses (very rapidly, probably within seconds), and depending on mass it will either become a neutron star or a black hole. The core is as dense as the nucleus of an atom (or denser), and as the outer shell collapses onto it, it rebounds with tremendous force. A shock wave travels through the outer layers of the star, and the star explodes with an intense flash of light. The star surface may shine for weeks with the intensity of a billion suns (a supernova) while its outer surface expands at a rate of several 1000 km per second. During this short time span the star may emit as much energy as throughout its previous existence.
The conditions in the outgoing shockwave are also conducive to the
formation of elements heavier than iron (e.g. neutron bombardment of iron produces gold,
gold is transformed into lead, neutron bombardment of lead leads to all the other elements
up to uranium). Because supernova explosions are the last step in a long history of
nucleosynthesis, and because they are comparatively rare, elements heavier than iron are
of small cosmic abundance.
Some newly released simulations suggest that a collision of neutron stars could also provide the conditions necessary for nucleosynthesis of elements heavier than iron.
|Nuclear fusion processes and the relative abundance of elements in the universe. Fusion from helium skipped over Li, Be, and B, to carbon and created all the elements up to iron. Note that abundances are shown on a logarithmic scale. Hydrogen and helium are about 8 to 9 orders of magnitude (100 million to a billion times) more abundant than all others. Thus, even today the universe is primarily composed of hydrogen and helium|
How do we know so much about the chemical composition of the universe? The answer is spectroscopy. Just as we can look at the spectrum of a single star or galaxy for spectral lines and chemical signatures, we also can conduct a systematic census of chemical composition. This was expressed more than hundred years ago one of the pioneers of astronomical spectroscopy in the following way:
One important object of this original spectroscopic investigation of the light of
the stars and other celestial bodies, namely to discover whether the same chemical
elements as those of our earth are present throughout the universe, was most
satisfactorily settled in the affirmative; a common chemistry, it was shown, exists
throughout the universe.'
- Sir William Huggins
Because the supernovas, unlike white dwarfs and red giants, propel large quantities of their more evolved fusion products into the surrounding space, they are very important contributors of elements heavier than helium to interstellar space. The "ashes" of nuclear burning and phantastic cataclysms are the materials we ourselves are made of (carbon, oxygen, etc.). Because first generation universes and stars (about 1 billion years after the Big Bang) would essentially have consisted of hydrogen and helium, most of the stars and galaxies we observe today must be younger than that, because they show spectra that reveal the entire periodic table. As our table above shows, the composition of the universe is mostly hydrogen and helium (more than 99%), with hydrogen being dominant. Thus, more than 10 billion years of nuclear fusion have converted only a small proportion of the initial material into heavier elements.
Because stars are in a way "factories" for heavier elements, scientists can look at the composition of stars (determined via spectroscopy), in order to figure out their relative age of formation within a galaxy.