Have you ever noticed how the sound of a motorcycle or a siren changes from higher pitch to lower as it goes by you? This change in sound is a result of a characteristic behavior of waves. (Sound travels as a pressure wave in the air. The boom of thunder that rattles your windows and your eardrums in created by rapid changes in pressure moving out from a lightening strike.) When the source of the sound wave and the observer are in motion relative to one another, the velocity of the wave doesn't change for the observer. Instead the observer "sees" a change in wavelength. This is different from our common experience with moving matter. Would you rather have a collision with a standing car or one moving at 90 mph? But waves, although they affect matter and may travel through matter, are a form of energy and so behave differently from what we might expect.

To understand this behavior, consider a siren. Imagine that both you and the siren are still. The siren is emitting a sound wave of a particular wavelength that your ear and brain interpret as pitch. The first diagram shows the siren emitting a single wave in very schematic fashion. Now change the scene and have the siren moving towards you. Because it takes time for the sound wave to be emitted, the siren has changed its position in the time it takes for a single wave to form. Because it is moving in the direction of the wave, the beginning and end of the wave are closer together than they would be if the siren were still . The wavelength is shorter and the sound higher in pitch. Similarly if the siren were moving away from you, the sound wave coming to your ear would be a little longer and the pitch lower. You can prove to yourself that the same results occur if the siren is still and you, the observer, are moving.

But what do sirens have to do with understanding the origins of the earth? The connection is that  we learn about the origins of the universe, stars, galaxies and planets by studying the light emitted by distant objects. Light is a wave form of energy, and like all waves, its wavelength is affected by the relative motion of the source and observer. If a star is moving away from the earth, its light is shifted to longer or "redder" wavelengths. If it moves towards the earth, its light appears bluer (shorter wavelength). But how can we know what the color was originally? Fortunately, elements emit light with characteristic wavelengths. So one can separate the light from a distance object into its spectrum and look for the characteristic wavelengths of hydrogen and helium, the two most common elements in the universe and so the easiest to find. When Edwin Hubble began to use shifts in wavelength to measure the velocity of galaxies in the 1920's, he discovered that except for the closest galaxies, those less than 100 million light years away, all the galaxies seemed to move away from the earth. To make matters even stranger, galaxies farther from the earth were speeding away faster than those that were closer. It was a troubling discovering because it seemed to put the earth back in the center of universe and there is no good reason for our being special. However, if you don't accept the earth as special, there is an alternative. Imagine dots on a balloon. As the balloon is inflated, each dot moves away from every other dot. Any observer standing on a particular dot would see all the other dots moving away from her with closer dots moving slowly and distant dots moving faster. However, the surface of the balloon does not have a center, it is the surface itself that is expanding. It would seem then that like the dots on the balloon, the galaxies are caught in an expanding universe.

[NOTICE HOW THE LONGER DISTANCES INCREASE MORE RAPIDLY THAN THE SHORTER.]

(An aside on light years -- Distances in the universe are so huge that light traveling at 300,000 km/sec or 186,000 mi/sec takes years, in fact often millions years to cross from one galaxy to another. So it was convenient for astronomers to measure distances in terms of the time it takes the light cross them. The nearest star in the night sky is about 3 light years away or 3 X 60 sec/min X 60 min/hr X 24 hr/day X 365.25 day/year X 300,000 km/sec, or about 30 trillion km or 20 trillion miles. In other words a light year is a long ways. One of the advantages of using this method to record distance is that we see the star or galaxy as it was in the past. When an astronomer photographs a galaxy 1 billion light years away, he is taking a picture of that galaxy as it looked a billion years ago because the light making the photograph left that galaxy 1 billion years ago. Using light years as a distance measure helps us to remember that we are looking into the past when we look far away.)

If the galaxies are now moving apart from each other, it is a simple step to imagine time running backward. If there is enough time, then all the galaxies collide at a single point. It is as if there was at some beginning of the universe a huge explosion that sent all the matter of the universe, the very universe itself, flying outwards. This is known as the Big Bang.

By estimating conditions in the universe at the time of Big Bang and shortly after, cosmologists, scientists who study these things, have estimated the original composition of the universe. It seems to have been almost all hydrogen and helium with a tiny amount of a third small atom, lithium. No iron, no carbon, no oxygen, no uranium, just those three elements. So is the theory wrong? Not necessarily. Is there a way in which an evolving universe might change in chemical composition?

When a star "burns" it is really not burning. It is combining 4 hydrogen atoms to make a single atom of helium. Because one atom of helium weighs a little less than 4 hydrogen atoms, a tiny bit of mass is converted to energy according to the equation E = mc². M is the mass that is transformed and c is the speed of light. Because c (300,000 km/sec) squared is a very large number even a small amount of matter produces a large amount of energy. Stars are huge. Even a relatively small star like our sun converts 600 million tons of hydrogen each second. It has been doing this for 4.5 billion years and is expected to be able to continue fusing hydrogen for another 4 to 5 billion years. That's a lot of hydrogen!!

This process of fusion requires the very high temperatures and high pressures of the interior of a star because the positive nuclei of the hydrogen atoms have to be brought close together to fuse. Only at extreme pressures can the nuclei overcome the repulsive force of their charge and get close enough to join. However, eventually a star uses almost all its hydrogen and fusion stops. Without fusion, the star cools a little bit because there is no new heat to replace the heat being radiated into space. As it cools, the star shrinks under gravity's pull. As it shrinks the helium nuclei are pushed closer and closer together until they begin fuse. (Why is it harder to get the helium nuclei close to each other?) Three helium nuclei make a single carbon atom. When our sun does this, the heat released from the helium fusion will cause a shell of hot gas to be expelled from the surface of the sun. This shell will expand outward and engulf and vaporize the four inner planets -- Mercury, Venus, Earth and Mars. But, don't lose sleep worrying about it because you are not likely to be here when it happens.

Like hydrogen before it, helium is depleted by the fusion process. The star cools, shrinks and new fusion reactions begin between the remaining hydrogen and helium and carbon or simply multiple carbon nuclei to produce nitrogen, oxygen, sulfur,..., all the elements between lithium and iron. However, iron represents the most stable element in that it has the lowest energy potential of any nuclei. If iron fuses with any other nuclei, it uses energy -- makes the surroundings colder. Iron is element # 26. So there is no way to use fusion to make any of the other naturally occurring elements (numbers 27 to 92) because both entropy and potential energy change oppose the action.

All the heavier elements form in the cataclysmic end of some stars. When the star has so much iron that no additional fusion can occur, it begins to shrink again but this time the shrinkage becomes a massive implosion. If the star is the right size, somewhat larger than our sun, the gravitational collapse heats the star to billions of degrees -- so hot that iron nuclei fall apart. (The entropy gain becomes more important than increase in potential energy.) Huge numbers of particles are released as the iron nuclei become free neutrons and protons causing a tremendous increase in pressure that blows the star apart. The explosion or super nova spreads the carbon, nitrogen, oxygen, iron and other elements of the outer part of the star into surrounding gases of the galaxy. Neutrons from the star's core speed into the surrounding gases. Collisions between the neutrons and the atoms in these gases build the heavier nuclei. Think about it; all the lead, all the copper, all the gold and silver you have ever touched or seen is the a record of an exploding star. And of course, the carbon, nitrogen and oxygen of our bodies are from the cores of stars that "died" billions of years ago. As Carl Sagan, astronomer and popular writer said, "We are truly the stuff of stars."

The super nova explosion also sends a shock wave through the clouds of gas and dust around it that are now enriched in heavier elements. About 5 billion years ago an explosion and shock wave of this type caused a cloud of gas on the edges of the Milky Way Galaxy to thicken just enough that it began to collapse under its own gravitational attraction. As it collapsed, it began to spin and grow hotter. The largest concentration of gas and dust collected in the center, but smaller concentrations formed in a disk of spinning matter that stretched out to a distance of several billion miles. The central concentration became the sun, the outer concentrations the planets. Originally all would have been mostly large balls of hydrogen and helium but with a significant amount of the heavier elements as well, that would have formed a solid core. The earth at this time probably looked much like Jupiter today, core of rock and iron surrounded by a thick atmosphere of hydrogen, ammonia, methane, water and helium.

Gravitational collapse also heated the concentrations of matter. Collisions between particles, much like hammering on a piece of metal or twisting a wire, changes mechanical energy to heat. In the sun temperatures and pressures rose enough to cause hydrogen to fuse, releasing even more heat. This heat was radiated throughout the solar system heating the atmospheres of the primordial planets and causing the gases of the inner planets to escape into space.

Atoms in a gas are constantly moving. They have to be. For example, atmospheric gases are always being pulled toward the surface of the earth by gravity. If they were not in motion, all the gas molecules would wind up on the ground -- essentially packed together like molecules in a solid or a liquid. Now consider what do you do if you want to slow a gas down enough so that it will form a liquid. Of course, you cool it. What happens to the motion of gas molecules when the gas is heated? Sure, they move faster and spread out. The earth's hydrogen and helium moved so fast and spread so far that they escaped the pull of the earth and are probably part of the sun today. (The sun can hold on to its hydrogen because it so massive that it has a much stronger gravitational pull.)

And the BEGINNING? Well the earth seems to have been a barren rocky planet, mostly iron, silicon and oxygen, exposed to continued bombardment by meteors and comets. No oceans and little if any atmosphere. Four and a half billion years of geologic and biological evolution have made it a nicer home. The early earth would have been an interesting place to visit, but I wouldn't want to have lived there.

Reading question:  How does a scientist determine the chemical composition of a star that is 10's of light years distant from the earth?

RETURN