Stardust

An Astronomy Lecture and a Best Man's Toast

I frequently give short astromony lectures as introductions to my talks on software. One person who has seen me do a few of these asked me to write one particular talk down so that he could use it as a source for a toast he was to give as the best man at a friend's wedding. You'll see why this is appropriate if you read on.

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In the late 1800s scientists had determined that there were few mysteries left. Newton had formulated comprehensive laws of mechanics and gravity, and those laws appeared to be virtually flawless. Maxwell had described the relationship between magnetism and electricity, and in so doing had discovered what light itself was. So, indeed, there were few questions remaining to be answered, and some felt that science was slowing down and would become a matter of filling in ever-smaller details. The age of big discoveries was over.

But there was one question that had not been answered. What powered the Sun? Eddington had put forth a guess. He calculated the energy that would be radiated by a cloud of hydrogen gas as it collapsed under it's own gravity. Assuming that it started with a diameter equal to the Earth's orbit, he showed that it could radiate energy at the rate of the Sun for 10 million years before the collapse ended. So if his idea was correct, the Earth could be only a few million years old.

However, there was clear evidence from geology that there were rocks that were dozens of times older than this. What's more, some of those rocks had fossils of plants in them. So, clearly the Sun must have been shining, at something close to it's current output, for much longer than Eddington's 10 million years. The problem was that nobody knew what source of energy could maintain such an output for such a long time.

The problem got its first big clue in the early 1900s when Roentgen accidentally put a rock on top of a photographic plate and then noticed that the plate was fogged. He found that this particular rock would fog photographic plates just as though there were exposed to light. Clearly this rock must be emitting rays of some kind. He called them X-Rays.

Roentgen realized that the rock was very old, and that if it had been radiating X-Rays for all that time, it must have a vast supply of energy. This opened up a line of research that made great discovery after great discovery. It revolutionized our view of matter and energy. It showed us what atoms were made of, and what the pieces of atoms were made of. And, for better or for worse, it gave us access to the energy source that powers the Sun.

The Sun shines through a process known as hydrogen-fusion. The Sun is a ball of hydrogen a million miles in diameter. The vast weight of that hydrogen presses down on the core of the Sun pushing the atoms of hydrogen very close together. What's more the temperature is an unimaginable 30 million degrees, so those hydrogen atoms are moving too quickly to hold on to their normal shield of electrons.

Hydrogen atoms, without there electrons, are simply protons. Protons repel each other furiously like two repelling magnets. The closer they get to each other, the harder the repulsion. Yet the temperatures and pressures in the core of the Sun are so very great that a few protons get close enough to react.

Protons repel each other through the electromagnetic force. This is the same force that holds pieces of paper onto balloons, or gives us shocks in the wintertime. We call it static electricity. But protons carry another force called the strong nuclear force. This force has a very limited range. It can only be felt at atomic distances (10e-13cm). However, once within that distance, it is 1000 times stronger than the electromagnetic force.

Every second, 600 million tons of protons in the core of the Sun are driven to within the critical distance of each other, and they strongly attract one another through the strong nuclear force. This attraction is fierce, and lot of energy is released as the two protons slam into each other and fuse. This energy of fusion creates heat, and this heat is what makes the Sun shine.

The heat does something else. It makes the core of the Sun expand. The hotter the core, the more it expands. But the weight of the Sun presses down so hard that it stops this expansion. And so the Sun sits in a state of dynamic equilibrium. Like Atlas holding the weight of the world on his shoulders, the heat of the fusion reactions in the core of the Sun holds back the terrible weight of the rest of the Sun. But this can go on only so long as there is enough hydrogen in the core to keep the fusion reactions going.

The Sun has been shining for 5 billion years. During that time it has fused half the hydrogen in the core into helium. (Which means "The Stuff of the Sun"). The Sun will continue to shine at its current rate for another 5 billion years. Then, with the hydrogen running out, the fusion rate will begin to slow.

As the reaction rate slows, the core will cool. As it cools, the gravity of the Sun will begin to win the ten billion year old battle; Atlas will tire. The core will start to collapse. But as it shrinks, the pressures and temperatures will rise to the point that the helium atoms can start to fuse. This new birth of fusion will hold back the gravity of the Sun for another billion year or so; but at a terrible cost.

The heat produced by the helium fusion is so great that the outer layers of the Sun will expand. Eventually the surface of the Sun will collide with the Earth. Long before that, the oceans will have boiled away, and the mountains will have melted. Glowing bright red, the Earth will enter the Sun, and its crust, mantle, and core of iron will become vapor. The Earth will be no more.

Deep in the core of the Sun, helium is fusing into carbon and oxygen. Eventually, even this reaction will run out of fuel, and the core will begin to contract again. This contraction is the end for the Sun, because it is not heavy enough to force carbon and oxygen to fuse. And so in one last agonal spasm, the outer layers of the Sun will be blown away from the carbon core, forming one of the most transient and beautiful of sights: the wispy colored and layered cloud of a planetary nebula.

For 10 thousand years this beautiful death shroud will be a spectacular grave marker for the Sun and Earth. Then it will dissipate, leaving behind the rapidly spinning white hot cinder of carbon, the core of our Sun, now become a white dwarf star the size of the Earth, with the mass of the Sun; so dense that a teaspoon of it's material would weigh thousands of tons.

Other stars, more massive than our Sun, end in a different fate. They are heavy enough to force carbon and oxygen to fuse, indeed some are so massive that they can force any nucleus lighter than Iron to fuse.

Imagine a star that weighs 10 times more than our Sun. As such a star ends its life its insides look like an onion. The outer shell is hydrogen that is not fusing. Within that there is a shell of hydrogen fusing to helium. Within that there are shells that fuse (among other things) helium into carbon, carbon into oxygen, oxygen into silicon, and silicon into iron.

Iron, however, will not fuse. Or rather, when it fuses it does not release energy. In fact, fusing iron takes more energy than it produces. So iron is the dead end for such a star.

On the last day of this star's life, a nugget or iron begins to form at its very core. This nugget grows rapidly as the silicon burning shell rapidly fuses all the silicon into iron. Eventually the iron core grows so heavy, and is pressed down by the horrific mass of the star above it, that the electrons and protons within the iron are suddenly forced to react to become neutrons.

This reaction proceeds very quickly and produces a massive amount of energy in the form of neutrinos. Moreover the reaction causes the iron core to collapse by a factor of 1000. In a split second the iron core shrinks down from thousands of miles in diameter to perhaps 10 miles in diameter. The material above it rushes in to follow, moving at perhaps half the speed of light, only to encounter the barrage of neutrinos slamming into it from below.

The result is a SHOCK WAVE that roars outward through the body of the star, fusing everything it encounters. The over-pressures in this shock wave are so intense, and the flux of neutrons is so great that it can force any atomic nucleus within it's grip to fuse into heavier elements. Iron fuses into mercury, copper, tin, silver, gold, platinum, and uranium. Virtually every element on the periodic chart, and a few we haven't discovered yet, is formed in this gargantuan explosion. And in the midst of all this creation, the guts of the star are blown all over the sky in an event so violent that if we were within 50 light years our chances of survival would be nil. We call such explosions supernovae.

Supernovae are so bright that at 6500 light years they cast hard shadows at night, and they are visible in the daytime sky. The last to be that close to us was in 1055AD, where we now see the crab nebula.

The material blown away from the supernova is laced with heavy elements. It travels for centuries at a sizable fraction of the speed of light until it happens to plow into another cloud of hydrogen gas. The odds are that this collision will compress that cloud enough so that it can start to collapse under its own gravity, causing it to form a Sun, laced with heavy elements that just might become the substance of planets.

And this is the final gift of a dying massive star. In its death lay the seeds of the birth for many others stars, and the elements from which to build planets and people. The gold in my wedding ring, the iron in my hemoglobin, the calcium in my bones, the carbon in my proteins, had to have been formed by this event. No other event is energetic enough to form these atoms and distribute them across the sky. In Genesis it says that we are formed from the dust of the Earth. In truth we are stardust; made from the ashes of dead stars.



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