17. Of stars and element factories

(continued) Chapter 2. Long Evolution: Universe Emerging

 

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With the first generation of stars populating the young galaxies, millions – eventually uncountable billions – of these new beacons poured their light energy into the universe. But not yet did a single planet orbit any star. To make planets, gravity needs more than just hydrogen and helium to work with. At least some heavier elements are required.

 

Star formation actively continues today in the parts of galaxies that are richest with gas and dust – particularly out in the arms of spiral galaxies, of which there are quite a few. The Milky Way’s “Orion” spiral arm, home to our sun and its planets, has been called a nursery for star formation. The birth, life and death of stars is analogous to the birth, life and death of humans and other life forms, it just takes longer and is more boring. But you could say it the other way around – that human birth, life and death are analogous to the life cycle of large-scale structures in the universe – and then ask:  Is it just analogy, or is it a universal pattern? A pattern? Yes, it certainly seems to be.

 

In the pattern familiar to us, stars are born, live vigorously, grow old, and die. An older generation’s death helps give birth, phoenix-like, to the next generation of stars. Which generation a star happens to be born into determines which elements will be available for it to use as fuel, how long it will live, and what sorts of elemental products it will leave behind when it exits – often by dramatic explosion.

 

Stars are element factories. Those in the first generation were necessarily composed of only the elements produced directly by the big bang: a great deal of hydrogen, much less helium, a dab of lithium. By orders of magnitude, hydrogen was the most plentiful element in the early universe, so it became the primary fuel for making stars. Hydrogen burned intensely hot and relatively fast, so first-generation stars had short lifetimes of only a few million years. Their enormous mass also contributed to exhausting their fuel.

 

As hydrogen atoms burned in a star’s atomic furnace, the intense heat and pressure transformed them, alchemy-like, into increasing numbers of helium atoms. In turn, heat and pressure then turned some of the helium into lithium – and so the process continued, in turn transmogrifying each product element into another “new” element. The atoms produced by the first stars were mainly lighter elements, “lighter” meaning the atoms’ nuclei contained fewer protons and neutrons than do “heavier” elements. But the process could go only so far in these first stars, for when their primary hydrogen fuel became completely used up most stars collapsed in upon themselves and exploded.

 

When a star exploded, its newly-forged elements were broadcast far across its galaxy. From this debris, the new elements gradually formed new clouds. As the clouds grew slowly larger, enriched with elements produced by the first-generation stars, their growing gravity drew in still more gas from nearby smaller clouds. Once again, cloud centers became the compressed seeds for formation of stars – a second generation. When enough light-element debris from previously exploded stars was incorporated into the new generation of protostars, they too ignited as nuclear furnaces, but with a difference.

 

The products of burning and churning in these newer stars were elements that were heavier (more protons and neutrons in the nucleus) than the first-generation elements had been. Successive stellar generations gradually produced more and more heavier elements. After lithium and deuterium, along came oxygen, nitrogen, silicon, sulfur, and many others including carbon – the very carbon of which our bodies are made. Our human bodies are formed of dust – immortal elements – that were forged in the hearts of stars. And you know what that’s called, as did the immortal Hoagie Carmichael. Stardust.

 

The burning process inside a star has been compared to layers in an onion. The outer layers are supported (in the sense of “held out away from the hot core”) by the energetic nuclear reactions at the core. When the hydrogen fuel which began the process is used up, helium moves into the core and all other layers collapse a bit inward. This collapse increases pressure and temperature at the core, sustaining the fiery furnace, keeping it blazingly active. Further collapses produce ever new layers of fuel, slowly migrating inward, one at a time as each layer is slowly consumed. As the process repeats in later-generation stars, burning ever heavier elements as fuel, successive collapses of layer after layer produce still heavier elements in and around the core – until the process ends.

 

The heaviest element of all, the last to be produced by this process, is iron. One might think of it as alchemy designed by God, and  – unlike those old medieval alchemists who failed in their aspiration to turn lead into gold – God’s alchemy works just fine. When all the onion-like layers have moved inward, each taking its turn to be burned into something heavier, a dense core of solid iron at last forms, and quickly. When this happens no power in the universe can prevent the remaining residue from crashing violently inwards. The star succumbs to instantaneous total collapse. This in turn generates a shock wave the likes of which we mere mortals lack sufficient imagination to conceive. The rapid collapse is immediately followed by titanic explosion. Every last atom of every element that was produced by the sequential layered burnings is flung outward, blasted away in all directions far across the wide universe. Over eons of many such explosions, thus does the universe become gradually enriched with the periodic table of the elements.

 

The light generated by such a cataclysmic dying-star explosion outshines the combined light of its entire galaxy. Indeed – it temporarily lights the entire universe like a great beacon which may blaze many days before fading to darkness. Ancient earthlings remembered and passed down their awe-struck legends about the great lights which occasionally appeared in the skies. Their descendants call such lights supernovas.

 

And that’s just the supernovas

Not all stars explode. Some end up becoming low-mass red giants – i.e., after a lifetime of burning brightly, they run low on fuel, cool down, swell up – way up – and are seen to be red in color. This will be our own sun’s fate in about 5 billion more years. A variation on this, red supergiants are stars much more massive than our sun in which temperature becomes so hot the outward pressure forces outer layers of the star to be shed and the dying star ends up being surrounded by a beautiful shell of glowing hot gas. When red supergiants eventually collapse, they leave behind a compact, inert mass of carbon and oxygen which eventually cools to invisibility and is called a white dwarf.

 

Some other stars, very massive, become black holes. These collapse with such force that exponentially accelerating density and gravity overcome all outward pressure, making it impossible for them to explode. The star’s huge body of matter is compressed down into a ball so small and super dense that not even light can escape its ferocious inward pull. Gravity is at its most extreme. Think of a big bang in reverse though on a much smaller scale. Is not this implosion as fully mysterious as the explosion which was the big bang?

 

Our universe’s real mysteries are far more mysterious than any fiction we can conceive.

 

On elements

Almost all stars of any age produce at least some heavier elements during their lifetime. Overall, each succeeding generation of stars, burning the elements produced by preceding generations, further enriches the universe with ever heavier elements in ever increasing quantities. Thus the earliest (older) stars produced primarily light elements, while the younger stars – including our sun – produce primarily heavier elements. Except for the hydrogen, helium and traces of light elements present in the very early universe, all the elements in the universe today – all that cosmic “gas and space dust” of which we so often speak – are in fact the ashes of long dead stars.

 

On Earth, we know some of them as the air we breathe (nitrogen, oxygen, hydrogen as water vapor, argon, carbon as carbon dioxide), the iron we mine to make steel girders and cars (the non-plastic parts these days), and the carbon fossil fuels we unearth and burn as oil, coal and “natural” gas. The latter residue of ancient life fuels our ongoing industrial revolution, now in its third century, while also fueling global warming which may well take down the industrial revolution in a collapse of human civilization. More on that later.

 

Nearly fourteen billion years after the big bang, this process continues – stars burning the elemental products of previous stars and producing newer, heavier elements which they disperse in final death explosions. After all this, hydrogen still remains by far the most plentiful element in the universe. Fortune and eternal fame await the man, woman or teen prodigy who figures a way to efficiently convert our essentially infinite hydrogen supply to a permanent energy replacement for polluting, Earth-destroying fossil fuels.

 

All these elements taken together, lightest to heaviest, constitute the Periodic Table of the Elements seen in every high school science room. The Periodic Table should be of interest to us all. Notice the dietary supplements among the heavier elements – iron, copper, selenium – some of which you may be taking in pill form today. Manufacturing them depended on exploding stars – and then eventually they appeared on your grocery shelf in little bottles. Perhaps you have a gold filling, or own a hammer made of iron forged into steel. Our ancestors figured out how to melt copper and tin together to forge a harder alloy called bronze, and so our human history includes an early “Bronze Age.” It was ended by the Iron Age; iron was harder – and sharpened better.

 

The Periodic Table of the Elements is a star-generated library of new and used parts for building the universe and everything in it, including our bodies. Stars are the source of most of the universe’s carbon, which we know to be essential for life on this planet. Our Earth-life bodies are literally descendants of the stars, and would not now exist if routinely exploding stars and supernovae hadn’t been out there burning, creating light, building heavier elements out of lighter elements. We are carbon-based bipeds who breathe oxygen and have iron in our blood. Other tidbits of star and supernova debris float around in our veins, our brains, in our very cells – the cosmic seeds of life on earth. Truly, in the most literal sense possible, we are stardust…that has been passed through Earth dust, thence into the plants growing from the earth, which our ancestors ate to feed their earth bodies so they could be robust, and breed, and produce descendants.

 

These are words you can indeed interpret literally. And God sees that it is good, for it had to be He who kicked off the whole simple process of self organizing rising complexity via an ingenious process which we humans, lately arrived, have chosen to call evolution.

 

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Of stars and planets, Sol and Earth

Even as the universe continued expanding outward in all directions, creating its own space as it went (most normal people don’t quite get that part because of our limited three-dimensional perspective), the attractive force of gravity caused great clouds of elemental gas and dust to condense and self organize into galaxies and stars within galaxies. But it didn’t stop there, because gravity is constant. While the new suns were forming, protoplanets were also forming in orbits around them. The local star system within which we happen to live went through the same planet-forming process as trillions of others. A vast ball of hot gas gradually condensed until its parts became our sun, Sol, and eight planets – four inner rocky balls and four outer gas giants orbiting a central fireball far bigger than all of them put together. How did this miracle proceed?

 

Imagine our solar system when it was barely beginning. No sun, no planets, only an enormous cloud of heavy-elements gas and dust, light years across, containing a few wrinkle-like perturbations of density, remnants left over from the big bang. There was a germinal tendency for the entire cloud to slowly turn, a rotating tendency which would grow as time passed, aided and abetted by the gravity of neighboring protostar clouds in nearby regions of the galaxy. Over long distances they reached out to each other, influencing each other from afar with the gravity they brought to a focus. They still do.

 

Our gas cloud had motion within it. Atoms moved, collided, some joining to become molecules, others moving on solo. The moving gas/dust throughout the cloud had clumpiness, its density was not the same everywhere – there were local variations on the same theme we saw in formation of galaxy clusters, galaxies and stars. Wherever density grew a tad greater than somewhere else, stronger gravity resulted apace.

 

The largest clump was of course at the center. With its greater gravity the center pulled in vast amounts of the surrounding gas and dust. Eventually the center condensed and grew dense enough, superhot enough, to ignite and become our sun. This happened some 4.6 billion years ago – a bit more than nine billion years after the big bang. In other words, our star was born when two-thirds of the history of the universe had (so far) elapsed. But don’t forget, our universe is still young – much is yet to happen.

 

Like all stars, our sun is a huge ball of incandescent gas. As stars go it is unremarkable, ordinary. It has an expected lifetime of about nine billion years, half of which has already elapsed. Many stars are bigger than ours, and yet Sol, only average in size, could hold more than a million planets the size of our earth. Its very ordinary surface temperature averages about 10,000 degrees Fahrenheit. A little farther out, the temperature of its corona is around a million degrees. Deep down inside at its core, where the alchemy of elements changing into other elements is going on, temperatures reach 15 million degrees. The sun appears yellow to us because its temperatures are intermediate. In comparison with other stars, Betelgeuse appears red because it’s cooler than our sun, Orion appears blue-white because it is hotter.

 

But our sun’s ordinariness is just one perspective. As the parent body of the solar system, our star is so massive that it contains more than 99 percent of all the matter in the system. That remaining tiny one percent fraction was more than enough to build all the planets, all their respective moons, and all the comets, meteors, rocks and debris now in orbit around the sun. Everywhere you turn, our universe is breathtaking in its extremes.

 

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…to be continued in one week…

 

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