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"WE ARE ALL STAR STUFF"

All living things are composed of elements created deep within stellar furnaces.
By Neil F. Comins

When my children were young, I used to do their night feedings. I can't count the number of times in the middle of the night, watching these wonderfully complex combinations of simple atoms, I marveled at the universe's complexity. It was early one morning in 1991 when one of the connections between life and the cosmos struck home. I was feeding Joshua and casually stroked his cheek. Feeling the warmth and the motion, I couldn't help but wonder about how the elements in Joshua came to be. How did the matter in the universe evolve from its initial hydrogen, helium, and lithium into the variety of elements essential for life?

I knew the answer, of course, but I had never made the connection between it and our lives as vividly as I did that morning. Somehow it all came down to cars. The first car that I can remember our family owning was a slate-blue 1959 Chevy Biscayne. It had horizontal wings just about as wide as the ones on a Boeing 747. But the combination of a weak, finicky 235-cubic- inch, 108-HP, 6-cylinder engine and the power-hogging two-speed Powerglide transmission gave it little acceleration, making it dangerous to even pass cars on four-lane roads, much less attempt to fly.

The lackluster Biscayne took a back seat to my mother's 1961 sky-blue Thunderbird convertible, powered by a 390-cubic-inch, 300-HP V8. This was the Thunderbird shaped just like a horizontal V2 rocket and virtually identical to the 1963 T-bird featured as the flying car in the remake of Flubber. The car accelerated like greased lightning and could run rings around the Chevy. The difference between the two cars was highlighted by the difference in fuel consumption. The powerful T-bird guzzled it, while the Chevy merely sipped leisurely.

Different rates of fuel consumption also characterize how energy is generated in stars and how different elements get into the cosmos. Stars make energy by nuclear fusion. This energy eventually rises through the stars' surfaces and escapes as starlight and other radiation. Energy is only created where the overlying matter provides enough pressure to heat the star to about 10 million degrees kelvin. At or above such temperatures, various nuclei can be forced close enough together to bond or fuse, releasing energy and forming heavier elements. For example, hydrogen fuses into helium, and helium fuses into carbon. The sun shines due to energy created by hydrogen fusion.

Stars come in all masses, ranging from 0.08 solar mass to about 120 solar masses. All stars begin their lives by fusing hydrogen into helium in their cores. Stars with less than about 0.5 solar mass don't have enough matter to compress the helium further. Once such a star's hydrogen has been fused into helium, fusion stops and the helium star just cools off. More massive stars create enough internal pressure to compress their helium-rich cores until the temperature there is high enough to fuse helium into carbon.

Like more powerful car engines, the fusion rate and the resulting energy output of more massive stars can run rings around the output of the sun. For example, while fusing hydrogen into helium in its core, a star with 10 times the sun's mass emits 10,000 times as much energy per second as the sun. This occurs because their greater mass enables more massive stars to crush inward with more force than can lower mass stars. Greater compression yields greater heating of the core, which in turn leads to more rapid fusion than in less massive stars. While the sun will take about 10 billion years to convert its core entirely into helium, a star with three times as much mass will complete the job in just half a billion years because of the higher fusion rate in the more massive star. The sun is a Chevy Biscayne among stars. This is just as well, because the evolution of a life-supporting Earth takes more time than the entire lifetimes of more massive, Thunderbird-like stars.

When a star's hydrogen core is finally converted into helium, fusion stops there because the core temperature isn't high enough for helium fusion to begin. Helium fusion requires temperatures of at least 100 million degrees kelvin, ten times the fusion temperature for hydrogen. Therefore, the nonfusing core is compressed and heated by the pressure created by the star's ever-present gravity. At the same time, gas in a shell just above the core is being compressed to the point where the hydrogen in it begins fusing. This "shell fusion" creates more helium.

The cores of stars are their most exotic, hottest, and most active regions. Astronomers see in them a variety of elements created by fusion. Though you might think these are the elements we're made of, they're not. The key to our existence is the shells of fusion that occur outside the core. The star in which we were first leavened had many shells, as we will see.

Eventually, stellar cores become hot enough to fuse helium into carbon. For stars with masses less than about 8 solar masses, this activity is their swan song. When the cores of such low-mass stars are all carbon, fusion stops there and the core never heats up enough to fuse carbon. They slowly shed their outer layers as planetary nebulae, revealing their carbon cores, which we call white dwarfs. Because the outer layers of such low-mass stars consist primarily of hydrogen and helium, they don't have anything to do directly with Joshua's chemistry, or yours, either.

When a star starts out with more than 8 solar masses and evolves a carbon core, it does have enough gravity to compress and heat its core until it begins fusing carbon into nitrogen and oxygen. The temperature just above the carbon core is so high that carbon-core fusion is accompanied by two layers of shell fusion: The helium that was previously created in the shell just outside the core, as described above, fuses into carbon, and, likewise, the hydrogen just above this carbon-forming shell fuses into helium.

When the carbon core entirely converts into oxygen, fusion ceases there and the oxygen core compresses further by the star's gravity. This compression leads to even more fusion outside the core. Then three major shells of fusing matter exist. The inner one, rich in carbon, creates nitrogen and oxygen, the middle one, rich in helium, creates more carbon, while the outer one begins fusing hydrogen into helium. Other elements, including neon, sodium, and magnesium, are also being created simultaneously in these shells. The oxygen core eventually heats to 1 trillion degrees kelvin and also begins fusing. By now, however, time is running short for the star.

Each time the core converts into a heavier element, fewer atoms are left than were in the preceding cores. For example, after the first core fusion stage only one quarter as many helium atoms occupy the core as the number of hydrogen atoms it previously had. In the next stage only one-third as many carbon atoms exist as there were helium atoms, and so on. This decrease in the number of particles to fuse, combined with higher temperatures in the core at each successive stage, causes the time it takes to convert the core from one element to the next to drop precipitously. Consider a 25 solar mass star. It takes about 7 million years to convert its core hydrogen into helium; about 700,000 years to convert that helium to carbon; about 600 years to convert that carbon into oxygen; about a year to convert that oxygen into silicon; and about a day to convert the silicon core into iron.

In every massive star the core temperature increases each step of the way, reaching nearly 3 trillion degrees kelvin by the time iron forms. Additional layers of shell fusion also develop until the massive star's interior resembles an onion. A variety of elements exists in these shells, but certainly not all the elements that exist on Earth. For example, there is no iron, nickel, copper, zinc, selenium, silver, tin, gold, mercury, lead, or dozens of other heavy elements. Without many of them, such as iron for our blood and for chlorophyll synthesis, copper for enzyme activation and the production of hemoglobin and bone, zinc for enzyme and insulin activity, and selenium for fat metabolism, life would be completely different, if it could exist at all.

The blue-white star Rigel in Orion (his right foot, diagonally across Orion's belt from red Betelgeuse) is an onion-skinned star. The fusion in its core and numerous shells creates 700,000 times as much energy each second as does the sun. The fuel in Rigel's core is nearly gone and the star does not have much more time to shine. In cosmic terms, Orion will "shortly" lose a foot.

The end of stellar evolution for a massive star occurs because iron created in its core absorbs energy when particles fuse with it, rather than emitting energy like other elements. That, as the saying goes, changes everything. The result is catastrophic. At all previous stages, the pressure from the outer layers caused the core to compress and heat, thereby starting fusion. The energy created by fusion, primarily in the form of gamma rays, provided an outward pressure that stopped the collapse. This is the state of affairs inside the sun today and explains why it is not now collapsing.

Once the core is all iron, fusion ceases. The star's gravity compresses the core, which is another way of saying that gravity supplies the core with energy. This compressed iron quickly becomes so dense that the normal laws of physics that describe the behavior of matter on Earth no longer apply.

As the iron core collapses, many of the electrons in it fuse with protons in the iron. This process converts the protons and electrons into neutrons and particles called neutrinos. Neutrinos are also created copiously in the sun. Neutrinos interact little with normal matter like you and me. Billions of neutrinos from the sun pass through your body every second. Normal matter is nearly transparent to neutrinos just as glass is nearly transparent to visible light. However, the incredibly dense matter in a collapsing, massive star is not transparent to neutrinos. The iron dissociates into neutrons as the core collapses and the neutrinos fly outward into the fusing shells of matter that are following the core inward.

The neutron-rich core falls inward at velocities up to a quarter the speed of light. It collapses from a couple of thousand miles to about 20 miles across in a matter of seconds. Of course, the neutrons are packing closer and closer together during this time, until they reach a density so great (more than ten trillion times denser than water) that a repulsive force between neutrons called neutron degeneracy pressure turns on. This force is powerful enough to overcome the inward rush and cause most of the core to stop and rebound violently.

The rebounding core slams into the shells, as do the neutrinos emitted by it. The outward forces imparted from the expanding neutron core and the neutrinos being absorbed by the incredibly dense shells have the effect of stopping the shells from collapsing and then pushing them outward with great force. All this takes place within a matter of seconds after the core begins collapsing. The force supplied to the shells is so enormous that their expansion outward is explosive - we call it a supernova. Indeed, the energy released in this type of supernova (there is at least one other kind) is so great that the exploding star can outshine a billion normal stars in its galaxy.

Now comes the part about where the elements in our bodies originate. As you have probably surmised, a variety of relatively light elements found on Earth was produced in the shells before the core collapsed. These include helium, carbon, nitrogen, oxygen, neon, sodium, magnesium, and silicon, among others. But virtually all the iron created in the star exists now as neutrons, and dozens of elements heavier than iron have not been produced in the shells at all - yet.

The pressure exerted by the neutrinos and rebounding core heats the shells sufficiently to drive a tremendous burst of additional fusion in them. As one example, let's see how the iron in our bodies came about. As the shells undergo that burst of fusion, up to half a solar mass worth of a radioactive nickel isotope is created. This nickel decays to radioactive cobalt, which in turn decays to stable iron. Many other fusion reactions are occurring and creating the other heavy elements.

For completeness, it's worth mentioning that the neutron core stops expanding on impact with the shells. The core then recollapses. If it has between 1.4 and 3 solar masses of neutrons, neutron degeneracy pressure will stop its collapse and the core becomes a stable neutron star. If the neutron core contains more than 3 solar masses, even that force can't prevent the neutrons from overlapping, collapsing further, and creating a black hole. These are concentrations of matter so dense that their only effect on the outside comes from their gravitational attractions. Once matter or energy enters a black hole, it cannot escape. However, the details of what happens to the material entering the black hole are not yet known.

Supernova remnants are among the most spectacular objects in the galaxy. In 1987, a supernova in the Large Magallenic Cloud, a galaxy 160,000 light-years from Earth, was visible to the naked eye in the night sky of the Southern Hemisphere. Nearly 7,000 years ago a star "went supernova" in our own Milky Way Galaxy at a distance of 6,000 light-years from Earth.

The gas and dust that compose supernova remnants expand free of the core and spread out into the galaxy. Many of the radioactive isotopes in the remnant decay to stable elements, creating even more of the heavy elements that exist on Earth. Eventually enough of this gas cools or clumps together with other interstellar gas to begin recollapsing and forming a new generation of stars and planets. Earth is an especially concentrated residue of a supernova. Our planet started as gas and dust orbiting the newly forming sun. Lighter elements in our neighborhood, such as hydrogen and helium, were pushed away by the sun, leaving only the heavy stuff created in the massive star and its supernova orbiting and condensing to form the planets Mercury, Venus, Earth, and Mars. If the sun hadn't rid our region of the solar system of the light elements, then Earth would be like Jupiter and the other giants, a planet surrounded by thousands of miles of liquid hydrogen and helium.

The stardust I cradled in my arms was warm and soft. Joshua looked up at me with big blue eyes. While such times are special in and of themselves, they were made even more wonderful for me. While we may not know where we are going, at least science provides us with knowledge about where we came from.

Neil Comins, a longtime contributor to Astronomy, is the author of What if the Moon Didn't Exist? and coauthor of Discovering the Universe.

"Reproduced by permission. Copyright 2001, ASTRONOMY magazine, Kalmbach Publishing Company."

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