We Are Made of Stardust

Throughout most of human history, people considered themselves and the planet they lived on as being apart from the rest of the universe. After all, the heavens were clearly unreachable and therefore had to remain the abode of the deities found in the numerous mythologies that enriched ancient civilizations. It is only with the rise of modern astronomy, exobiology, and space technology that scientists have been able to investigate properly the chemical evolution of the universe. And the results are nothing short of amazing.

While songwriters and poets often suggest that a loved one is made of stardust, modern scientists have shown us that this is not just a fanciful artistic expression; it is quite literally true. All of us are made of stardust! Thanks to a variety of astrophysical phenomena, including ancient stellar explosions that took place long before the solar system formed, the chemical elements enriching our world and supporting life came from the stars. This section provides a brief introduction to the cosmic connection of the chemical elements.

The chemical elements, such as carbon (C), oxygen (O), and calcium (Ca), are all around us and are part of us. Furthermore, the composition of planet Earth and the chemical processes that govern life within our planet's biosphere are rooted in these chemical elements. To acknowledge the relationship between the chemical elements and life, scientists have given a special name to the group of chemical elements that they consider to be essential for all living systems—whether here on Earth, possibly elsewhere in the solar system, or perhaps on habitable planets around other stars. As previously mentioned, scientists refer to this special group of life-sustaining chemical elements as the biogenic elements.

Biologists focus their studies on life as it occurs on Earth in its numerous, greatly varied, and interesting forms. Exobiologists extend basic concepts about carbon-based life here on Earth to create their scientifically based speculations about the possible characteristics of life beyond the terrestrial biosphere. When considering the biogenic elements, scientists usually place primary emphasis on the elements hydrogen (H), carbon, nitrogen (N), oxygen, sulfur (S), and phosphorous (P). The chemical compounds of major interest are those normally associated with water (H2O) and with other organic chemicals in which carbon bonds with itself or with other biogenic elements. There are also several "life-essential" inorganic chemical elements, including iron (Fe), magnesium (Mg), calcium, sodium (Na), potassium (K), and chlorine (Cl).

All the natural chemical elements found here on Earth and elsewhere in the universe have their ultimate origins in cosmic events. Since different elements come from different events, the elements that make up life itself reflect a variety of astrophysical phenomena that have taken place in the universe. For example, the hydrogen found both in water and hydrocarbon molecules formed just a few moments after the big bang event that started the universe. Carbon, the element that is considered to be the basis for all terrestrial life, formed in small stars. Elements such as calcium and iron formed in the interiors of large stars. Heavier elements with atomic numbers beyond iron, such as silver (Ag) and gold (Au), formed in the tremendous explosive releases of supernovae. Certain light elements, such as lithium (Li), beryllium (Be), and boron (B), resulted from energetic cosmic-ray interactions with other atoms, including the hydrogen or helium nuclei that are found in interstellar space.

Following the big bang explosion, the early universe contained the primordial mixture of energy and matter that evolved into all the forms of energy and matter that scientists observe in the universe today. For example, about 100 seconds after the big bang, the temperature of this expanding mixture of matter and energy fell to approximately 1 billion kelvins—"cool" enough so that neutrons and protons began to stick to each other during certain collisions and form light nuclei, such as deuterium and lithium. When the universe was about three minutes old, 95 percent of the nuclei were hydrogen, 5 percent were helium, and there were only trace amounts of lithium. At the time, the nuclei of these three light elements were the only ones that existed.

As the universe continued to expand and cool, the early atoms (mostly hydrogen and a small amount of helium) began to form through the capture of electrons and then gather through gravitational attraction into very large clouds of gas. For millions of years these giant gas clouds were the only matter in the universe because neither stars nor planets had yet formed. Then, about 200 million years after the big bang, the first stars began to shine, and the creation of important new chemical elements started in their thermonuclear furnaces.

Stars form when giant clouds of mostly hydrogen gas, perhaps light-years across, begin to contract under the attractive force of their own gravity. During millions of years, various clumps of hydrogen gas would eventually collect into a giant ball of gas that was hundreds of thousands of times more massive than Earth. As the giant gas ball continued to contract under its own gravitational influence, an enormous pressure arose in its interior. Consistent with the laws of physics, the increase in pressure at the center of this "protostar" was accompanied by an increase in temperature. Then, when the center reached a minimum temperature of about 15 million kelvins, the hydrogen nuclei in the center of the contracting gas ball moved fast enough so that when they collided, these light (low-mass) atomic nuclei would undergo fusion. This is the very special moment when a new star is born.

The process of nuclear fusion releases a great amount of energy at the center of the star. Once thermonuclear burning begins in a star's core, the internal energy release counteracts the continued contraction of stellar mass by gravity. The ball of gas becomes stable—as the inward pull of gravity exactly balances the outward radiant pressure from thermonuclear fusion reactions in the core. Ultimately, the energy released in fusion flows upward to the star's outer surface, and the new star "shines." It is this continuous, radiant outflow of energy from a parent star that provides the energy necessary to sustain life on any habitable planets that may orbit the star.

Stars come in a variety of sizes, ranging from about one-tenth to 60 (or more) times the mass of our parent star, the Sun. It was not until the mid-1930s that astrophysicists began to recognize how the process of nuclear fusion takes place in the interiors of all normal stars and fuels their enormous radiant energy outputs. Scientists use the term nucleosynthesis to describe the complex process of how different size stars create different elements through nuclear fusion reactions.

Astrophysicists and astronomers consider stars that are less than about five times the mass of the Sun to be medium- and small-size stars. The production of elements in stars within this mass range is similar. Small-and medium-size stars also share a similar fate at the end of life. At birth, small stars begin their stellar life by fusing hydrogen into helium in their cores. This process generally continues for billions of years, until there is no longer enough hydrogen in a particular stellar core to fuse into helium. Once hydrogen burning stops, so does the release of the thermonuclear energy that produced the radiant pressure, which counteracted the relentless inward attraction of gravity. At this point in its life, a small star begins to collapse inward. Gravitational contraction causes an increase in temperature and pressure. As a consequence, any hydrogen remaining in the star's middle layers soon becomes hot enough to undergo thermonuclear fusion into helium in a "shell" around the dying star's core. The release of fusion energy in this shell enlarges the star's outer layers, causing the star to expand far beyond its previous dimensions. This expansion process cools the outer layers of the star, transforming them from brilliant white hot or bright yellow in color to a shade of dull glowing red. Quite understandably, astronomers call a star at this point in its life cycle a red giant.

Gravitational attraction continues to make the small star collapse, until the pressure in its core reaches a temperature of about 100 million kelvins. This very high temperature is sufficient to allow the thermonuclear fusion of helium into carbon. The fusion of helium into carbon now releases enough energy to prevent further gravitational collapse—at least until the helium runs out. This stepwise process continues until oxygen is fused. When there is no more material to fuse at the progressively increasing high temperature conditions within the collapsing core, gravity again exerts it relentless attractive influence on matter. This time, however, the heat released during gravitational collapse causes the outer layers of the small star to blow off, creating an expanding symmetrical cloud of mate rial that astronomers call a planetary nebula. The expanding cloud may contain as much as 10 percent of the small- or medium-size star's mass. The explosive blow-off process is very important because it disperses into space the elements created in the small star's core by nucleosynthesis.

The final collapse that causes the small star to eject a planetary nebula also liberates thermal energy. But this time, the energy release is not enough to fuse other elements, so the remaining core material continues to collapse until all the atoms are crushed together and only the repulsive force between the electrons counteracts gravity's relentless pull. Astronomers refer to this type of condensed matter as a degenerate star and give the final compact object a special name—the white dwarf star. The white dwarf star represents the final phase in the evolution of most low mass stars, including the Sun.

If the white dwarf star is a member of a binary star system, its intense gravity might pull some gas away from the outer regions of the companion (normal) star. When this happens, the intense gravity of the white dwarf causes the inflowing new gas to reach rapidly very high temperatures, and a sudden explosion occurs. Astronomers call this event a nova. The nova explosion can make a white dwarf appear up to 10,000 times brighter for a short period of time. Thermonuclear fusion reactions that take place during the nova explosion also create new elements, such as carbon, oxygen, nitrogen, and neon. These elements are then dispersed into space.

In some very rare cases, a white dwarf might undergo a gigantic explosion that astrophysicists call a Type Ia supernova. This happens when a white dwarf is part of a binary star system and pulls too much matter from its stellar companion. Suddenly, the compact star can no longer support the additional mass, and even the repulsive pressure of electrons in crushed atoms can no longer prevent further gravitational collapse. This new wave of gravitational collapse heats the helium and carbon nuclei in a white dwarf and causes them to fuse into nickel, cobalt, and iron. However, the thermonuclear burning now occurs so fast that the white dwarf completely explodes. During this rare occurrence, nothing is left behind. All the elements created by nucleosynthesis during the lifetime of the small star now scatter into space as a result of this spectacular supernova detonation.

Large stars have more than five times the mass of the Sun. These stars begin their lives in pretty much the same way as small stars—by fusing hydrogen into helium. However, because of their size, large stars burn faster and hotter, generally fusing all the hydrogen in their cores into helium in less than one billion years. Once the hydrogen in the large star's core is fused into helium, it becomes a red supergiant—a stellar object similar to the red giant star previously mentioned, only much larger.

However, unlike a red giant, the enormous red supergiant star has enough mass to produce much higher core temperatures as a result of gravitational contraction. A red supergiant fuses helium into carbon, carbon and helium into oxygen, and even two carbon nuclei into magnesium. Thus, through a combination of intricate nucleosynthesis reactions, the supergi-ant star forms progressively heavier elements up to and including the element iron. Astrophysicists suggest that the red supergiant has an onionlike structure—with different elements being fused at different temperatures in layers around the core. The process of convection brings these elements from the star's interior to near its surface, where strong stellar winds then disperse them into space.

Thermonuclear fusion continues in a red supergiant star until the element iron is formed. Iron is the most stable of all the elements. Elements lighter than (below) iron on the periodic table generally emit energy when joined or fused in thermonuclear reactions, while elements heavier than (above) iron on the periodic table emit energy only when their nuclei split or fission. So from where did the elements that are more massive than iron come? Astrophysicists postulate that neutron capture is one way in which the more massive elements form. Neutron capture occurs when a free neutron (one outside its parent atomic nucleus) collides with another atomic nucleus and "sticks." This capture process changes the nature of the compound nucleus, which is often radioactive and undergoes decay, thereby creating a different element with a new atomic number.

While neutron capture can take place in the interior of a star, it is during a supernova explosion that many of the heavier elements, such as iodine, xenon, gold, and the majority of the naturally occurring radioactive elements, are formed by numerous, rapid neutron capture reactions.

What happens when a large (greater than five solar masses) star goes supernova? The red supergiant eventually produces the element iron in its intensely hot core. However, because of nuclear stability phenomena, the element iron is the last chemical element formed in nucleosynthesis. When fusion begins to fill the core of a red supergiant star with iron, thermonuclear energy release in the large star's interior decreases. Because of this decline, the star no longer has the internal radiant pressure to resist the attractive force of gravity. And so the red supergiant begins to collapse. Suddenly, this gravitational collapse causes the core temperature to rise to more than 100 billion kelvins, smashing the electrons and protons in each iron atom together to form neutrons. The force of gravity now draws this massive collection of neutrons incredibly close together. For about a second, the neutrons fall very fast toward the center of the star. Then they smash into each other and suddenly stop. This sudden stop causes the neutrons to recoil violently, and an explosive shock wave travels outward from the highly compressed core. As this shock wave travels from the core, it heats and accelerates the outer layers of material of the red supergiant star. The traveling shock wave causes the majority of the large star's mass to be blown off into space. Astrophysicists call this enormous explosion a Type II supernova.

A supernova will often release (for a brief moment) enough energy to outshine an entire galaxy. Since supernovae explosions scatter elements made within red supergiant stars far out into space, they are one of the most important ways the chemical elements disperse in the universe. Just before the outer material is driven off into space, the tremendous force of the supernova explosion provides nuclear conditions that support rapid capture of neutrons. Rapid neutron capture reactions transform elements in the outer layers of the red supergiant star into radioactive isotopes that decay into elements heavier than iron.

This section could only provide a very brief glimpse of our cosmic connection to the chemical elements. But the next time you look up at the stars on a clear night, just remember that you, all other persons, and everything in our beautiful home planet is made of stardust.

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