Journey Work of the Stars: Origin of the Chemical Elements

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Stars manufactured the elements necessary for life on Earth. Image by: NASA

I believe a leaf of grass is no less than the journey-work of the stars.”
Walt Whitman in Leaves of Grass

Sometimes poets get it before scientists do. Walt Whitman was probably speaking poetically when he wrote this line in the nineteenth century.

If taken literally and out of context, however, Whitman got it exactly right long before astronomers realized the connection between stars and the atoms around us.

How do stars make the atoms of the various elements we see around us?

The Big Bang: In the Beginning

During the Big Bang, matter initially formed as individual protons so that the only element present was hydrogen. Elements other than hydrogen consist of atomic nuclei containing multiple protons.

To form heavier elements protons combine via nuclear fusion. As the universe expanded and cooled, it soon reached temperatures and densities comparable to the cores of stars. Under these conditions, hydrogen fusion reactions convert hydrogen into helium, releasing energy, both in the early universe and in stellar cores.

Nuclear fusion reactions converting helium into heavier elements, such as carbon, require higher temperatures and densities than those required for hydrogen fusion reactions. By the time the universe contained enough helium for helium atoms to fuse into carbon atoms, the universe was too cool and low density for helium fusion reactions to occur.

Hence the Big Bang could not form significant quantities of elements heavier than helium. Shortly after the Big Bang the universe consisted of about 75% hydrogen, about 25% helium, and trace amounts of lithium and beryllium. No other elements formed at this time.

How then did the other elements originate? Stars are key.

Stellar Nucleosynthesis

Stars generate their energy by nuclear fusion reactions. Initially stars convert hydrogen to helium in their cores using reactions similar to the reactions that manufactured helium shortly after the Big Bang. All fuel sources must, however, eventually run out.

Stars exhaust their hydrogen fuel leaving a helium core, but the stars don’t die out just yet. The helium core collapses gravitationally and increases its temperature, density, and pressure until helium burning fusion reactions ignite. Two helium atoms fuse to form a beryllium atom. A third helium atom quickly joins to form a carbon atom. Occasionally a fourth helium atom combines with the carbon to form an oxygen atom. These fusion reactions deep in stellar cores manufactured the carbon and oxygen atoms in our bodies long before the solar system formed.

Stars having about the same mass as the Sun do not have enough gravitational force to compress the carbon/oxygen core sufficiently to ignite carbon fusion reactions. In these low mass stars, stellar nucleosynthesis stops with the carbon/oxygen core and the dying stars collapse into white dwarf stars. As the remaining heat dissipates over several billion years, low mass stars “go gentle into that good night.” (Dylan Thomas)

Just before a massive star explodes as a supernova, nuclear fusion reactions form an iron core. Image by: NASA

Massive Stars

Massive stars, however, “rage against the dying of the light.” (Dylan Thomas)

Dying violently, they fuse and recycle heavier elements. Fusion reactions involving carbon manufacture heavier elements including oxygen, neon, sodium, and magnesium. These atoms in turn fuse into even heavier elements up to iron and elements near iron on the periodic table.

When massive stars have iron cores, however, the fusion reactions stop. Iron does not fuse into heavier elements because fusion reactions manufacturing elements heavier than iron do not release energy. They require energy. Iron is therefore the boundary between fission and fusion. How do stars make elements heavier than iron?

Stars manufacture elements slightly heavier than iron on the periodic table via the s (slow) process. In the s process, a neutron collides into an iron or other heavy atomic nucleus. The neutron then decays into a proton and an electron, in a process physicists call beta decay. The electron escapes the nucleus. The proton stays in the nucleus transforming the atom into the next heavier element on the periodic table. Repeating this process slowly manufactures elements up to bismuth on the periodic table.

The Role of Supernovas

Stars rapidly manufacture all the elements heavier than bismuth by the r (rapid) process during supernova explosions. A type II supernova occurs when the iron core of a massive star collapses gravitationally because the nuclear reactions that helped generate the outward pressure cease. The core then rebounds like a ball falling to the ground and bouncing back. The rebound triggers a type II supernova explosion, which releases approximately as much energy in about a year as our Sun releases in its entire 10 billion year lifespan.

The supernova also creates very large numbers of neutrons which slam into atomic nuclei and transform them into the next heavier elements by decaying into protons. In this manner massive stars manufacture all elements on the periodic table, including those heavier than bismuth.

The Crab Nebula is the remnant from a supernova explosion that occurred in 1054. Image by: NASA, HST

Supernovas also simultaneously recycle the heavy atoms they manufacture. If the heavy elements that stars manufacture remain trapped in stellar cores, they don’t do the universe much good. Supernovas blast atoms heavier than hydrogen and helium back into interstellar space. These atoms then recycle into the nebulas that form the next generation of stars. Planets, such as Earth, orbiting second and third generation stars therefore contain the heavy elements required for life but not manufactured in the Big Bang.

Stars: Cosmic Cauldrons

Stars are the cosmic cauldrons whose fires forge all the elements beyond hydrogen and helium, including those in our bodies. Rather than slipping gentle into that good night, massive stars die violently. Raging against the dying of the light, they sow seeds for life in the universe. We are all the journey-work of the stars.

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