Building blocks of universe got start in the stars

Advertisement

It is awesome to imagine the principal elements making up our bodies, carbon, oxygen, nitrogen and a few others, have their origins in the incredibly hot cores of stars.

The bulk of the physical universe is composed of basic elements. Ninety-two elements are generally recognized as occurring naturally. Humans have synthesized about two dozen more. Scientists have a firm grasp of how the naturally occurring elements came to exist.

Throughout human history, people have tried to develop a means for transforming some abundant and easily obtained material into something rare and valuable, like gold. Nuclear physicists in the 20th century learned how to perform just such transmutations.

No one is going to get wealthy turning one element into another. The cost of the equipment is prohibitive and the amount of material produced is very small. But the knowledge gained in such endeavors has proven invaluable to understanding the requirements for stable atomic nuclei.

Meanwhile astronomers have been learning about the nature and life cycle of stars. Within the core of stars gravity creates incredible pressures and temperatures. Also, stars have different elemental compositions.

Stars have been observed in various stages of exploding and spewing their contents into their surroundings. The burned out hulks of stars that have exhausted their fuels are well cataloged. Additionally, astronomers see evidence that seems to suggest some black holes are the remnants of stars.

Putting all this knowledge together gives a coherent picture of what happened following the Big Bang". It explains how the heavy elements came to exist and why some elements are more abundant than others.

Throughout the universe simple hydrogen atoms are by far the most abundant. Large telescopes allow us to look at distant objects and see the universe as it was when it was very young. At these early times, we see an even greater dominance of hydrogen.

Hydrogen is the simplest of all atoms. Its nucleus has just one proton surrounded by a single electron in a cloud-like shell. More complex and massive elemental atoms have greater numbers of protons and electrons. Nuclei also have neutrons that contribute to their stability and give additional mass.

Hydrogen is the principle fuel of young stars. "Burning" hydrogen fuel is unlike burning wood, which is a chemical reaction yielding modest amounts of heat and light. Burning hydrogen in the core of a star is a much more energetic nuclear fusion reaction.

High energy collisions enable nuclei to fuse together. The composite nucleus is held together by the strong nuclear force which overwhelms the electrical repulsion of the positively charged protons. Several products are possible from these reactions but helium, with two protons and two neutrons, is most important.

The mass of the helium nucleus is less than the sum of the initial hydrogen masses. The missing mass has been turned into energy in accordance with Einstein's famous equation expressing the energy equivalence of mass.

High energy collisions arise from the tremendous pressures and temperatures in a star's core. Such conditions are brought about by gravitational forces acting to compress the star. Heat generated from gravitational compression and nuclear fusion creates a counteracting expansion. This keeps the star from collapsing.

So long as the star has enough hydrogen fuel to burn in its core, the star is in a relative state of equilibrium. As the heavier helium is created, it sinks to the star's core. Temperatures and pressures in the core are too low to "ignite" nuclear fusion of helium nuclei.

Once most of the hydrogen in the core is used hydrogen fusion moves outward away from the core. Subsequently the core cools and contracts. The burning shell of hydrogen expands and becomes brighter. This is the Red Giant phase.

Core contraction eventually causes temperatures and pressures to begin rising again. If the star is large enough, sufficient pressure will be generated to cause helium to begin fusing. For very large stars this hiccup phenomenon will cyclically repeat itself.

In each cycle, the lighter element fuel available at the beginning of the cycle becomes exhausted. Then the star's core cools and contracts. The resulting increase in core temperature and pressure ignites nuclear fusion between heavier nuclei produced in the earlier phase of the cycle.

Only a portion of the new fuel will burn in each cycle. The proportion of usable fuel becomes progressively smaller in succeeding cycles. Also, the amount of energy released in nuclear reactions between progressively more massive nuclei is less. Consequently, most of a star's nuclear energy is released in the conversion of hydrogen to helium.

At the close of each cycle the violent hiccup causes the star to lose some of its outer hydrogen envelope. Astronomers have seen stars surrounded by several expanding waves of hydrogen gas expelled from preceding hiccups.

The relative abundance of heavier elements in the universe reflects the dynamics of these successive nuclear cycles in stars. The high abundance of carbon, oxygen, neon and magnesium are a direct consequence of nuclear reactions involving helium. Nitrogen's high abundance is attributed to conversions made from carbon and oxygen.

The physics behind these processes is well understood. However, they only account for the origin of atoms up to the mass of iron. Several different mechanisms contribute to creation of more massive elements.

One, called the s-process, involves the slow capture of neutrons by iron nuclei in star cores. Other, r, rp, and p processes, occur rapidly in spectacular supernova events. Supernovas are the most energetic events witnessed by humans.

They occur after a star 5 to 10 times the mass of the Sun has gone through all the cycles leading to the production of iron. In the stable phases of each cycle, high internal temperature and pressure generated, in part, from energy released in fusion reactions keeps the star from collapsing.

In the final cycle, iron fusion actually consumes energy. Removing energy cools the core until it suddenly, violently collapses. The subsequent rebound of the implosion gives rise to the supernova explosion. More massive nuclei are created when existing nuclei are bombarded with energetic neutrons and protons spewed out in the explosion.

For several days the explosion can be brighter than an entire galaxy. What is left, depending on the mass of the original star, is a white dwarf, neutron star or black hole. Smaller stars are never hot enough to pass through all the cycles leading to production of iron. They eventually become white dwarf stars or the planetary nebula so colorfully shown in Hubble telescope photos.

I have outlined the processes that have been underway since about 380,000 years after the Big Bang. From that point, simple hydrogen has undergone a series of transmutations creating the stuff of which we and our environment are made.

Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at steveluckstead@charter.net.

Comments

Use the comment form below to begin a discussion about this content.

Sign in to comment

Click here to sign in