Chapter 13: Star Birth and Death

The Periodic Table of Elements

The major subplot in the life stories of stars is the creation of elements. Main sequence stars like the Sun create helium from hydrogen. More massive stars can also make carbon by the triple-alpha process. However, there are many elements in the periodic table heavier than carbon. Are these created in stars or were they present at the birth of the universe? How can we explain the existence and the vastly varying cosmic abundance of different chemical elements? It is the result of the abilities of stars of different masses, and at different stages in their evolution, to create elements up the periodic table.

 

William Fowler

Sir Fred Hoyle

The question of the origin of the elements was taken up in the 1950s by four young astrophysicists working at the California Institute of Technology. Three of them were from England — the husband-and-wife team of Geoffrey and Margaret Burbidge, and the iconoclastic Fred Hoyle. The fourth was William Fowler, a genial expert in nuclear physics (with Scottish roots). In a massive paper in 1957, they succeeded in explaining most aspects of the cosmic abundance of elements. They wrote: "the problem of element synthesis is closely allied to the problem of stellar evolution." All the members of the team had stellar careers. The Burbidges each became directors of a major observatory, Hoyle was knighted, and Fowler received the Nobel Prize. Their paper told the story of how stars had created the materials of the everyday world — the calcium in our bones, the nitrogen and oxygen in the air we breathe, the metals in the cars we drive, and the silicon in our computers.

Extreme temperatures are required to fuse heavy elements. Recall that the core of a giant star is contracting. As the core contracts and gets hotter, it can initiate reactions involving even heavier elements. The elements synthesized depend on the mass of the star. Imagine a set of massive red giants with different masses: 4, 6, 8, 10 solar masses, and so on. Those of about 4 or 6 solar masses will have helium-rich cores hot enough to ignite the helium nuclei and fuse them into carbon in the triple-alpha (helium nuclei are also called alpha particles) process just described. Those of around 8 solar masses will have hot enough cores to further ignite the carbon and fuse it into heavier elements such as oxygen, neon, and magnesium.

The reactions in massive stars where helium nuclei are fused to heavier nuclei include:

₁₂C + ₄He → ₁₆O + photon

₁₆O + ₄He → ₂₀Ne + photon

These reactions require temperatures above 500 million K. Heavy nuclei can also fuse with each other, although in this case, the electrical repulsion between protons in the nuclei is stronger, and a temperature above 1 billion K is needed for the reactions to proceed:

Carbon Fusion:

₁₂C + ₁₂C → ₂₄Mg + photon

Oxygen Fusion:

Cutaway diagram of massive star pre-collapse. Based on a tracing of Public Domain NASA diagram Nucleosynthesis in a star.gif color key version)

₁₆O + ₁₆O → ₃₂S + photon

 

The final stage of heavy element creation in main sequence stars, which happens only in stars whose cores reach temperatures of a few billion K, is called Silicon Fusion. Although the overall result of this is to turn silicon and sulfur into iron, it proceeds in a very different way from previous burning stages. The silicon made by oxygen burning is "melted" down by the extreme temperatures in the core to He, neutrons, and protons. These particles then rearrange themselves by hundreds of different reactions into elements like ₅₆Fe, an iron nucleus.

If a star is massive enough, the core-building process leads to a core consisting of shells with increasingly heavier elements, surrounding an inner core of iron. We are used to thinking of iron as a solid metal, but the state of iron at the center of a massive evolved star is quite different. Stellar iron is actually denser than terrestrial iron, but the electrons have been stripped from all the atoms and the nuclei are not held together in a lattice. Fantastic pressure keeps the iron as a high-temperature gas. The shell structure in these stars can be so complex that there can be different fusion reactions happening on the surfaces of different shells all at the same time.

Estimated abundances of the chemical elements in the Solar system. Hydrogen and helium are most common, from the Big Bang. The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers, and (2) a general decrease in abundance, as elements become heavier.

With iron, the fusion process reaches an insurmountable obstacle. Iron has the most stable nuclear configuration of any element. This means that energy is consumed, not produced, as iron nuclei fuse into heavier elements. This accounts for the steep fall in abundance of the elements heavier than iron. Thus the iron nuclei in stars do not continue to ignite and fuse as the core contracts and gets hotter. The heart of a star is like an iron tomb that traps matter and releases no energy to counter the continuing collapse.

 

But what about the elements heavier than iron? If even the most massive stars can only fuse elements up to iron, where do the rest of the elements (like the gold and platinum in jewelry and the uranium that is involved in terrestrial nuclear power) come from? In addition to the nuclear fusion that stars use as their main source of power, there are side processes that go on, in shell burning in giant stars and in the supernova explosion process, that make these elements heavier than iron. Although they are rare, their existence requires us to explain their production. This gives astronomers yet another test of their stellar models.

Observations, combined with computer models, have illuminated two ways that elements heavier than iron are created. In the r-process, neutrons are rapidly captured by iron nuclei and nuclei of similar atomic numbers. Since neutrons have no electric charge, they can easily insert themselves into an atomic nucleus given sufficient temperature and pressure. The r-process occurs with blinding speed in the blast wave that results from the death of a massive star, an event called a supernova. The s-process is a slower neutron capture process that occurs in evolved stars. Whereas the r-process takes seconds, the s-process takes thousands of years. Slow neutron capture happens after a star from 0.6 to 10 times the mass of the Sun leaves the main sequence, in a stage of evolution called the asymptotic giant branch. Until a few years ago it was thought that half of the heavy elements beyond iron are produced by each process.

However, our understanding of heavy element production was scrambled by the observation of neutron star collisions by the Laser Interferometer Gravitational-Wave Observatory (LIGO). Observations and computer simulations suggested that the cataclysm of neutron stars colliding could be a prodigious source of heavy elements, all the way up to gold and platinum. Meanwhile, clues had been accumulating that supernovae were not as efficient factories for heavy elements as had been thought. LIGO had been detecting black hole collisions, but in August 2017 it saw an event that was different. It had the hallmarks of a neutron star collision and it was followed by a burst of gamma rays with an afterglow that was caught by ground-based telescopes. Astronomers saw a radioactive waste cloud that started off the size of a small city but, moving at a few tenths of light speed, it grew in a day to the size of the Solar System. They estimated that the neutron star collision created 200 Earth masses of pure gold and 500 Earth masses of pure platinum! The field is still in flux, but it now appears that neutron star collisions generate most of the heavy elements in the universe.

Stars are like factories whose main business is the creation of heavy elements from lighter ones. Remember that main sequence stars release less than a percent of the mass of a hydrogen atom as pure energy when they make helium. The same is true of the nuclear reactions at later stages of a star's life. So the heat and light we see from a star are just minor by-products of the fusion process, just as the heat and light we see from a factory do not reflect its primary purpose of making large objects from smaller components. Stars are the "factories" of the matter we see around us. In order to return these newly created elements to interstellar space, stars need to eject them via mass loss - winds, planetary nebulae, and supernovae. The dense cores of these objects lock up material that will never be incorporated into the gas that makes a new generation of stars. White dwarfs hold on to their helium, carbon, and oxygen, and neutron stars keep the material in iron cores from ever returning to interstellar space. Black holes are even more enigmatic; their heavy elements are trapped within an event horizon which stops any information about that material from reaching us.