The Hoyle state is produced through a “triple alpha process” inside stars. Two alpha particles fuse to form a beryllium atom, and then a third alpha quickly fuses with it, creating the Hoyle state. This primordial nuclear state marks the starting point for most of the elements necessary for life. Image: Illustration: Courtesy of Forschungszentrum Jülich GmbH
Billions of years ago, all of Earth’s carbon erupted into existence inside distant, dying stars. At first, each atom’s nucleus arose in a swollen, squashed state with little chance of survival. For every 2,500 that immediately fizzled, only one shape-shifted into a stable form capable of supporting life.
That primordial, unstable nuclear state, called the Hoyle state, was discovered more than 50 years ago, but it has taken the rise of modern supercomputers and the development of new mathematical techniques to figure out just how the laws of physics cook it up. In work first detailed in May 2011 and further refined in a paper to be published this month in Physical Review Letters, a group of theoretical physicists in Germany and the United States applied the forces of physics to a computer-simulated set of subatomic particles to build the structure of the Hoyle state nucleus from scratch.
“It looks like a bent arm,” said Dean Lee, a professor of nuclear and particle physics at North Carolina State University and a study co-author.
Physicists say knowing the structure of the Hoyle state will help reveal how it gives rise to carbon, oxygen, nitrogen and other light elements that compose the complex molecules of living things. The synthesis of these elements enables the genesis of life, but it also drives the evolution of stars.
“The carbon-oxygen-nitrogen cycle is simply crucial for the formation of almost all the other elements, and for understanding how stars live and how stars decay and fade away,” said Morten Hjorth-Jensen, a professor of theoretical nuclear physics at the University of Oslo and Michigan State University, who was not involved in the research. “And, of course, without the Hoyle state we wouldn’t be here.”
The quest to unravel the Hoyle state started in 1954 with what the astrophysics writer Marcus Chown has called “the most outrageous prediction” ever made in science. The theoretical astrophysicist Fred Hoyle reasoned that his own existence meant that an unknown, exotic state of the carbon atom with about 7.65 million electron volts of extra energy must arise inside dying stars, even though no one had ever detected spectral emissions from such an atom.
“Hoyle postulated that this 7.65 MeV carbon had to exist in order for there to be life,” Hjorth-Jensen said. “Then, four or five years later, an experimental group in Caltech actually found this Hoyle state in emissions.”
Just as predicted, almost all the key elements of life descend from that fleeting form of carbon. When midsize stars like our sun run low on hydrogen to fuse into helium, their outer layers expand and redden, and their cores shrink. During this inner contraction, helium nuclei (also called alpha particles), each containing two protons and two neutrons, are thrust together so forcefully that they fuse, forming a four-proton, four-neutron atomic nucleus called beryllium-8. In the ten-thousandth of a trillionth of a second before the beryllium decays back into two alpha particles, a third alpha particle sometimes smacks into the beryllium, fusing with it to form an excited, plus-size carbon-12 nucleus: the Hoyle state. In addition to carbon’s usual six protons and six neutrons, this state packs an extra bundle of energy.