Some stars have the element neon to thank for their ultimate, explosive demise, according to astrophysical research.
Astronomers love studying the life cycles of stars, including which stars die in which ways. Less-massive stars, like our Sun, expand and shed their layers as they transition into white dwarfs, while much bigger stars explode in violent supernovae, and their cores turn into black holes or neutron stars. But questions surround the ending of intermediate-size stars that have between seven and 11 times the mass of the Sun. Do they shed their layers or explode? And if they do go supernova, what’s the end product? Understanding these stars in part relies on understanding the behaviour of the element neon.
As dying, intermediate-mass stars burn through their hydrogen and resulting helium, simulations have shown that they could form cores made of the elements oxygen, neon, and magnesium. These stars might either lose some of their outer hydrogen envelope and become dim white dwarf stars, or, if the core becomes large enough, collapse into a neutron star.
But these cores are weird objects, where the pressure from the crushing gravity is offset by the quantum mechanical rules that govern electrons. Two electrons cannot share the exact same quantum properties, limiting how closely you can squish them together and exerting a “degeneracy pressure” on the core. Crucial to the evolution of this process is the rate that neon atoms in the core capture electrons. This process releases energy that can set off the oxygen in the star, creating an explosion. But when the energy release and subsequent explosion happen can change the star’s fate.
One recent paper, led by Oliver Kirsebom of Dalhousie University in Canada, studied the reverse of the neon electron-capture process, in which a fluorine atom spits out an electron and becomes neon. They did this by slamming a carbon foil with a fluorine beam from the JYFL Accelerator Laboratory in Finland. By determining the odds that fluorine decayed to neon, they could calculate the reverse: how often neon would capture an electron in the oxygen-neon-magnesium core. The rate they calculated was much higher than previous measurements, setting off the oxygen while the core was at a lower density and resulting in a thermonuclear explosion and a white dwarf, rather than a neutron star.
“What’s remarkable is that it’s a singular nuclear transition, and a very rare transition that you normally neglect,” Kirsebom told Gizmodo. “Under the specific conditions in these stars, it could have a profound effect on the evolution.”
The team’s measurements were “a milestone in precision nuclear astrophysics,” Carla Frohlich of the Department of Physics at North Carolina State University (who was not involved in the research) wrote in a Physics viewpoint. She wrote that the results cap a decades-long search to measure this “forbidden transition,” a kind of atomic process rare on Earth but perhaps more common in the extreme cores of stars.
In another study, published in the Astrophysical Journal and led by Stockholm University postdoctoral research Shuai Zha, scientists built a computer model of the death of a star 8.4 times the Sun’s mass. The energy released from the electron capture causes an ignition of oxygen, which burns off the core’s other metals and sets off an explosive wave. The paper found that the fate relied on the number of electrons and the value of a critical density, above which the core collapses into a neutron star and below which it rips itself apart in a thermonuclear explosion.
The researchers’ estimates of the core’s density are higher than the critical density, and therefore, they think that neon precipitates the collapse of the cores into neutron stars. However, their work predated Kirsebom’s, and they plan to make comparisons in a forthcoming paper.
Kirsebom explained to Gizmodo that there are still open questions about these stars. Most notably, a major uncertainty surrounds scientists’ theoretical understanding of convection in stellar cores, or how moving matter transports heat around. There are other difficult-to-study nuclear processes that probably play a role as well.
“It’s fair to say that there are conflicting opinions on the final fate of these stars and that better understanding of especially convection... is needed to make progress,” he told Gizmodo. He hopes that better accelerator laboratories will help scientists study ever more unstable exotic particles and isotopes. Additionally, astronomical studies might reveal the presence of white dwarfs with more heavy elements than expected. These might be a physical remnant of an oxygen-neon-magnesium core blowing up.
Featured image: NASA and ESA; Acknowledgment: J. Hester (ASU) and M. Weisskopf (NASA/MSFC)