Our Sun is a star like no other. I have discussed the Sun’s amazing design in one published book, a forthcoming book,1 and in various articles (see resource list below). Our Sun uniquely makes possible advanced life and advanced civilization on Earth.
Despite substantial evidence for design, astronomers know there’s more to uncover. One limiting factor has been a solar physics issue; namely, accounting for elements such as carbon, nitrogen, and oxygen. The discipline of solar physics is a testament to the biblical principle that the more we learn about nature the more evidence we uncover for the supernatural handiwork of God. The primary factor preventing the discovery of yet more design features of the Sun is the solar abundance problem—in particular a discrepancy in the amount of carbon, nitrogen, and oxygen in the Sun’s interior. Thanks to results published by the Borexino Collaboration in a recent issue of Nature,2 this problem is now well on the way to resolution. (Not all readers will need the technical details. Feel free to skim, glean what you can, and pick up again at More Solar Fine-Tuning to Come.)
Solar Abundance Problem
Previously, astronomers possessed just two sets of methods for determining the relative abundances of elements in the Sun. One set is through spectroscopic measurements of the Sun, three-dimensional hydrodynamic models of the Sun’s atmosphere, and laboratory measurements of spectral line wavelengths and strengths. The second set is through helioseismology measurements and theoretical models of how the Sun’s known fusion reactions in its nuclear furnace affect the Sun’s interior structure over time.
Helioseismology is the study of the Sun’s interior structure and dynamics through detailed observations of the Sun’s oscillations. The Sun’s oscillations are akin to earthquakes. Just like terrestrial seismology provides geophysicists with insights about Earth’s interior structure, so too, helioseismology offers astronomers a window into the Sun’s interior.
The match between theoretical models of the Sun’s interior and implications of the Sun’s interior through helioseismic measurements was, until a decade ago, astonishingly good. This spectacular match persuaded astronomers they possessed an accurate picture of the Sun’s interior structure and elemental composition. This deduced elemental composition was in agreement with solar spectroscopic observations, laboratory spectral line measurements, and solar atmosphere models. Then in 2009, an update of these spectroscopic studies3 yielded a lower solar metallicity (abundance of elements heavier than helium) than the older spectroscopic results. The difference between the spectroscopic update and the older spectroscopic and current helioseismology results was especially noticeable for carbon, nitrogen, and oxygen. This disparity is known as the solar abundance problem.4
Resolving the Solar Abundance Problem
Right away astronomers set about proposing solutions to the solar abundance problem. Of five possible solutions originally proposed, only two provided the potential of sufficient modification to resolve the solar abundance problem: (1) spectroscopic analysis adjustments and (2) radiative opacity adjustments. Both the opacity of the Sun’s outer layers and the Sun’s spectra are directly impacted by the Sun’s metallicity, especially the Sun’s abundance levels of carbon, nitrogen, and oxygen.
The major contribution of the Borexino Collaboration is to provide a third, independent tool for determining the Sun’s metallicity, a tool with the potential to adjudicate between the low- and high-metallicity solar models. Borexino is a neutrino observatory located underneath Mount Gran Sasso, otherwise known as the Great Rock of Italy (see figure 1). The observatory is situated in a hollowed-out chamber measuring 100 x 20 x 18 meters (330 x 66 x 60 feet) beneath 1,400 meters (4,600 feet) of solid rock. The neutrino detector consists of a stainless steel sphere containing 2,212 very sensitive photomultipliers that surround 300 tons of ultra-pure liquid scintillator (a material that reabsorbs energy as light) where the sphere is shielded by 2,400 tons of water and a thick thermal blanket (see figure 2).
The Sun’s nuclear fusion reactions produce an enormous quantity of neutrinos. With the passing of every minute, about 16 trillion solar neutrinos pass through the thumbnails of every human being. Neutrinos interact so weakly with protons, neutrons, electrons, and photons that virtually all of them approaching Earth pass through the planet without being perturbed in any way.
Even though the neutrino interactions are extremely rare and weak, the volume of scintillator fluid and the enormous number of solar neutrinos emitted enables the Borexino Collaboration to detect nearly a hundred neutrino events per day. The challenge for any neutrino detector is to distinguish between neutrino events and signals from cosmic rays and background radioactivity. The 4,600 feet of sedimentary rock, the water shielding, and the thermal blanket effectively block out any cosmic rays and minimize background radioactivity.
The Borexino neutrino detector ranks as the most sensitive in the world. It can detect neutrinos down to an energy limit of 100 kilo-electron volts. The neutrino energy levels from stellar nuclear fusion chains range from 190 to 16,000 kilo-electron volts.
The Borexino Collaboration has been making solar neutrino observations for slightly more than a decade. During that time, the detector has measured the entire sequence of proton-proton chain nuclear reactions whereby protons are fused to form helium. These studies established that at least 99% of the Sun’s energy output is produced through sequences of nuclear fusion that convert hydrogen into helium.5
The same studies included measured fluxes of solar beryllium-7 and boron-8 neutrinos. The high-metallicity solar models and the low-metallicity models predicted different fluxes of these neutrinos. The Borexino Collaboration’s measurements favored the high-metallicity models and disfavored the low-metallicity models at a 96.6 percent confidence level.
Several years ago, the Borexino Collaboration began major upgrades to their neutrino detector with the goal of measuring the solar neutrino flux from the carbon-nitrogen-oxygen (CNO) nuclear fusion cycle. The CNO cycle is a process of nuclear fusion where stars fuse hydrogen into helium via a six-stage sequence of reactions that involve protons being fused step-by-step to carbon, nitrogen, and oxygen to produce helium. Readers can find the specific reaction details here.
More Solar Fine-Tuning to Come
In their November 26, 2020, paper the Borexino Collaboration reported on their discovery of solar neutrinos produced by the CNO cycle. Their measured flux of CNO cycle neutrinos indicated that the the CNO cycle is responsible for about 1% of the Sun’s total energy output. The cycle also yielded a direct measure of the quantities of carbon, nitrogen, and oxygen in the Sun. These quantities were not measured precisely enough to distinguish definitively between the low- and high-metallicity models. They did, nevertheless, favor the high-metallicity model and were consistent with the conclusions drawn from the beryllium-7 and boron-8 neutrino fluxes.
The Borexino results favoring the high-metallicity solar model are supported by two independent findings. In 2017, a team of eight astronomers led by Núria Vinyoles concluded that the best opacity determinations for the Sun’s radiative zone resolve the solar abundance problem for the original high-metallicity solar model but not for the low-metallicity solar model published in 2009.6 In 2018, two nuclear physicists showed that forthcoming calculations relevant to atomic spectroscopy likely will revive the high-metallicity model.7
The Borexino Collaboration is far from finished. Their goal is to measure the flux of solar CNO cycle neutrinos with sufficient precision to resolve once and for all the solar abundance problem and to uncover more of the fine-tuned features of the Sun’s nuclear furnace. The results they have produced so far combined with the two independent findings described above already go a long way toward resolving the solar abundance problem. This progress sustains the fine-tuned features of the Sun that make advanced life and advanced civilization possible. We can look forward to yet more evidence for a fine-tuned Sun that testifies of a supernatural, benevolent Creator.
Articles on Our Sun’s Uniqueness
- “It Takes a Dull Star to Have a Great Party” (July 6, 2020)
- “Our Special Star: Resolving the G-Dwarf Problem” (June 8, 2020)
- “How the Sun’s Mass Affected Earth’s History of Life” (December 31, 2018)
- “One Uniquely Smooth, Regular, and Stable Star” (April 16, 2018)
- “Bad Flare Days” (November 6, 2017)
- “Our Sun Is Still the One and Only” (April 17, 2017)
- “Middle Age Is Good, Especially for Our Sun” (October 24, 2016)
- “Long-Term Habitability Requires a Sun-Earth Twin” (August 8, 2016)
- “No Bad Flare Days for the Sun” (February 6, 2014)
- “Why Our Sun Is Spot On” (August 19, 2013)
- “Sun’s Rare Birth” (September 3, 2012)
- “Enhanced Activity in Solar-Type Stars” (February 1, 2010)
- “Rare Solar System, Rare Sun” (December 14, 2009)
- “Search for the Sun’s Twin” (March 17, 2008)
- Hugh Ross, Cosmic Interior Designs (working title), (Covina, CA: RTB Press, forthcoming), chapter 9.
- Borexino Collaboration, “Experimental Evidence of Neutrinos Produced in the CNO Fusion Cycle in the Sun,” Nature 587 (November 26, 2020): 577–82, doi:10.1038/s41586-020-2934-0.
- Martin Asplund et al., “The Chemical Composition of the Sun,” Annual Review of Astronomy and Astrophysics 47, no. 1 (September 2009): 481–522, doi:10.1146/annrev.astro.46.060407.145222.
- Aldo M. Serenelli et al., “New Solar Composition: The Problem with Solar Models Revisited,” Astrophysical Journal Letters 705, no. 2 (November 10, 2009): L123–L127, doi:10.1088/0004-637X/705/2/L123.
- Borexino Collaboration, “Comprehensive Measurement of pp-Chain Solar Neutrinos,” Nature 562, no. 7728 (October 25, 2018): 505–10, doi:10.1038/s41586-018-0624-y; Borexino Collaboration, “Neutrinos from the Primary Proton-Proton Fusion Process in the Sun,” Nature 512, no. 7515 (August 28, 2014): 383–86, doi:10.1038/nature13702.
- Núria Vinyoles et al., “A New Generation of Standard Solar Models,” Astrophysical Journal 835, no. 2 (February 1, 2017): id. 202, doi:10.3847/1538-4357/835/2/202.
- Anil K. Pradhan and Sultana N. Nahar, “Recalculation of Astrophysical Opacities: Overview, Methodology, and Atomic Calculations,” Workshop on Astrophysical Opacities, Astronomical Society of the Pacific Conference Series 515, Proceedings of the Conference held August 1–4, 2017, at University of Michigan, Kalamazoo, ed. by Claudio Mendoza, Sylvaine Turck-Chiéze, and James Colgan (San Francisco: Astronomical Society of the Pacific, 2018): 79–88, https://www.researchhub.com/paper/25520/recalculation-of-astrophysical-opacities-overview-methodology-and-atomic-calculations.