Solar evolution

The Sun, the closest representative of the stars that populate the universe, has been a reliable benchmark for testing theories of stellar astrophysics for centuries. Much has been learned about the Sun’s formation, present status, and future evolution from the developments of modern physics, their powerful theoretical models, and a wealth of observations of both this star and others like it.

Overview

The Sun was born a little over 4.5 billion years ago from a ten-kelvin interstellar cloud, roughly 10¹⁴ kilometers across, of cold atomic and molecular gas. Triggered by an event like a nearby supernova, supersonic turbulence in the cloud caused a compressed region to collapse under its own gravity, fragmenting into smaller (on the order of 10¹² kilometers) pieces on a timescale of about two million years. These clumps flattened into disks from their angular momenta, feeding the disk centers as they collapsed. In the disk that would become our solar system, the center accumulated one to two solar masses after an additional 10⁴ years and became opaque to its own radiation. Consequently, the central temperature rose to about 10,000 kelvins, and the collapsing mass became a protostar within another 10⁵ years.

This protostar, about the size of Mercury’s orbit and several times the Sun’s current luminosity, contracted further over the next million years. In the process, the early Sun entered the T-Tauri stage of evolution, exhibiting strong protostellar winds and bipolar outflows of jets that became less and less collimated as the surrounding disk flattened and dissipated. These jets’ compositions concordantly evolved from being primarily molecular to atomic as the young Sun’s temperature rose. This activity subsided as the Sun’s protostellar evolution slowed over the next 10⁷ years, and gravity struggled to compress the hot, ionized stellar material further. When the core reached a temperature of ten million kelvins, fusion of hydrogen into helium through the proton-proton chain was possible.

Following a thirty-million-year period of slight contraction, the Sun settled into its twenty-first-century state on the main sequence of stellar evolution, with a radius of 6.96 10⁵ kilometers, a luminosity of 3.83 10³⁶ watts, and surface and central temperatures of 5,780 kelvins and 15 10⁶ kelvins, respectively. According to computer models and meteoritic evidence, 4.5 billion years have elapsed since that time. Early in its main sequence history, the Sun increased its luminosity by about 30 percent as the core temperature and fusion reaction rates rose with the increasing mean atomic weight of the nuclear end products deposited in the interior.

The Sun is expected to remain on the main sequence for another 5.5 billion years, maintaining its normal solar cycles and associated magnetic activity. The Sun’s luminosity will also continue to increase slowly as more helium “ash” settles in the core. After a total lifetime of ten billion years on the main sequence, the Sun’s core will be composed of enough helium to shut down hydrogen fusion at its center. Though fusion will still occur in a shell surrounding the depleted core center, hydrodynamic equilibrium will no longer be maintained, and the core will begin to collapse under its own gravity. The subsequent release of gravitational energy will heat the hydrogen-burning shell, increasing the nuclear reaction rates, which, in turn, will increase the gas pressure on the surrounding solar layers.

Arriving at the subgiant stage in its evolution, the Sun’s surface temperature will fall to about 4,000 kelvins, and the solar envelope will expand to a radius three times its current size. The Sun will spend 10⁸ years in this stage before becoming a red giant star, 100 solar radii in size—about the size of Mercury’s orbit—and several hundred times its present luminosity. The surface temperature will not change appreciably during this time, however.

About 10⁵ years afterward, the core will have contracted to the point where the central temperature will reach 100 million kelvins, and helium fusion can occur through the triple-alpha process. The central density of this compressed core will be an extremely high 10⁸ kilograms/meter³, and electron degeneracy pressure will stabilize the core against further collapse. Since a degenerate core is largely insensitive to the additional energy generated by helium fusion, the core will not expand as it is heated, and a runaway “helium flash” will ensue for several hours. The tremendous amount of energy dumped into the core will then heat it to the point where thermal pressure can take over, expanding the core and establishing a new balance with gravity for the next 10⁵ years.

The triple alpha process will then proceed at a steady rate in the core, surrounded by a lower-temperature hydrogen-fusing shell. The Sun will reach the horizontal branch of stellar evolution with a slightly higher surface temperature. The solar envelope will also shrink back to ten main sequence solar radii. When the core’s helium fuel is exhausted, it will be left as carbon ash, surrounded by concentric shells of helium-fusing, then hydrogen-fusing, layers. As before, the core will contract until electron degeneracy pressure dominates over thermal pressure, reaching a central density of 10⁸ kilograms/meter³ and a temperature of 250 million kelvins.

At this red supergiant stage, the central temperature is insufficient to fuse carbon, but the compression will drive the surrounding helium- and hydrogen-burning shells to higher temperatures and luminosities. This will cause the outer layers to expand to 500 solar radii, or about the size of Earth’s orbit, cooling to a surface temperature of 4,000 kelvins. The exact size of the Sun at this point is unknown, depending on the severity of mass loss from winds ejected in the red giant phase. The Sun will last a relatively short 10⁴ years in this stage. Shell helium burning will happen in a series of violent spurts, causing the solar envelope to fluctuate in size. Additionally, photons produced by electron-nuclei recombinations in the envelope will push the layers out farther with each expansion phase. Eventually, the outer layers will be ejected as a planetary nebula, enriching the surrounding Interstellar medium and leaving behind the Sun’s compact carbon core. As a white dwarf, this Earth-sized object will cool in a leisurely manner over many billions of years to become a black dwarf at a temperature very nearly that of absolute zero.

Knowledge Gained

Our understanding of the Sun’s history is gleaned mostly from theoretical models. In 1644, René Descartes proposed the theory of vortices, roughly outlining solar genesis from infalling swirling gas. Later, Emanuel Swedenborg’s 1734 nebular hypothesis postulated that the Sun was formed by a rotating nebula, an idea further explained in Immanuel Kant and Pierre-Simon Laplace’s independently formulated nebular hypotheses. In 1755 and 1796, respectively, they invoked the conservation of angular momentum to picture a collapsing cloud rotating and contracting into a protostellar disk.

In the early twentieth century, James Jeans established the physical criteria governing hydrodynamic equilibrium and the conditions necessary for a cloud to collapse. Models of star formation favor supersonic turbulence in the parent interstellar cloud, possibly from the shocks of a nearby supernova, as the spark necessary to trigger gravitational collapse. This idea was originally posited by Carl von Weizsäcker in 1944 and Dirk ter Haar in 1950, and it resurfaced in the 1990s with the advent of modern computational power. While still a vaguely understood subject, it is theorized that turbulence was responsible for defining the structure and evolution of the presolar molecular clouds, providing the high compression and transport of angular momentum required for gravity to induce further collapse. The actual isothermal collapse was investigated as a simple case by Richard Larson and Michael Penston in 1969 and also by Frank Shu, who explored the inside-out collapse model that produces protostars. More rigorous investigations of star formation have since been conducted, with thought given to the roles of complex magnetic effects, turbulent viscosities, chemical compositions, and rotation.

Observationally, astronomers have compared these predictions with Sun-like stars in various stages of development. Radio observations of the M20 Nebula provide images of many stages of stellar evolution, from the parent cloud, fragmentation, and collapse to emission nebulae lit by the first generation of high-mass stars. The 1970s and 1980s saw the discovery of successively lower mass protostars closer and closer to the solar system. For example, observations by the Infrared Astronomical Satellite (IRAS) identified Barnard 5, a currently forming solar-type star. Radio and infrared observations of hydrogen and carbon monoxide have found winds of 100 kilometers per second, as well as expanding knots of water and bipolar radio jets characteristic of protostars.

At higher energies, Chandra, XMM-Newton, and Einstein Observatory X-ray satellites have also observed nascent solar-type stars and star-forming regions for clues to our Sun’s past. Although the evolution of the Sun’s X-ray luminosity depends on its poorly known initial rotation, astronomers know that it declined gradually from the outset of the main sequence for 100 million years and then dropped by a factor of 1,000 until the present day. This decline is connected to the decline of the solar corona’s temperature with time. As for the emerging Sun’s immediate environment, it has been suggested that the abundance of neutron-rich iron 60 (Fe⁶⁰) in some meteorites implies that supernovae were nearby. This would indicate that the Sun was born in a fairly crowded environment similar to the active star-forming regions in the Orion nebula.

The isolation of the solar system is probably a result of a series of gravitational interactions with other protostars that ultimately ejected the emerging solar system from its crowded neighborhood. Similar isotope-decay analyses have been applied to primordial gas-rich meteorites, deducing the composition and strength of the solar wind within one billion years of the Sun’s formation. Observations of T-Tauri stars in both the X and ultraviolet ranges indicate that the Sun emitted energetic particles and winds as flares at this stage, producing precompacted, irradiated grains with peculiar Isotope ratios in the circumstellar disk. Excess neon 21 (Ne²¹) in meteoritic grains is often seen as evidence for this process. After reaching the main sequence, ancient meteoritic evidence further shows that the solar wind flux gradually declined to its present value. Furthermore, radionuclides in lunar rock samples show that the Sun’s proton emission has remained relatively constant over the past five million years, aside from variations from the eleven-year solar cycle. This is also true for heavy ions ejected in flares over this timescale, with the exception that the most ancient flares had an overall enrichment in the trans-iron group of nuclei.

Verification of the Sun’s lifetimes in its stages of evolution comes from confirming the computed standard model of solar structure, specifically the nuclear reactions occurring in the core and their observable properties. For instance, the Solar and Heliospheric Observatory (SOHO), launched in 1995, probed the interior composition and temperature structure of the Sun by “listening” to internal pressure waves reflecting off the photosphere. This application of helioseismology has indirectly validated the lifetimes of its various evolutionary stages by substantiating the standard model’s predictions of solar composition with depth. Further insight into the Sun’s future is gained through observing the evolution of other stars. The Ring and Helix nebulae, for example, are photogenic examples of planetary nebulae ejection and the death throes of Sun-like stars.

Context

Interestingly, theories of solar formation and evolution seem historically motivated by coincidental developments in physics, taking advantage of the increasing availability and sophistication of quantitative measurements and computations. Early speculations of star formation prior to the nineteenth century, without the advanced physics needed to support them, were easily discarded. In the 1840s, J. Robert Mayer and John James Waterson realized that recently studied chemical and electrical energy sources would be unable to provide the Sun’s luminosity for any reasonable timescale. Bolstered by the triumph of thermodynamic principles like energy conservation, William Thomson and Hermann von Helmholtz advocated gravitational contraction as the Sun’s energy source. In the twentieth century, solving this problem required the synthesis of two separate fields of science—astronomy and atomic physics—into the new field of astrophysics.

Arthur Eddington and Henry Russell analyzed the interplay of pressure and gravity to understand high-temperature stellar interiors and radiative equilibrium, while Ernest Rutherford and Niels Bohr laid the foundation for solar physics by establishing quantum mechanics. In 1917 and 1920, Eddington and Harlow Shapley concluded that stars must have ages greater than the tens of millions of years allotted by the gravitational collapse scenario.

From 1920 onward, favor shifted from electron-proton annihilation to fusion reactions; these would offer lifetimes measured in trillions and billions of years, respectively. The latter solution won with arguments for a multibillion-year universe from Edwin Hubble’s research in receding galaxies, quantum mechanical arguments for the possibility of fusion in hot stellar cores, and Hans Bethe’s robust proposal of the proton-proton chain and carbon-nitrogen-oxygen (CNO) cycle (a series of thermonuclear reactions) in 1938. This energy-generation mechanism, along with the associated main sequence lifetime and later evolution, has since been supported by a plethora of increasingly sensitive observational data across the electromagnetic spectrum.

Further progress has been made, especially for uncovering the Sun’s early evolution, with the launch and operation of the James E. Webb Space Telescope in 2021. This satellite, along with other post-Hubble telescopes, probes dust-obscured interstellar clouds at infrared wavelengths and at high resolution to help elucidate the intricate picture of low-mass star formation. Additionally, space probes, such as the Parker Solar Probe and the European Space Agency’s Solar Orbiter, have taken the closest yet images of the Sun, providing scientists with valuable data.

Bibliography

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