Solar interior

The nuclear fusion reactions that power our Sun occur in its interior. The nature of this interior determines conditions at the surface of the Sun, affecting the rest of the solar system.

Overview

The Sun is composed primarily of hydrogen and helium. Though these substances are gaseous on Earth, the conditions in the Sun are such that they do not behave inside the Sun the way that they do on Earth. The Sun’s gravity compresses and heats these gases in the Sun’s interior. About one-fifth of the way below the surface of the Sun, the gases are so hot that they are ionized—that is, they have been stripped of their electrons. The closer to the center of the Sun these gases are, the more compressed and heated they are. Almost 94 percent of the Sun’s mass is contained within the innermost half of the Sun’s radius.

Near the center of the Sun, the temperature is more than fifteen million kelvins. Gases are compressed to a density of about 160 grams per cubic centimeter (g/cm³), more than fourteen times denser than lead. At such density and temperature, hydrogen nuclei begin to fuse into helium. Every second, close to 600 million tons of hydrogen is consumed in this fusion process. The result of this fusion is the production of about 596 million tons of helium. The difference in these two mass figures, four million tons, is converted into energy via the equivalence of 2 [e equals mc squared]>mass and energy given by Albert Einstein’s famous equation E = mc². This energy released in the deep interior of the Sun is what supports the Sun against gravity and keeps it from collapsing further under its own weight.

Density and temperature are greatest at the center of the Sun. This is where nuclear fusion occurs at the fastest rate. The density and temperature decrease with distance from the center of the Sun. Therefore, the rate of nuclear fusion decreases with increasing distance from the center of the Sun. Beyond about 25 percent of the distance from the Sun’s center to its surface, the density and temperature are too low to support fusion at any appreciable rate. This innermost portion of the Sun, where nuclear fusion occurs, is called the core of the Sun. The temperature at the top of the core is about seven million kelvins. However, there is no well-defined edge to the core. Rather, the farther from the center of the Sun, the lower the rate of fusion.

Throughout the core, energy is produced through nuclear fusion. Much of this energy is initially in the form of high-energy gamma rays. The gamma rays produced in this manner travel only an extremely short distance before they collide with an electron. Gamma rays are a form of electromagnetic radiation. While often described as waves, Electromagnetic radiation also acts like particles called photons. These photons carry momentum. Therefore, when the gamma rays collide with electrons, they scatter off of the electrons in a process called Compton scattering. In the scattering process, the gamma rays lose momentum and energy, and the electrons gain momentum and energy. These collisions between the gamma rays and the electrons are what support the interior of the Sun. Gamma rays continually rebound from electron to electron, scattering in random directions. Gamma rays continually lose energy, eventually becoming X-rays. The radiation scatters in random directions with each collision. In this manner, called radiative diffusion, the energy from the nuclear fusion in the Sun’s core gradually works its way outward from the middle of the Sun. Because of the large number of collisions in random directions, it takes a long time for the radiation to travel outward. On average, the radiation diffuses outward at a rate of about fifty centimeters per hour and takes nearly 170,000 years to make its way out of the Sun.

Radiative diffusion dominates until a distance from the center of about 71 percent of the Sun’s radius. At that point, the temperature has dropped to about two million kelvins, and the gas density has dropped to about 0.2 g/cm³ (about 150 times denser than Earth’s sea-level atmospheric density). Here, some electrons are captured by the atoms. Instead of the light scattering off of electrons, it is absorbed by atoms, heating the gases. The hot gas then expands and rises. When the gas reaches the surface of the Sun, the photosphere, it cools by radiating light into space. The cooler gas then sinks until it is again warmed by absorption of more energy. Heat transfer in this manner is called convection. Since convection dominates the energy transfer mechanism in the upper portion of the Sun, the top 29 percent of the Sun’s radius is called the convective region. The lower 71 percent of the Sun’s radius, including the core, is called the radiative region because radiative diffusion is the chief mechanism for energy transfer.

Between the radiative region and the convective region is a small region that acts as an interface between the radiative and convective regions. This zone is called the tachocline. Below the tachocline, atoms are almost all ionized, and radiative diffusion dominates. Above the tachocline, atoms have electrons, and convection dominates, but the transition is not sharp. There is no set distance from the center of the Sun where the transition between radiative diffusion and convection occurs. The lower portion of the tachocline has more radiative diffusion than convection, and the upper portion of the tachocline has more convection than radiative diffusion. The tachocline is important to solar astrophysicists because this is the region where the Sun’s magnetic field is believed to be produced.

The top of the convective region is the photosphere. This is often regarded as the visible surface of the Sun. However, because the Sun is not solid, it really has no “surface.” Rather, when the density and temperature of the gases that make up the Sun drop to low enough values, the gases become transparent to light, and the heat energy then shines out into space as thermal (blackbody) electromagnetic radiation. The photosphere itself is not a sharp boundary but a rather thin zone. The distance from the center of the Sun to the top of the photosphere is generally regarded as the Sun’s radius.

Methods of Study

The interior of the Sun is difficult to study. It cannot be imaged directly. However, it is important because the Sun’s magnetic field is produced deep inside the Sun, and the Sun’s magnetic field and magnetic behavior are responsible for solar activity, solar storms, and considerable interaction of the Sun with the rest of the solar system.

Though the Sun’s interior cannot be imaged directly, it can be studied by the way that it influences the surface of the Sun. The Sun resonates in certain vibrational modes. This can be thought of as being analogous to the ringing of a bell, only at far lower tones because the Sun is so large. These vibrations can be recorded on the surface of the Sun through observations of the rise and fall of the Sun’s surface using the Doppler shift of spectral lines in the Sun’s spectrum. The study of the motions of the surface of the Sun in this manner is called helioseismology. As shock waves pass through the interior of the Sun, they diffract (or bend) through the different layers of the Sun. Shock waves can also reflect from different layers in the Sun. Motion of material within the Sun can also distort the shock waves. Helioseismology can, therefore, yield a great deal of information about the Sun’s interior structure.

Solar astrophysicists have learned much about the motion of material in the Sun’s convective region beneath the photosphere. The interior of the Sun has been found to rotate at a slightly different rate than the surface layers of the Sun. This is believed to play a role in the Sun’s magnetic behavior.

A further probe into the Sun’s interior is through the study of neutrinos streaming from the Sun. Nuclear fusion occurring at the Sun’s core produces not only gamma rays but also tiny, weakly interacting particles called neutrinos. These particles then flow outward from the Sun and can be detected on Earth. Studies of those neutrinos can yield information about the nuclear fusion processes going on in the Sun’s core. Early studies of the neutrinos appeared to show far fewer neutrinos than had been predicted by nuclear theory. However, emerging studies suggest that the neutrinos oscillate, or change form, between three types, and the early experiments only detected one type of neutrino.

Context

The Sun is the nearest star to the Earth, and it is the center of the solar system. It dominates everything else in the solar system. The interior structure of the Sun is determined by its composition and mass, and that interior structure determines the observational characteristics of the Sun. Understanding the interior structure of the Sun, therefore, is important to understanding the Sun itself and, ultimately, its interactions with its planets, including Earth.

For many years, it was assumed that because the Sun is hot and bright, like fire, it must be shining through some sort of burning process. Burning, though, was unable to account for the Sun’s energy. Astronomers theorized that perhaps the Sun was shining through the release of its gravitational energy. That, too, failed to account for the Sun’s energy output. Finally, by the early twentieth century, physics had advanced to the point where astrophysicists understood nuclear fusion to be the Sun’s energy source.

Likewise, the interior structure of the Sun could not be understood until physicists understood the physics of materials under conditions such as exist inside the Sun. Theoretical astrophysicists describe the Sun’s structure through a set of equations called a solar model. Helioseismological studies match the theoretical models to a very high degree of accuracy. Scientists continued to expand their knowledge of the Sun and its interior into the twenty-first century. Launched in 2018, the Parker Solar Probe flew into the Sun’s upper atmosphere in 2021, sampling particles and magnetic fields in the corona. The information gleaned from the probe offers hope for new discoveries regarding the interior of the Sun. The NASA NuSTAR mission, or Nuclear Spectroscopic Telescope Array, has also produced images that scientists hope will shed new light on the inner workings of the Sun.

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