Sun

V

Inside the Sun

The energy that the Sun produces in its core must travel to the Sun’s surface to make the Sun glow. The mechanisms that transport radiation from the center to the surface of the Sun define the structure and behavior of the layers inside the Sun.

A

Radiation and Convection

Nuclear fusion releases energy deep down inside the Sun’s high-temperature core, which extends from the center to about one-quarter of the radius of the Sun. The layers above the core produce no energy, so the core, which makes up only 1.6 percent of the Sun’s volume, produces all of the Sun’s energy. Energy moves from the core to the rest of the Sun through two spherical shells that surround the core. The inner shell is called the radiative zone, and the outer one is called the convective zone. Radiation and convection are two ways that energy can travel from one place to another (see Heat Transfer).

Radiation involves the movement of energy, but not the movement of material. The radiative energy spreads out in all directions and can move between objects that are not connected. Radiation can be absorbed by another substance. In the process of convection, matter moves energy. Convection occurs when a liquid or gas moves into contact with an object at a different temperature.

Energy moves from the core of the Sun to the next innermost layer, the radiative zone, through radiation. The radiative zone spans from the outer edge of the core, which is 174,000 km (108,000 mi) from the Sun’s center, to 496,000 km (308,000 mi) from the Sun’s center. The radiation diffuses outward in a haphazard, zigzag pattern. Particles in the radiative zone repeatedly absorb, radiate, and deflect photons of energy. The matter in the radiative zone stays in the same place while the energy moves through it. Because of this continued ricocheting in the radiative zone, about 170,000 years, on average, are required for a photon of energy to work its way outward from the Sun’s core to the bottom of the convective zone.

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The Sun’s interior cools with increasing distance from the center, as the heat and radiation of the core spread outward into an ever-larger volume. At the base of the convective zone, the temperature is about 2.2 million degrees C (about 4.0 million degrees F). At the boundary of the cooler convective zone, the radiative energy has lost too much intensity and the material is too cool and dense to allow the energy to pass through. The layers of material at the bottom of the convective zone heat up with blocked radiation and become less dense than surrounding material. This heated material then moves up through the convective zone, carrying energy toward the atmosphere of the Sun. When the material reaches the atmosphere—a layer that is much less dense than the convective zone—the energy can radiate into space. The material at the top of the convective zone becomes cooler and therefore denser when it releases its energy, falling back down to the bottom of the zone to pick up more energy. The length of time needed for a particle to pass through the convective zone, from the innermost to the outermost edge, is about ten days.

B

The Oscillating Sun

The behavior of the outer, visible layer of the Sun allows scientists to glimpse the structure of the interior of the Sun. The visible part of the Sun is called the photosphere. The photosphere heaves in and out with a rhythmic motion. The material in the photosphere can reach a height of 50 km (30 mi) and speeds of 500 m/s (1,600 ft/s). The time each oscillation takes to go from its highest point to its lowest point and back again is called its period. Each oscillation has a period of about five minutes.

The oscillations in the photosphere are actually caused by sound waves from the convective zone. Sound waves, whether on Earth or in the Sun, are waves carried by matter. They travel by compressing matter in their path. Because they rely on matter, sound waves cannot travel through a vacuum, or an area in which no matter is present. Air carries most of the sound we hear on Earth. The hot plasma of the Sun carries sound waves within the Sun. Hot gas churns in the convective zone, producing a noise like that of a jet airplane or a pot of boiling water, but much, much louder. When these sounds strike the photosphere and rebound back down, they disturb the gases there, causing them to rise and fall.

The sound waves are trapped inside the Sun and cannot travel through the vacuum of space. Even if they could reach Earth, the Sun’s sounds are too low-pitched for the human ear to hear. A period of five minutes corresponds to 0.003 vibrations per second. The lowest sounds that even a sensitive human ear can hear have a frequency of about 25 vibrations per second.

Scientists can “listen” to the Sun’s vibrating notes indirectly by watching the rhythmic motions of the photosphere. Sensitive instruments detect the Sun’s oscillations by recording periodic changes in the wavelength of the Sun’s light. Motion at the solar photosphere changes the wavelength of the light that it emits. When oscillations on the Sun’s photosphere move its material away from Earth, the Sun’s light shifts to longer wavelengths. This shift occurs because each successive wave has farther to travel than the one before it did in order to reach Earth, so the distance between waves (the wavelength) becomes longer. Photospheric oscillations that move material toward Earth make the wavelengths shorter, because each wave has a shorter distance to travel than the one before it did. These changes in wavelength due to motion are called the Doppler effect.

C

Helioseismology

Helioseismology is the study of the interior of the Sun. Measuring the speed of sound waves in the Sun helps scientists determine the temperature and composition of the Sun. The speed of sound depends on the temperature and composition of the material through which the sound passes. Helioseismologists exploit this relationship to establish how the Sun’s temperature, density, and composition vary with distance from the center. Experimental measurements of the density and temperature of the Sun’s layers agree almost perfectly with theoretical models. Measurements of the Sun’s core temperature are very close to the predicted value, showing that the predicted number of solar neutrinos should also be correct.

Scientists also use oscillations in the photosphere to study the movement of the interior of the Sun. About 10 million separate sounds—each traveling in a different, defined section of the solar interior—combine to produce the oscillations in the photosphere. Scientists can separate all of the different vibrations, trace them back to their origins, and look into the heart of the Sun.

D

Rotation

Like Earth, the Sun rotates, or spins, around an imaginary line that runs through its center. This line is called the Sun’s axis, and the top and bottom of this line mark the Sun’s north and south poles, in the same way that Earth’s axis marks the North Pole and South Pole on Earth. Earth, the Sun, and the other planets in the solar system all lie on one plane, and the Sun’s north pole and Earth’s North Pole are oriented in roughly the same direction relative to the plane. The Sun’s equator, like Earth’s, is an imaginary line halfway between the north and south poles that runs east and west. Like Earth, the Sun rotates from west to east when viewed from above the north pole, but unlike Earth, different parts of the Sun rotate at different rates. In the photosphere, the areas near the north and south poles of the Sun rotate more slowly than the areas nearer the solar equator. A spot at the Sun’s equator takes 25 days to rotate completely, while a spot 15° from the poles takes 34 days to make a complete rotation. This phenomenon is known as differential rotation.

Scientists use helioseismology to measure the Sun’s internal rotation. Sound waves moving in the direction opposite to the rotation of the Sun appear to move more slowly than those moving with the rotation of the Sun. Helioseismologists can pinpoint the origins of fluctuations on the Sun’s surface and compare sound waves that have taken different paths to the surface. Armed with this sensitive indicator, helioseismologists have shown that the differential rotation exhibited by the photosphere persists throughout the convective zone. These differences disappear in the underlying radiative zone, where the rotation speed becomes uniform from pole to pole. At the boundary where the convective and radiative zones meet, the different rotation speeds cause the material in the zones to rub together. Scientists suspect that the forces generated by the two zones moving against each other may create the Sun’s magnetic field.

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