Stellar Oscillations And Helioseismology

Type of physical science: Astronomy; Astrophysics

Field of study: Stars

Helioseismology is the study of the oscillations that take place within the sun. These periodic vibrations are caused by sound waves, which originate within the convective zone of the sun. By analyzing the motion of these waves, scientists can image the interior of the sun and develop a more accurate model of the sun.

Overview

It was discovered in the early 1960's that areas of the sun's surface are periodically oscillating up and down. This discovery, like many other scientific discoveries, was made somewhat by accident. The evidence for solar quakes was obtained while astronomers were attempting to measure the oblateness of the sun to verify another theory.

It was known for quite some time that the orbit of the planet Mercury did not follow the precise path predicted by the solution to Sir Isaac Newton's laws of gravity and motion. In fact, as Mercury orbited the sun, its orbit would also revolve around the sun. Although some precession was predicted, a percession in Mercury's orbit of 43 arc seconds per century was not predicted. It was proposed by some scientists that there was another planet within the orbit of Mercury. Others suggested that, after so many accurate predictions, Newton's law of gravity needed some alteration. In 1916, Albert Einstein proposed that a massive object, such as the sun, warps the space-time around itself. Mercury's orbit would then follow the curvature in space-time caused by the sun. Einstein's general theory of relativity accurately accounted for the motion of Mercury. According to an opposing theory of gravity, the scalar-tensor theory of Carl Brans and Robert H. Dicke, the sun is not a perfect sphere but has a slight equatorial bulge caused by the rotation of its core. Dicke proposed that a distortion of 0.05 percent of the sun's surface would explain the observed behavior of Mercury's orbit.

At the University of Arizona, astronomer Henry Hill built a telescope that was designed specifically to detect a distortion in the shape of the sun. When the telescope became operational and the sun's surface was studied, no evidence of a distortion was observed. Upon further observation and an additional series of measurements, Hill and his colleagues discovered the periodic oscillations of the sun's surface.

Light from the sun is analyzed by the use of a spectrometer. This device, similar to a glass prism, breaks up sunlight into its component colors. Since the cooler gases of the sun's atmosphere absorb some wavelengths of light, a spectrum, which contains several dark lines, is produced. These dark lines in the sun's spectrum form the chemical signature of the various elements that compose the sun.

The discovery of surface oscillations was made by observing the Doppler shift of various spectral lines in the light from the sun. By analyzing light, astronomers can determine whether the source is moving toward or away from the observer. If the spectral lines have been shifted to the blue (or short-wavelength) end of the spectrum, the source of the light is moving toward the observer. If the shift is toward the red (or long-wavelength) end of the spectrum, the source is moving away from the observer. By observing various points on the surface of the sun, astronomers were able to show the periodic oscillation of those points, as spectral lines would be alternatively redshifted and then blueshifted.

It was first observed that these periods of oscillation were about five minutes in duration. Since then, it has been determined that the entire surface of the sun is in a state of constant oscillation, with periods varying between minutes and hours. It might be said that the sun is ringing like a bell. In this case, however, the bell is being struck continuously.

Astronomers believe that the origin of these waves is the convective zone beneath the photosphere or "surface" of the sun. These huge boiling columns of gas carry heat from the sun's interior to the surface. The tops of these convecting cells produce the granulation seen in solar photographs. These granules may each be hundreds of kilometers across. The gas rises toward the surface accompanied by a tremendous roar. These sound waves oscillate through the sun and cause its surface to rise and fall periodically.

As sound waves travel downward into the sun, they encounter higher temperatures and pressures. These changing physical conditions result in the wave's velocity being increased.

Eventually, the waves begin to bend upward toward the surface of the sun. When they reach the bottom of the photosphere, they are reflected back into the interior of the sun. The depth that the wave travels depends upon its length. The wavelength also determines how far a wave will travel around the sun before it hits the surface.

The sun's interior is conducting waves with virtually millions of different wavelengths and frequencies. Some waves have the exact length necessary to make an even number of bounces before they return to where they began. Astronomers categorize these waves by the number of times that they strike the surface in one complete cycle of the sun. For example, a wave with the designation I-4 strikes the surface in three places before it bounces back to its starting position. Once it returns to its origin, it has struck the surface of the sun four times.

Scientists have found that waves with low I numbers travel deep within the sun and may be a key to revealing physical characteristics there, while waves with higher I numbers may be used to probe the shallow zones of the sun's interior.

Applications

During an earthquake, various types of seismic waves are generated. These waves are received at seismic stations around the world and their arrival times are noted. By determining the path and velocity of these waves, scientists learn much about the interior of the earth. Similar methods are used by exploration geophysicists to study the subsurface when searching for mineral deposits or potential oil traps. In this case, the seismic waves are generated by explosive charges or other artificial means. The waves then travel into the earth and are reflected as they strike various rock layers. Scientists can then determine the depth of these layers.

Scientists hope to be able to use solar seismic waves to image the interior of the sun, just as geophysicists use seismic waves to study the interior of the earth. Prior to this new development in solar physics, the processes that occur within the sun and the locations of various boundaries within the sun were theoretical. It is hoped that future helioseismic studies will continue to increase human knowledge of the sun.

Theorists believe that the sun is a giant ball of gas that is sustained by a thermonuclear fire burning within its core. Because of the great pressure and high temperature within this region of the sun, the nuclei of hydrogen are fused together to form helium. During this process, which is known as the proton-proton cycle, 600 million tons of hydrogen are consumed each second and turned into some 590 million tons of helium. The tonnage that is not converted into helium is transformed into energy. Most of the energy leaves the core in the form of high-intensity radiation called γ rays. The remainder is in the form of chargeless, massless, subatomic particles called neutrinos.

By studying the rate at which neutrinos are emitted from the core, scientists can gain some insight into the processes that are occurring within the core. In the late 1960's, an experiment was set up deep within an abandoned gold mine in Lead, South Dakota. Conducted by Raymond Davis of the Brookhaven National Laboratory, this ongoing experiment uses a 378,000-liter tank of a chlorine solution to detect the solar neutrinos. As the neutrinos pass through the solution and strike individual atoms of chlorine, argon nuclei are formed. Since these argon nuclei are radioactive, they can be detected easily and the neutrino flux can be calculated.

The results of the Davis experiment have touched off one of the major debates in modern astrophysics. The experiment is detecting only about one-third as many neutrinos as the theory indicates should be detected. Astrophysicists are wondering what is happening to the missing neutrinos. Several theories have been proposed to solve the problem. It has been suggested that perhaps the Davis experiment has flaws. This possibility has been examined thoroughly; the high-technology equipment involved in this experiment has been checked several times and the data have been interpreted and reinterpreted. The problem of the missing neutrinos remains.

There is a possibility that the model of the solar interior is incorrect. The possibility exists that the core of the sun is not as hot as current theory indicates. If the core is cooler, then the number of neutrinos emitted from the sun would be less, and this would account for the missing neutrinos. A particle known as WIMP, or weakly interacting massive particle, has been proposed as a possible solution. According to modern theory, WIMPs were formed in the early universe, when matter was in a very dense state and temperatures were extremely hot. These particles migrated toward the center of newly forming stars, where they remain today. It is believed that WIMPs may circulate within the interior of the sun, carrying heat away from the core. As a result of this process of carrying away heat, the fuel cycle would be slowed somewhat, thus reducing the number of neutrinos emitted.

There are still other possibilities to be considered. One interesting possibility is that the sun's core is rotating much faster than its outer layers. If this is true, the core temperature would be lower and, thus, the rate of neutrino emission from the core would be lower. A potential problem with this solution is that a rapidly rotating core would cause an equatorial bulge, and no such bulge has been detected.

It is obvious that the only way to determine precisely the cause of the missing neutrinos and whether the core is rotating is to have images of the interior of the sun. Helioseismic waves may provide the answer. It has already been determined by studying solar seismic patterns that the sun's convection zone is deeper than previously believed. Apparently, it composes about 30 percent of the solar radius. New information has been found that links the rate of rotation of the interior of the sun to the formation of sunspots.

More extensive studies using helioseismology have been planned. The Global Oscillation Network Group (GONG) at the National Solar Observatory in Tucson, Arizona, was founded to begin such studies. Also, the European Space Agency (ESA) plans to install helioseismology instrumentation on its Solar and Heliospheric Observatory (SOHO), which will be able to take relevant data for years to come.

The GONG program has outlined several specific goals for their solar studies program.

The first goal is to determine the internal temperature, pressure, and composition of the sun from the surface down to the core. A second goal is to determine the rotational rates of internal layers of the sun. In addition to determining rotation rates, scientists will again attempt to detect any solar oblateness. This test will allow the accuracy of the general theory of relativity to be checked again and quite possibly, between the oblateness and rotation tests, the problem of the missing neutrinos can be solved. Scientists also hope to use helioseismic studies to investigate how energy is transferred from the solar surface to the chromosphere and corona. It is currently believed that intense magnetic fields, along with acoustic shock waves from the tops of convecting cells, are responsible for temperatures of 400,000 Kelvins in the chromosphere and temperatures of 2 million Kelvins in the corona.

GONG's plans include building a chain of 50-millimeter refracting telescopes to be stationed at various locations on the earth. These locations will ensure that at least two telescopes will be gathering data from the sun at all times. The network will observe the sun continually for a three-year period. This large amount of data is necessary if astronomers are to determine the path of sound waves through the sun and convert that information into a model of the solar interior. Each of the telescopes that will be deployed by the GONG project contains an instrument called a Fourier tachometer. This device is capable of measuring extremely small Doppler shifts at more than sixty-five thousand different points on the surface of the sun. By observing these shifts, astronomers can determine oscillation periods of these various points and form a detailed model of the solar disk.

Context

The discovery that the surface of the sun is oscillating was made in the early 1960's. At the time, scientists were gathering data on the oblateness of the sun in an attempt to determine which theory--the scalar-tensor theory of Dicke and Brans or Einstein's general theory of relativity--could best explain the motion of the planet Mercury. As it turned out, there was no noticeable oblateness of the sun, but subsequent observations revealed a Doppler shift in the solar spectra taken from various points on the sun. This Doppler shift provided evidence for periodic oscillations. Further investigations have revealed that the sun is ringing as if it were a large bell that is continuously being struck.

The millions of different wavelengths and frequency combinations of waves are believed to originate within the sun's convective zone. In this area, the tremendous heat from the core is flowing outward toward the surface. This method of heat flow, convection, is found only in a gas or a liquid. Here, the material toward the base of the zone is extremely hot. Hot liquids and gases are also less dense than cooler liquids or gases. As a result, the less dense material rises toward the surface, carrying the heat. At the base of the zone, cooler material flows in to fill the void. This material will be heated subsequently and will begin to move toward the surface. The sound waves given off by this movement of huge amounts of hot gases are the waves that cause the sun to vibrate.

The discovery of helioseismic waves and, therefore, the science of helioseismology will enable solar scientists to map the interior of the sun and perhaps solve some of the perplexing problems in solar physics. For example, if it can be determined whether the core is rotating or if the sun has an equatorial bulge, it may be possible to solve the problem of the missing neutrinos. Helioseismic imaging will also make it possible for astronomers to determine the boundaries for such zones as the convective zone, radiative zone, and the core itself. In addition, the accumulation of data from the GONG program will, over a period of years, enable astronomers to form an accurate model of the solar surface and interior.

Principal terms

CONVECTION: the flow of heat in a liquid or a gas; the hot material is less dense and flows upward, and the cooler material flows toward the bottom of the convection cell

FUSION: a thermonuclear reaction that powers the sun and the stars; in the sun, hydrogen nuclei are fused into helium, generating energy

NEUTRINOS: chargeless, massless subatomic particles that are created during the fusion process; virtually unstoppable, they pass easily through the earth

OBLATENESS: a somewhat spherical object that is flattened at the poles

Bibliography

Gamow, George. A STAR CALLED THE SUN. New York: Viking Press, 1964. This very readable volume describes the sun, solar processes, and energy generation within the sun. Stellar evolution is also discussed. Some basic algebra is used in illustrations. Well suited for the layperson.

Lopresto, James Charles. "Looking Inside the Sun." ASTRONOMY 17 (March, 1989): 20-28. A somewhat technical article discussing the origin of the study of helioseismology and its possibilities in solar research. Although very little mathematics is used, the article contains an abundance of technical terms. The reader should have a background in basic physics and astronomy.

Mitton, Simon. DAYTIME STAR: THE STORY OF OUR SUN. New York: Charles Scribner's Sons, 1981. A nontechnical volume accessible to the general reader. Mitton discusses the sun, its structure, its processes, and its future.

Pasachoff, Jay M. ASTRONOMY: FROM THE EARTH TO THE UNIVERSE. Philadelphia: Saunders College Publishing, 1991. A fairly technical volume covering topics in stellar and solar system astronomy. This volume is used as a text for college freshman-level astronomy courses but would be accessible to the informed reader. Contains an excellent unit on the sun.

Seeds, Michael A. FOUNDATIONS OF ASTRONOMY. Belmont, Calif.: Wadsworth, 1990. This general astronomy textbook contains a section on the sun. Suitable for the general reader.

Seeds, Michael A. HORIZONS: EXPLORING THE UNIVERSE. Belmont, Calif.: Wadsworth, 1991. Although this volume is intended for use as a college-level general astronomy text, it is accessible to the general reader. Contains an excellent discussion on the sun.

Thermonuclear Reactions in Stars