Stellar Magnetic Fields

Resources

Stellar magnetic fields are an array of forces that can be observed surrounding and at the surfaces of stars like the sun. They are similar in nature to the effect of the well–known dipolar magnets found in science laboratories, classrooms, and toys, but far more powerful and infinitely more complex. They are an important part of the physical makeup of stars because they affect their interiors, atmospheres, and immediate environments. Observations of the sun show that it has a dynamic overall magnetic field and also smaller, but often much stronger pockets of magnetism associated with sunspots. The influence of these more localized magnetic fields can sometimes be quite dramatic when they are involved in the creation, shaping, and size of solar prominences, flares, and some features in the solar atmosphere (the corona). The large–scale magnetic field of the sun helps determine processes by which chemical elements are transported within and around the sun, and even the spin, or rotation, of the stellar surface. The study of the sun’s magnetic fields, particularly their large–and small–scale structures, helps in understanding their origins. It is assumed by astronomers that when magnetic fields around stars other than the sun are studied in detail they will show similar features and dynamics. Knowledge gained about the magnetic fields of stars can lead to an understanding of their potential impact on long–term stellar evolution.

Exactly how stellar magnetic fields work is somewhat of a mystery. The most widely accepted explanation for them is called the dynamo model. The dynamo principle is used in generators on the Earth, but may be thought of as the reverse of what is happening in a star. In a simple emergency generator, a gas engine spins a magnet within a coil of wire. The interaction of the moving magnetic field within the coil generates electricity in the wire, which is then sent out to a connector that provides electrical power to devices outside the generator. This is how hydroelectric power is generated at dams like the famous Hoover Dam in the Black Canyon of the Colorado River (between Arizona and Nevada). However, instead of a gas engine, water under high pressure provides the motion required to make the generator work.

In a stellar dynamo, rather than electricity being generated because of a moving magnetic field, a magnetic field seems to be generated by two major motions within the star. The first motion is the movement of the gases in the convection zone, which makes up the upper layer of the star. In this region, material at and just beneath the surface moves up as heat is transferred outward from lower layers to the surface by a process in which hot gas rises just as hot air does on the Earth. Once some of the heat of the gas is released at the surface of the Sun, that gas drops down again as it is replaced by hotter gases from below.

The second motion is caused by the simple fact that the sun is made of gas. Because of this, it does not rotate at the same speed everywhere as would a solid object like a planet. This is called differential rotation and it causes the material at the equator to move faster than material at the poles. While scientists have not worked out all the details, it appears that these two effects together create the basic stellar magnetic field of the sun and other stars. However, to be able to create a full picture, it would be necessary to describe accurately all the physical processes operating on the surface of and in the interior of every area of the sun including small– and large–scale turbulence. In addition, the overall magnetic field and sunspot fields themselves effect the movements of the convection zone, creating a situation far more complex than the highly unpredictable weather patterns of the Earth. A deeper understanding of the causes of stellar magnetic fields will require observations of many more stars and a more complete understanding of processes within them.

On the sun, more localized magnetic fields can be found and are made visually obvious by the appearance of sunspots, which were first recorded by ancient Chinese astronomers. They can be so large that they can indeed be observed, with proper filtering, with the naked eye. In the 1600s, Italian astronomer and physicist Galileo Galilei (1564–1642) and his contemporaries rediscovered sunspots shortly after the start of telescopic astronomy (astronomy that was supported by the use of telescopes). Sunspots are regions on the solar surface that appear dark because they are cooler than the surrounding surface area (photosphere) by about 2,200°F (1,200°C). This means they are still at a temperature of about 7,600°F (4,200°C). Even though they look dark in photographs of the sun, they are still very bright. If a piece of sunspot could be brought to the Earth, it would be extremely hot and blinding to look at just as any other piece of the sun. Sunspots develop and persist for periods ranging from hours to months, and are carried around the surface of the Sun by its rotation. Sunspots usually appear in pairs or groups and consist of a dark central region called the umbra and a slightly lighter surrounding region called the penumbra. The rotation period of the sun was first measured by tracking sunspots as they appeared to move around the sun. Galileo used this method to

KEY TERMS

Corona —The outermost layer of the sun’s atmosphere, seen during total solar eclipses as a glowing irregular halo.

Dipolar magnet —The common bar magnet that has opposing north and south magnetic fields.

Flare —A sudden burst of electromagnetic energy and particles from a magnetic loop in an active region of the sun. Sends material out into the solar system that can disrupt electronic devices even on the Earth.

Galileo Galilei —Italian physicist and astronomer (1564–1642) who is credited with first turning a telescope to the sky. Galileo discovered the moons of Jupiter, providing the first observational evidence of smaller celestial bodies moving around larger ones. For stating that the Earth definitely must, therefore, move around the sun, he was placed under house arrest for the latter part of his life.

George Ellery Hale —American astronomer (1868– 1938) best known for his contribution to the design and development of the world–famous 200–in (508–cm) telescope on Mt. Palomar in California.

Photosphere —The visible surface of the sun. The region from which light escapes from the sun into space.

Prominence —A cool cloud of hydrogen gas above the sun’s surface in the corona. Shaped by local magnetic fields of active regions on the sun.

Spectropolarimeter —A device that gathers information on the polarization state of individual chemical reactions from a star seen as lines in the star’s spectrum.

Zeeman–Doppler imaging —The process of using a spectropolarimeter to measure the Zeeman effect, the polarization of spectral lines and a shift in frequency of the lines due to the effect of magnetic fields on the light from a star.

deduce that the sun had a rotational period of about one month. However, because the sun is not a solid body, it does not have one simple rotational period. Modern measurements indicate that the rotation period of the sun is about 25 days near its equator, 28 days at 40° latitude, and 36 days near the poles. The rotation direction is the same as the motion of the planets in their orbits around the sun.

The magnetic causes of sunspots were not known until the early years of the twentieth century when American solar astronomer George Ellery Hale (1868–1938) mapped the solar magnetic field through its effect—called the Zeeman effect—on the detailed shape and polarization of spectral lines. Spectra show the chemical makeup of stars and are a major source of information for astronomers. They are created by spreading the light of a star into its component parts in the same way a prism creates a rainbow of colors from a light source. Chemical reactions in the star create lines of different intensities at predictable places along the spectrum allowing scientists to determine the makeup of the star. Since the chemical reactions would produce a spectral line in a given way in the absence of a magnetic field, scientists can see the effects of fields by comparison to the known spectrum of the reaction. The Zeeman effect is a change in the spectral lines caused by the sun’s magnetic field. The sun’s magnetic field has been mapped on a regular basis ever since Hale first did it, and it is now known that the 11–year sunspot cycle is just a part of an overall 22–year magnetic cycle. The shape of the sun’s magnetic field changes throughout the 11–year cycle, when during this period it reverses its magnetic polarity and begins the whole process over again. In addition to the differential rotation helping to cause the magnetic field of the Sun, it also stretches the north–south magnetic field lines until they run east–west during the first 11 years of the magnetic cycle. Rotating convection then somehow regenerates the north–south field, but with a reversed polarity, causing the process to start again for another 11 years. During these half–cycles, the number and intensity of sunspots increases and decreases with the changes in the overall magnetic field.

Using special high–resolution spectropolarimeters combined with other techniques, the magnetic fields of stars beyond the sun can be detected through the effect they have on the Zeeman signatures found in the shape and polarization state of spectral lines of those stars. Zeeman–Doppler imaging (ZDI) works best for moderate to ultra–fast rotating stars, for which the polarization of individual magnetic regions match the different speeds at which the surface of the star rotates. This method was used to detect the magnetic fields in cool stars other than the sun, showing that the same type of phenomena occur on other stars. Using Zeeman–Doppler imaging, astronomers have managed to detect and map the surface magnetic field of a few extremely active stars of about one solar mass (with ages ranging from a few million to more than ten billion years—twice the sun’s age). Some major differences were found between the alignment of the magnetic field lines of these stars and those of the sun, adding to the mystery of understanding stellar magnetic fields. The conclusion of astronomers studying these stars results is that the entire convection zone of these active stars is involved in forming the magnetic field rather than just the upper layers as appears to be the case with the sun.

These methods allow monitoring of the long–term evolution of the magnetic field shape and strength of other stars. Using them, astronomers hope to be able to detect the polarity switch of the large–scale field and observe a stellar analog of the solar magnetic cycle. If a change in magnetic field polarity is observed, it may indicate the approach of a polarity switch in the magnetic field of the star. Such observations would show that stellar magnetic fields are indeed very similar to those of the sun.

Resources

BOOKS

Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw–Hill, 2006.

Chaisson, Eric. Astronomy: A Beginner’s Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

Krumenaker, Larry, ed. The Characteristics and the Life Cycle of Stars: An Anthology of Current Thought. New York: Rosen Publishing Group, 2006.

Kundt, Wolfgang. Astrophysics: A New Approach. Berlin and New York: Springer, 2005.

Zelik, Michael. Astronomy: The Evolving Universe. Cambridge and New York: Cambridge University Press, 2002.

Clint Hatchett

Stellar magnetic fields

Stellar magnetic fields are an array of forces that can be observed surrounding and at the surfaces of stars like the Sun . They are similar in nature to the effect of the well-known dipolar magnets found in science laboratories, classrooms, and toys, but far more powerful and infinitely more complex. They are an important part of the physical makeup of stars because they affect their interiors, atmospheres, and immediate environments. Observations of the Sun show that it has a dynamic, overall magnetic field and also smaller, but often much stronger "pockets" of magnetism associated with sunspots . The influence of these more localized magnetic fields can sometimes be quite dramatic when they are involved in the creation, shaping, and size of solar prominences, flares, and some features in the solar atmosphere (the corona). The large-scale magnetic field of the Sun helps determine processes by which chemical elements are transported within and around the Sun, and even the spin, or rotation , of the stellar surface. The study of the sun's magnetic fields, particularly their large- and small-scale structures, helps in understanding their origins and it is assumed by astronomers that when magnetic fields around stars other than the sun are studied in detail they will show similar features and dynamics. Knowledge gained about the magnetic fields of stars can lead to an understanding of their potential impact on long-term stellar evolution .

Exactly how stellar magnetic fields work is somewhat of a mystery. The most widely accepted explanation for them is called the dynamo model. The dynamo principle is used in generators on Earth , but may be thought of as the reverse of what is happening in a star . In a simple emergency generator , a gas engine spins a magnet within a coil of wire. The interaction of the moving magnetic field within the coil generates electricity in the wire, which is then sent out to a connector that provides electrical power to devices outside the generator. This is how hydroelectric power is generated at dams like the famous Hoover Dam, but instead of a gas engine, water under high pressure provides the motion required to make the generator work.

In a stellar dynamo, rather than electricity being generated because of a moving magnetic field, a magnetic field seems to be generated by two major motions within the star. The first is the movement of the gases in the convection zone, which makes up the upper layer of the star. In this region, material at and just beneath the surface moves up as heat is transferred outward from lower layers to the surface by a process in which hot gas rises just as hot air does on earth. Once some of the heat of the gas is released at the surface of the sun, that gas drops down again as it is replaced by hotter gases from below.

The second motion is caused by the simple fact that the Sun is made of gas. Because of this, it does not rotate at the same speed everywhere as would a solid object like a planet . This is called differential rotation and it causes the material at the equator to move faster than material at the poles. While scientists have not worked out all the details, it appears that these two effects together create the basic stellar magnetic field of the Sun and other stars. However, to be able to create a full picture, it would be necessary to describe accurately all the physical processes operating on the surface of and in the interior of every area of the sun including small- and large-scale turbulence . In addition, the overall magnetic field and sunspot fields themselves effect the movements of the convection zone, creating a situation far more complex than the highly unpredictable weather patterns of Earth. A deeper understanding of the causes of stellar magnetic fields will require observations of many more stars and a more complete understanding processes within them.

On the Sun, more localized magnetic fields can be found and are made visually obvious by the appearance of sunspots, which were first recorded by ancient Chinese astronomers. They can be so large that they can indeed be observed, with proper filtering, with the naked eye . In the 1600s, Galileo Galilei and his contemporaries rediscovered sunspots shortly after the start of telescopic astronomy . Sunspots are regions on the solar surface that appear dark because they are cooler than the surrounding surface area (photosphere) by about 2,200°F (1,200°C). This means they are still at a temperature of about 7,600°F (4,200°C). Even though they look dark in photographs of the Sun, they are still very bright. If a piece of sunspot could be brought to Earth, it would be extremely hot and blinding to look at just as any other piece of the Sun. Sunspots develop and persist for periods ranging from hours to months, and are carried around the surface of the sun by its rotation. Sunspots usually appear in pairs or groups and consist of a dark central region called the umbra and a slightly lighter surrounding region called the penumbra. The rotation period of the Sun was first measured by tracking sunspots as they appeared to move around the Sun. Galileo used this method to deduce that the Sun had a rotational period of about a month. However, because the Sun is not a solid body, it does not have one simple rotational period. Modern measurements indicate that the rotation period of the Sun is about 25 days near its equator, 28 days at 40° latitude, and 36 days near the poles. The rotation direction is same as the motion of the planets in their orbits around the Sun.

The magnetic causes of sunspots were not known until the early years of the twentieth century, when George Ellery Hale mapped the solar magnetic field through its effect—called the Zeeman effect—on the detailed shape and polarization of spectral lines . Spectra show the chemical makeup of stars and are a major source of information for astronomers. They are created by spreading the light of a star into its component parts in the same way a prism creates a rainbow of colors from a light source. Chemical reactions in the star create lines of different intensities at predictable places along the spectrum allowing scientists to determine the makeup of the star. Since the chemical reactions would produce a spectral line in a given way in the absence of a magnetic field, we can see the effects of fields by comparison to the known spectrum of the reaction. The Zeeman effect is a change in the spectral lines caused by the sun's magnetic field. The sun's magnetic field has been mapped on a regular basis ever since Hale first did it, and it is now known that the 11-year sunspot cycle is just a part of an overall 22-year magnetic cycle. The shape of the sun's magnetic field changes throughout the 11-year cycle, reverses its magnetic polarity and begins the whole process over again. In addition to the differential rotation helping to cause the magnetic field of the sun, it also stretches the north-south magnetic field lines until they run east-west during the first 11 years of the magnetic cycle. Rotating convection then somehow regenerates the north-south field, but with a reversed polarity, causing the process to start again for another 11 years. During these half-cycles, the number and intensity of sunspots increases and decreases with the changes in the overall magnetic field.

Using special high-resolution spectropolarimeters combined with other techniques, the magnetic fields of stars beyond the Sun can be detected through the effect they have on the Zeeman signatures found in the shape and polarization state of spectral lines of those stars. Zeeman-Doppler imaging (ZDI) works best for moderate to ultra-fast rotating stars, for which the polarization of individual magnetic regions match the different speeds at which the surface of the star rotates. This method was used to detect the magnetic fields in cool stars other than the Sun, showing that the same type of phenomena occur on other stars. Using Zeeman-Doppler imaging, astronomers have managed to detect and map the surface magnetic field of a few extremely active stars of about one solar mass (with ages ranging from a few million to more than ten billion years—twice the Sun's age). Some major differences were found between the alignment of the magnetic field lines of these stars and those of the Sun, adding to the mystery of understanding stellar magnetic fields. The conclusion of astronomers studying these results is that the entire convection zone of these active stars is involved in forming the magnetic field rather than just the upper layers as appears to be the case with the Sun.

These methods allow monitoring of the long-term evolution of the magnetic field shape and strength of other stars. Using them, astronomers hope to be able to detect the polarity switch of the large-scale field and observe a stellar analog of the solar magnetic cycle. If a change in magnetic field polarity is observed, it may indicate the approach of a polarity switch in the magnetic field of the star. Such observations would show that stellar magnetic fields are indeed very similar to those of the Sun.

Resources

books

Introduction to Astronomy and Astrophysics. 4th ed. New York: Harcourt Brace, 1997.

Zelnik, Michael. Astronomy. 7th ed. Wiley and Sons, Inc. 1994.

Clint Hatchett

KEY TERMS

Corona

—The outermost layer of the sun's atmosphere, seen during total solar eclipses as a glowing irregular halo.

Dipolar magnet

—The common bar magnet that has opposing north and south magnetic fields.

Flare

—A sudden burst of electromagnetic energy and particles from a magnetic loop in an active region of the sun. Sends material out into the solar system that can disrupt electronic devices even on Earth.

Galileo Galilei

—The Italian physicist and astronomer (1564-1642) who is credited with first turning a telescope to the sky. Discovered the moons of Jupiter, providing the first observational evidence of smaller celestial bodies moving around larger ones. For stating that Earth definitely must, therefore, move around the Sun, he was placed under house arrest for the latter part of his life.

George Ellery Hale

—The American astronomer (1868-1938) best known for his contribution to the design and development of the world-famous 200-in (508 cm) telescope on Mt. Palomar in California.

Photosphere

—The visible surface of the Sun. The region from which light escapes from the Sun into space.

Prominence

—A cool cloud of hydrogen gas above the sun's surface in the corona. Shaped by local magnetic fields of active regions on the Sun.

Spectropolarimeter

—A device that gathers information on the polarization state of individual chemical reactions from a star seen as lines in the star's spectrum.

Zeeman-Doppler imaging

—The process of using a spectropolarimeter to measure the Zeeman effect, the polarization of spectral lines and a shift in frequency of the lines due to the effect of magnetic fields on the light from a star.

Stellar magnetic fields

Stellar magnetic fields are an assortment of powerful forces that can be observed at the surfaces of and surrounding stars like the Sun. Astronomers have yet to obtain a complete understanding of the magnetic fields of stars, but they continue to observe their activity in the hopes of understanding their effects on a star's interior makeup, atmosphere, rotation, and future evolution.

The mysterious magnetic field

A typical magnet—such as one commonly found on a refrigerator—is called a dipolar magnet. Dipolar refers to the two areas of the magnet from which it receives its power: opposing north and south poles. A star's magnetic field works in basically the same way, but it is much more complex. How stellar magnetic fields originate remains a mystery among astrophysicists. In space, there is no naturally occurring magnetic iron, yet astronomers know that magnetism does exist in space.

The most studied stellar magnetic field

The Sun, the only star our solar system, has show that it has a magnetic field that reaches all over its surface. Astronomers know that this magnetic field affects the rotation of the Sun and the movement of chemical elements around its surface. It has concentrated areas of magnetism called sunspots (dark areas on the Sun that produce magnetic storms).

Words to Know

Convection zone: Outermost one-third of the Sun's interior where heat is transferred from the core toward the surface via slow-moving gas currents.

Spectropolarimeter: A device that gathers information on the polarization state of individual chemical reactions from a star; these reactions are seen as lines in the star's spectrum.

Sunspot: A region of the Sun where the temperature is lower than that of the surrounding surface region and consequently appears darker. The presence of a strong, concentrated magnetic field produces the cooling effect.

Zeeman-Doppler imaging: The process of using a spectropolarimeter to measure the Zeeman effect.

Zeeman effect: A change in the spectral lines—their shape and polarization—caused by the magnetic field of the Sun.

While astronomers remain uncertain of exactly how the Sun's magnetic fields work, the most widely accepted theory involves a stellar dynamo. A stellar dynamo can be thought of like a generator (an engine usually fueled by gas that spins a magnet wrapped in coil, producing electricity). Astronomers theorize that in the case of the Sun, instead of producing electricity, the stellar dynamo generates a magnetic field in two ways, each involving powerful motions. The first involves the movement of gases in the convection zone. (A convection zone is the upper layer of a star.) In this zone, material close to the surface of a star rises as heat moves outward from the lower layers of the surface. This process results in hot gas rising from the surface, in a way that is similar to hot air rising on Earth. Upon the release of the heat of the gas at the Sun's surface, the gas drops down again as it replaced by the hotter gases below the surface.

The second type of motion in a stellar dynamo is a result of the Sun being made of gas (mainly hydrogen and helium). When the Sun rotates, its speed is varied due to its gassy composition; this differs from planets, whose solid composition produces a regular rotation. The irregular rotation of the Sun is called differential rotation. It causes the equator (the middle of the Sun) to spin faster than the poles (the top and bottom of the Sun).

Astronomers believe that the combination of the two stellar dynamo motions—involving convection zone gases and differential rotation—generate

the Sun's magnetic field. Continued observations of the Sun and other stars will help confirm this theory or bring forward other possibilities about how stellar magnetic fields are generated.

Methods used to study stellar magnetic fields

Astronomers study stellar magnetic fields by using a method known as the Zeeman effect. In this method, spectral lines are studied. Spectral lines are lights of a single frequency (wavelength) that are emitted by an atom when an electron changes its energy level. Chemical reactions in stars produce lines of varying intensities along a spectrum, thereby allowing scientists to recognize their chemical makeup. The Zeeman effect is a change in the spectral lines—their shape and polarization (a process that causes light waves to create a specific pattern)—caused by the magnetic field of the Sun.

Another method that astronomers use to study stellar magnetic fields is called Zeeman-Doppler imaging (ZDI). ZDI is the process of using a spectropolarimeter to measure the Zeeman effect of stars. A spectropolarimeter is a device that analyzes the polarization state of chemical reactions from stars; these reactions are viewed as spectral lines. Using this method, scientists can detect and map the surface magnetic field of active stars that range in age from a few million to more than ten billion years old.

Importance of stellar magnetic fields

Astronomers are still uncertain of the origins of stellar magnetic fields. But with continued observations, they believe they will learn more about the large and small structures of magnetic fields that should help them comprehend how and where those fields originate and how they affect the interiors and atmospheres of stars. Understanding stellar magnetic fields will help astronomers learn more not only about the physical makeup of stars, but about their future evolution, as well.

[See also Star; Sun ]