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.