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.
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 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).
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.
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.
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.