Mystery of Sun’s Magnetic Field Unraveled a Bit More


The inside of a star can be thought of as an incredibly complicated dance with a multitude of dancers. Some dancers move independently while others move with each other. While we know that this motion generates magnetic fields, it is extremely tricky to determine how it works in detail.

A star’s magnetic field strength can be measured by its X-ray emission. Researchers have now found that four old red dwarf stars are emitting X-rays at a much lower rate than expected.  The Sun’s mass is more than twice than that of the stars observed. This suggests that the magnetic fields of these stars is much weaker than previously thought.

Young stars of all masses have very high levels of X-ray emission that indicate a strong magnetic field.  While it is commonly acknowledged that the magnetic fields of stars like our Sun weakens over time, it was not expected to happen for low mass stars whose internal structure is very different.

Stars like the Sun are giant spheres of superheated gas. The magnetic field in the Sun is responsible for producing its 11-year cycle, powerful eruptions of particles from the solar surface and sunspots. Solar storms on the Sun can affect astronauts in space, knock out communications satellites, damage electrical power systems and produce spectacular auroras on Earth.

Lead author Nicholas Wright of Keele University in the United Kingdom notes that although it has been common knowledge for decades that the behavior of the Sun and other stars is influenced by their magnetic fields in a major way, many details remain a mystery. The results from the red dwarf studies is but one small step in the mission to understand the Sun and other stars fully.

A star’s magnetic field is influenced by both the flow of gas in its interior and its rotation. The rotation of stars similar to the Sun changes with depth below the surface and latitude (the poles versus the equator). Convection also plays a role in the generation of a star’s magnetic field. With convection, heat from the interior of the star is distributed to its surface in a circulating pattern of descending cooler gas and rising cells of hot gas, almost like the circulation of warm air inside an oven.

suns magnetic field
An artist’s illustration shows the interior of a low-mass star, such as GJ 3253, a low-mass red dwarf star that is around 31 light years away from Earth, seen in the inset of an X-ray image from Chandra.
Credits: X-ray: NASA/CXC/Keele Univ./N. Wright et al; Optical: DSS

The hot gas closer to the core of the star remains relatively stable and convection only occurs in the outer third of the Sun. The speed of rotation in these two regions is different, generating most of the Sun’s magnetic field. Many astronomers believe the magnetic fields between the core and the convection zone wind up and strengthen. As a star’s rotation slows with age, one would expect the magnetic field of such stars to weaken with time.

Convection occurs all the way into the core of stars less massive than the Sun. Therefore, the boundary between regions with and without convection does not exist for these stars. If this region is indeed crucial for generating the magnetic field in the Sun as is thought, the magnetic fields should not weaken much over time.

Wright and co-author Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass, used data from the ROSAT satellite to look at two of the stars and NASA’s Chandra X-ray Observatory to study the other two. This enabled them to test the hypothesis that interaction along the convection zone-core boundary dominates the generation of magnetic field in stars like our Sun. Their findings went against what they would have expected. The magnetic fields of the low mass red dwarf stars decrease as they age, even though they have no convection zone-core boundary.

These results imply that the generation of magnetic field in stars like our Sun is not dominated by the interaction along the convection zone-core boundary as was previously thought.

The full study was published in Nature journal.