Stellar Turbulence Shapes Activity of Stars

James Maynard
Mar 9 · 4 min read

Stellar turbulence plays a significant role in the production of starlight, a new study from the Max Planck Institute reveals.

Stars shine as a result of fusion reactions at their cores, transforming one element into another, heavier, type of atom. Energy from these reactions works its way outward, through various layers of the Sun, each with its own distinctive characteristics.

Outside the core lies the radiation zone, through which energy travels in the form of high-energy electromagnetic waves. As the energy cools, the main method of transport becomes plasma flows, forming the next layer out from the center of the Sun — the convection zone. Here, heated plasma (the fifth state of matter) rises, cools, then sinks back again, creating conditions similar to boiling water.

What gives rise to activity on the Sun and other stars? A new pair of studies seeks the answer. Image credit: LPICGR/Pixabay

Movement of charged materials in the Sun gives birth to the solar magnetic field. Lines of this magnetic field are then twisted by the rotation of the Sun (which is faster at the equator than near the poles), creating a twisting, convoluted series of lines of force.

“Unfortunately, we cannot look directly into the Sun and other stars to see these processes in action, but have to resort to more indirect methods”, says Dr. Jyri Lehtinen from the Max Planck Institute for Solar System Research (MPS) in Germany.

Fire Burn and Caldron Bubble

Two models of stellar dynamos are currently accepted by most astrophysicists. The first idea suggests that the rotation of stars provides the major impetus for convection, while the other holds that small-scale flows play the dominant role within this layer of stars.

Researchers carried out a pair of studies, comparing the activity of hundreds of stars to each other, as well as examining their rotational and convective properties. These studies were developed to determine which properties of stars have the greatest effect on stellar activity, revealing the nature of the dynamo process happening inside stars.

The 224 stars in the studies included younger stars, those in the prime of their lives (like the Sun), as well as older stars. Properties of stars change as they age, and more developed stars tend to have a thicker convection zone than younger stars. This layer can even, occasionally, completely replace the radiative zone of a star. These conditions slow down the rate at which energy passes through the body, and it (usually) reduces the speed at which the star rotates.

The team examined data showing the effects of calcium ions in the stellar plasma of the stars. Investigation showed these emissions were not correlated (in a simple way) to the activity level of the star, downplaying the role of rotation affecting the stellar dynamos.

“Our results demonstrate that turbulence plays a crucial role in driving stellar dynamos and suggest that there is a common turbulence-related dynamo mechanism explaining the magnetic activity of all late-type stars,” investigators describe in Nature Astronomy.

Run, Spot, Run

Like our stellar parent, many stars develop dark, heavily-magnetic regions (known as sunspots on the Sun).

“As a star rotates, these regions come into view and pass out of it leading to a periodic rise and fall in emission brightness,” Dr. Maarit Käpylä from Aalto University in Finland, states.

A look at how the solar dynamo within our own sun affects the magnetic fields of our parent star, Image credit: Solar Dynamics Observatory

As dark regions move across the surface of stars, the amount of light coming from stars fluctuates. Other effects can also alter the amount of energy we see from a star, making it challenging to measure the rotational rate of distant stars.

Rotational rates of the stars was inferred through advanced data processing, merging measurements taken at Mount Wilson Observatory in California with data on the composition, rotational rates, and developmental stage of each target.

The recent dimming of Betelgeuse seen from Earth over the last few months may or may not be processes within the star, or an effect of dust blocking our view of the red giant star.

This new study provides more evidence to the idea that convection, not rotation, is the major driving force behind the behavior of stars.

James Maynard is the founder and publisher of The Cosmic Companion. He is a New England native turned desert rat in Tucson, where he lives with his lovely wife, Nicole, and Max the Cat.

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