An international team of astronomers has for the first time figured out what goes on inside red giants. After examining what goes on below the surface of dozens of red giant stars, the scientists found that these objects have incredibly strong magnetic fields.

To peer inside these stars, they used a technique called asteroseismology, which uses waves on their surface to interpret the turbulence in the deeper layers. This is similar to medical ultrasounds which use sound waves to see within the human body. The scientists looked at two different types of wave: pressure waves from internal turbulence (akin to sound waves), and gravity waves which are driven by the buoyancy of the different layers. The different types can penetrate to different depths inside a star.

The findings will expand our understanding of the life and inner mechanisms of stars. The study focuses on a particular type of objects called red giant branch (RGB) stars.

RGB stars are objects in a late phase of stellar evolution. They form when a star of low to intermediate mass (from 0.3 to eight times the mass of the Sun) runs out of hydrogen to fuse in its core. When that happens, the star begins to contract under its own gravity. This free-fall contraction eventually generates enough pressure and heat to start fusion in a shell around the core. The sudden restart of fusion generates an outward force that makes the star swell. The newly formed red giant has a tenuous and inflated atmosphere, a dense helium core, and a radius even hundreds of times larger than the original star.

The dense core makes red giants perfect candidates for asteroseismology. The pressure waves do not bounce off the core, but they are actually transformed into gravity waves. These waves travel across the interior of the stars and cause the red giants to oscillate in different patterns. One of these patterns is called dipole mode and can be observed as one side of the star becomes brighter and the other becomes dimmer.

The team worked out that if there’s a strong magnetic field present inside a star, the gravity waves become trapped in the core in what the researchers call a “magnetic greenhouse effect.” For example, the dipole mode variation across the star becomes less striking in the presence of a strong magnetic field.

NASA’s Kepler space telescope has detected dipole-damping mode in several red giants, and using the data the team showed that the most likely explanation was a magnetic greenhouse effect produced by an internal magnetic field up to 10 million times stronger than Earth’s own magnetic field.

The discovery is very significant. “As far as we are aware, this is the first time astronomers have been able to detect and measure magnetic fields deep within the cores of stars. The discovery will allow us to understand the impact of strong magnetic fields on the lives and (sometimes dramatic) deaths of stars.” Dr Jim Fuller, who co-led the study, told IFLScience.

Dr Matteo Cantiello, one of the co-authors, added: “In 1926 the astrophysicist Sir Arthur Eddington famously lamented the apparent impossibility of looking inside stars, a place he thought was 'less accessible to scientific investigation than any other region of the universe'. Nearly 90 years have passed and now we can even detect something as elusive as magnetic fields in stellar interiors. If Eddington was here today, he would be pretty amazed.” 

The research is published in the October 23 issue of Science. 

^{Image Credit: Red Giant Star by Maxwell Hamilton, via Flickr. CC BY 2.0}