Kingston- Astronomers at Queen’s University in Kingston have discovered for the first time that Polaris – more commonly known as the North Star – is host to a remarkable magnetic field. Stellar magnetic fields exert forces on charged particles in their atmospheres, impacting how the star evolves and changes over time. Magnetic fields can alter the winds of stars, their internal structure, and their rotation speed. Polaris is a member of a foundational class of pulsating stars called the classical Cepheids, which allow astronomers to measure extragalactic distances and study the expansion of our Universe. Little is known about the influence of magnetic fields on Cepheid behavior and evolution, as their magnetic fields are very difficult to detect. Understanding the magnetic field of Polaris will help researchers understand the mysteries of the universe.

Polaris, the brightest star in the constellation Ursa Minor, is the closest star to the north celestial pole, lying almost directly above Earth's rotational axis. Historically, Polaris has been essential to astronomical navigation and due to its immobility in the sky, it became a symbol of steadfastness and reliability in many cultures.  Polaris pulsates with a four day period during which its diameter, temperature, and brightness change. As the nearest and brightest Cepheid Polaris should be a reliable cornerstone in setting the extragalactic distance scale. However, over decades Polaris has shown puzzling changes in its pulsation which astronomers have been unable to satisfactorily explain and may be related to its magnetic properties.

Queen’s PhD candidate James Barron (Physics, Engineering Physics and Astronomy) is leading a team of Canadian and international astronomers in exploring the magnetism of the classical or Type I Cepheids. It was during an observing run at the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii that Barron discovered a singular magnetic field of Polaris. Their discovery was recently published in the journal Monthly Notices of the Royal Astronomical Society. 

“I was analyzing the data that had been obtained the night before and I was excited to see that Polaris’ magnetic field had been observed. When I saw the results, I couldn’t believe my eyes! I actually checked with my thesis advisor that the results were real,” said Barron.

Similar to how the Earth has a magnetic field that can be detected with a compass, many stars are also hosts to magnetic fields. Stellar magnetic fields are generated by the motion of charged particles inside the star's interior in what is known as a convective dynamo. We see the powerful effects of magnetic fields in the sun through observable phenomena like sunspots and solar flares.

“This is the first magnetic field detection of Polaris and the signal is detected very clearly,” says Barron. “It is also a very different magnetic signature than we see in the other Cepheids and this may be due to its low amplitude pulsation, or differences in its evolution. It is also unclear whether its magnetic properties are related to its strange variability. These are all questions we want to explore.”

Barron and his team have started to monitor Polaris by obtaining more observations at CFHT. This will allow them to look for variations in the magnetic field over long periods of time. Ultimately, the aim is to combine the observations to develop a `map’ of its magnetic field by taking many observations as it rotates across our field of view.

 “Mapping Polaris’ magnetic field will provide the first view of the global magnetic structure of any Cepheid, and will serve as the basis for a deeper theoretical understanding of the role of magnetic fields in Cepheid evolution and behaviour” says Gregg Wade, Professor, Astromony, Astrophysics and Relativity, Physics, Engineering Physics and Astronomy at Queen’s University.

The findings are available in the journal Monthly Notices of the Royal Astronomical Society.

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