LET’S TALK ABOUT
LET’S TALK ABOUT... COSMIC MAGNETISM BY CASSANDRA CAVALLARO (SKAO) When you think about magnetism, your mind may return to school physics lessons, playing around with little magnets and iron filings. While magnetism is apparent to us at those very small scales, it also pervades the entire Universe. As one of the four fundamental forces of physics like gravity, it helps to govern everything around us on Earth. Our own magnetic field, the magnetosphere, acts like a protective blanket to shield us from harmful radiation from the Sun. The beautiful auroras that we see in the skies of both hemispheres are the result of this interaction – and these auroras have been spotted beyond Earth, too. We’ll come back to that later in the article. “Magnetic fields are everywhere!” says Dr George Heald of CSIRO Astronomy and Space Science, co-chair of the SKA’s Cosmic Magnetism Science Working Group. “Their influence can be felt in the space between objects, like at home when you feel the fridge door attracting a magnet before they are touching. When we talk about ‘magnetic fields’, it is a way to describe the ability for magnetic forces to influence objects or even light.”
Let’s start with the basics. Magnetic fields are generated whenever a charged particle like an electron or a proton moves, and space is full of moving charged particles – hence cosmic magnetism. “As far as we can tell, everything in the Universe is magnetic, or at least co-exists with magnetic fields,” George explains. “One of the big outstanding questions in astronomy is why magnetic fields are so strong and so organised across wide areas in space, especially inside objects like galaxies.” That strength is particularly extreme around the cores of dead stars (neutron stars or pulsars); we’re talking trillions of times stronger than on Earth. But that’s not the case everywhere. Vast regions between galaxies that we might think of as “empty” space still have magnetic fields, but here they may be a billion times weaker than on Earth.
“The study of magnetic fields in all these different environments is of paramount importance to track the full history of how magnetism has evolved in the Universe,” says Dr Valentina Vacca of INAF, co-chair of the working group alongside George. “Magnetic fields in evolved systems, like planets, stars, galaxies and clusters of galaxies, reflect this evolution, while those in more diluted environments such as filaments and voids between clusters of galaxies can give us precious information about more ‘pristine’ magnetic fields.” While there’s much we don’t yet understand about magnetism’s role in the Universe, we know that it influences star formation, affecting how many stars are created from gas clouds inside galaxies, and how massive they become. It’s also possible magnetic fields affect the evolution of galaxies, for example influencing the formation of the arms of spiral galaxies. Of course, we can’t see magnetic fields, so how do we study them? “The only way we have to see them is through their actions,” says Valentina. “There are three physical mechanisms we can observe at radio wavelengths, so radio telescopes allow us to study magnetic fields with great detail compared to other telescopes.”
Spiral galaxy NGC 628, around 32 million light years from Earth in the constellation Pisces. The flow lines overlaid on this optical image trace the orientation of the magnetic fields that run through the disk of this galaxy, as traced by radio observations. Credits: Optical image: Steve Mazlin and Vicent Peris, Calar Alto Observatory; Radio data: Mulcahy et al. 2017, Jansky Very Large Array (VLA) 22
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