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The Milky Way's Magnetism Is Messier Than We Thought, Detailed Spiral Arm Mapping Reveals

The galactic magnetic field is about 100,000 times weaker than the Earth’s, but that doesn’t mean it’s not important.

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Stephen Luntz

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Stephen Luntz

Freelance Writer

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

Edited by Francesca Benson
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Francesca Benson

Copy Editor and Staff Writer

Francesca Benson is a Copy Editor and Staff Writer with a MSci in Biochemistry from the University of Birmingham.

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A portion of the Sagittarius Arm, marked in this map, appears out of line with the ancipated overall field, probably as a result of past events like supernovae

A portion of the Sagittarius Arm, marked in this map, appears out of line with the ancipated overall field, probably as a result of past events like supernovae.

Image Credit: NASA/JPL-Caltech/Federal University of Rio Grande do Sul

Our galaxy has a weak – but immense – magnetic field that stretches across most of it. Although we know its broadest outline, the fine-scale detail is a mystery. Now, the magnetism or a small portion of has been revealed at finer resolution, revealing there is a lot more jumble to it than smooth models previously suggested.

The galactic magnetic field is not strong enough that you could use it to stick something to your fridge, let alone generate electricity from a turbine. Nevertheless, it shapes the way stars and planets form by causing the raw material to clump more than gravity would on its own. The field polarizes light passing through it, which is how we have detected and measured it.

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Unfortunately, when we look through the galaxy, we see a combined effect of all the fields in our line of sight, rather than a three-dimensional map. 

“Until now, all observations of magnetic fields within the Milky Way led to a very limited model that was uniform all over and largely matched the disc shape of the galaxy itself,” said study author Dr Yasuo Doi of the University of Tokyo in a statement

The fact stars and planets can produce local fields that are much stronger (and usually in other directions) than the galactic field is known, and the field around some stars has been measured. There’s been a gap, however, between detecting specific local fields and the large-scale shape, with little idea of what the field looked like on scales of tens or hundreds of light years.

Doi and colleagues combined data from the Gaia satellite and Earth-based measurements of polarized light to find signs of magnetism at finer scales. To do so over the whole galaxy would be an epic task – so the team focused on a portion of the Sagittarius Arm, one of the four great spiral arms of the galaxy. The Sun and Earth are located in the smaller Orion-Cygnus spur, possibly an offshoot of the major Perseus Arm, but it’s much harder to map something you’re inside compared to mapping a neighbor.

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The team measured the polarization of hundreds of stars within their chosen field and used Gaia to locate these stars precisely. This allowed them to identify the contributions made by five vast clouds of magnetized gas within the field.

Each cloud has a field that is smooth on scales of 15-30 light years and greater, but is often orientated quite differently from the galaxy as a whole.

The white linesshow the polarization, which correlates with the orientation of local magnetic field lines. It reveals the galactic field is far from homogenous and combined, this information builds a detailed map of the magnetic field in the Sagittarius arm of the galaxy.
The white lines show the polarization, which correlates with the orientation of local magnetic field lines. It reveals the galactic field is far from homogenous and combined, this information builds a detailed map of the magnetic field in the Sagittarius arm of the galaxy.
Image Credit: Doi et al/The Astrophysical Journal CC-By-ND



Three clouds within the Sagittarius arm have fields with broadly similar alignments to each other (40°-58° away from Galactic north), but another cloud is at roughly right angles to these three. A fifth cloud, that lies between us and the Sagittarius arm, has an angle similar to the outlier among the Sagittarius clouds. That puts the clouds up to 60° out of alignment with the galactic plane, with which the galactic magnetic field is thought to align. Their direction probably reflects the effects of some major past event, such as an ancient supernova explosion that left a magnetic legacy.

“I am personally intrigued by the foundational process of star formation, pivotal to the creation of life, including ourselves, and I aim to grasp this phenomenon in its entirety with time,” Doi said. For that, he thinks it is necessary to understand galactic magnetic field lines better, and hopes to do more mapping of the way they cause gas to accumulate prior to the birth of stars.

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The study is published open access in The Astrophysical Journal.


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spaceSpace and PhysicsspaceAstronomy
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  • Milky Way,

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  • polarized light,

  • Astronomy,

  • spiral arm,

  • galactic magnetic field

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