From Earth, the galaxy seems almost still. Stars shift only slightly, and the Milky Way drifts across the night sky, creating an illusion of quiet stability.
But that calm is misleading. The galaxy is under constant strain, with gas in motion and forces at work every second to keep it from falling apart.
One of the most important of those forces is invisible. It is the magnetic field that runs through the Milky Way, helping the galaxy hold its shape and change over time.
Astronomers have known for decades that this magnetic field must exist. Without it, gas and dust would collapse inward under gravity and the galaxy would not survive in its current form.
What remained unclear was what that field actually looks like, or how it twists and shifts across the galaxy. That picture is finally coming into focus.
The galaxy’s invisible force
A team led by Dr. Jo-Anne Brown, a physics and astronomy professor at the University of Calgary, set out to map the Milky Way’s magnetic field across the northern sky.
“Without a magnetic field, the galaxy would collapse in on itself due to gravity,” said Dr. Brown. “We need to know what the magnetic field of the galaxy looks like now, so we can create accurate models that predict how it will evolve.”
This effort resulted in two newly published studies and something even more valuable for the future. The research produced a complete, high-quality dataset that astronomers around the world can now use.
Alongside that dataset is a new model that helps explain how the Milky Way’s magnetic field has changed over time.
Listening to the galaxy in radio waves
To build this map, the team used a new radio telescope at the Dominion Radio Astrophysical Observatory in British Columbia.
The telescope scans the sky at multiple radio frequencies, which allows scientists to detect subtle changes caused by magnetic fields in space.
“The broad coverage really lets you get at the details about the magnetic field structure,” said Dr. Anna Ordog, lead author of the first study.
The data came from a project known as the Global Magneto-Ionic Medium Survey, or GMIMS. This survey focuses on how magnetic fields interact with charged particles spread throughout the galaxy.
The result is one of the most detailed views ever created of the Milky Way’s magnetic environment.
Tracing the Milky Way’s magnetic field
The key measurement behind the work is an effect called Faraday rotation. It describes how radio waves change as they pass through magnetic fields and clouds of electrons in space.
“You can think of it like refraction. A straw in a glass of water looks bent because of how light interacts with matter,” said Rebecca Booth, a PhD candidate and lead author of the second study.
“Faraday rotation is a similar concept, but it’s electrons and magnetic fields in space interacting with radio waves.”
By carefully tracking these changes across the sky, the researchers could trace the direction and structure of the magnetic field.
A strange flip inside the Milky Way
One region stood out. The Sagittarius Arm of the Milky Way behaves differently from the rest of the galaxy.
“If you could look at the galaxy from above, the overall magnetic field is going clockwise,” said Dr. Brown. “But, in the Sagittarius Arm, it’s going counterclockwise. We didn’t understand how the transition occurred.”
“Then one day, Anna brought in some data, and I went, ‘O.M.G., the reversal’s diagonal!’”
That diagonal pattern was unexpected. It showed that the change in direction is not abrupt or simple.
Instead, it cuts across the galaxy at an angle, revealing a more complex structure than previous models suggested.
Booth expanded on this discovery by focusing on how the magnetic field flips direction within the Sagittarius Arm.
“My work presents a new three-dimensional model for the magnetic field reversal. From Earth, this would appear as the diagonal that we observe in the data,” noted Booth.
A starting point for future studies
Magnetic fields are always at work in a galaxy, even though we can’t see them. They help guide the movement of gas, influence where stars take shape, and affect how galaxies change over long stretches of time.
A detailed map of the Milky Way’s magnetic field gives scientists a dependable starting point for future studies, many of which will look far beyond our own galaxy.
What looks like empty space is anything but empty. Thanks to this work, the hidden structure holding the Milky Way together is no longer just a theory. It is now something astronomers can study, test, and build upon for years to come.
The full study was published in the journal The Astrophysical Journal.
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