Milky Way spiral arms may be larger than previously thought, study finds
A team of astronomers has discovered that the spiral arms of the Milky Way galaxy may extend further into space than previously known.
Using data from two telescopes orbiting high above Earth’s atmosphere—NASA’s Chandra, the most powerful X-ray telescope ever built, and the European Space Agency’s XMM-Newton—scientists made precise distance measurements of the dust clouds in the arms.
Researchers took advantage of rare, powerful gamma-ray bursts in distant galaxies. As the X-rays from these explosions traveled through the Milky Way, some of the light was reflected from the dust clouds, creating rings that could be measured with unusual precision.
An artist’s rendering of what the Milky Way galaxy looks like when its arms are further away in space. (NASA)
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“This is a very direct way, relying solely on geometry, to precisely measure distances to the spiral arms of the Milky Way,” said Italian PhD student Beatrice Vaia, who led the study. “Most other methods rely on assumptions about how the Milky Way rotates, and this is becoming increasingly uncertain in the outer regions of our galaxy.”
According to the data the team collected, the dust cloud in the farthest arm of the Milky Way is estimated to be approximately 3,500 light-years across.
Astronomers have known about the Milky Way’s arms for at least a century, but mapping them has always been difficult because Earth lies within one of them.
A view of the Milky Way Galaxy near the Mesquite Flat Sand Dunes in California’s Death Valley National Park on May 29, 2025. (Tayfun Coşkun/Anadolu via Getty Images)
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But the latest breakthrough in studying gamma-ray bursts—a method not hampered by Earth’s location within the galaxy—could have major impacts on how we conceptualize our home in the universe.
“The differences are small, but any review of these distances is important because they are so fundamental to understanding our galaxy,” said study co-author and PhD student Ilaria Fornasiero. “For example, this might mean that astronomers need to revise estimates of the galaxy’s mass, because this affects how wide the arms are.”
The Chandra X-ray Observatory being built on February 10, 1999. (NASA)
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The only drawback of the technique is that proper gamma-ray bursts are extremely rare. Researchers have found only a few in the last 25 years that are bright enough and positioned to measure the Milky Way’s spiral arms.
“We will continue to look for more,” said co-author Andrea Tiengo.
