NASA’s Chandra telescope reveals Milky Way’s outer reaches may stretch farther than previously known

The scientists made precise distance measurements of dust clouds in the arms 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.

Researchers took advantage of rare, powerful gamma-ray bursts in distant galaxies. As X-rays from those bursts traveled through the Milky Way, some of the light bounced off dust clouds, creating rings that could be measured with unusual precision.

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“This is a very direct way — relying only on geometry — to precisely measure distances to the Milky Way’s spiral arms,” said Beatrice Vaia, the Italian Ph.D. student who led the study. “Most other methods rely on assumptions about how the Milky Way rotates, which become increasingly uncertain in the outer regions of our galaxy.”

The dust cloud in the most distant arm of the Milky Way was estimated to be about 3,500 light-years wide, according to the data the team collected.

Astronomers have been aware of the Milky Way’s arms for at least a century, but mapping them has always been difficult since Earth is positioned inside one of them.

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But the recent breakthrough with studying gamma-ray bursts — a method not hampered by Earth’s position inside the galaxy — could have massive implications on how we conceptualize our home within the universe.

“The differences are small, but any revision of these distances is important because they are so fundamental for understanding our galaxy,” said Ilaria Fornasiero, a Ph.D. student and co-author on the study. “For example, this could mean that astronomers have to revise estimates of the mass of the galaxy because that affects how wide the arms stretch.”

The only downside to the technique is that suitable gamma-ray bursts are rare. Researchers have found only a handful over the past 25 years that were bright enough and positioned in a way that allowed them to measure the Milky Way’s spiral arms.

“We will continue to be on the lookout for more,” said co-author Andrea Tiengo.