What primary method is used to map the spiral arms by penetrating interstellar dust?

Answer

Studying radio waves emitted by cold hydrogen (21 cm)

Astronomers study the distribution of hydrogen gas, which emits radio waves at a 21 cm wavelength, because these radio waves can easily pass through the thick layers of dust obscuring visible light.

What primary method is used to map the spiral arms by penetrating interstellar dust?

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