The 21-centimeter radiation easily passes through the interstellar dust clouds that prevent optical measurements from being made deep inside the galactic center, enabling the charting of the galaxy's spiral structure.
Hydrogen line:
A change in the energy state of neutral hydrogen atoms results in the formation of the hydrogen line, also known as the 21 centimeter line, H I line, or H I line. Its frequency is exactly 1420 in this electromagnetic radiation.
The charting of the galaxy's spiral structure is made possible by the 21-centimeter radiation's ability to easily pass through the clouds of interstellar dust that prevent optical investigations deep inside the galactic center.
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