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Hydrogen line
The
hydrogen line
, 21 centimetre line or
HI line
refers to the
electromagnetic radiation
spectral line
that is created by a change in the energy state of neutral
hydrogen
atoms. This electromagnetic radiation is at the precise frequency of 1420.405751786
MHz
, which is equivalent to the vacuum wavelength of 21.10611405413
cm
in
free space
. This wavelength falls within the
microwave
radio
region of the
electromagnetic spectrum
, and it is observed frequently in
radio astronomy
, since those
radio waves
can penetrate the large clouds of interstellar
cosmic dust
that are
opaque
to
visible light
.
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