Wiktionary
n. (context physics astronomy English) a spectral line of atomic hydrogen having a wavelength of about 21cm; used extensively in radio astronomy
Wikipedia
The hydrogen line, 21-centimeter line or H I 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 , which is equivalent to the vacuum wavelength of 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.
The microwaves of the hydrogen line come from the atomic transition of an electron between the two hyperfine levels of the hydrogen 1 s ground state that have an energy difference of . The frequency, , of the quanta that are emitted by this transition between two different energy levels is given by the Planck–Einstein relation . The constant of proportionality, , is known as the Planck constant.
Usage examples of "hydrogen line".
Many species have used the cold hydrogen line to search for other minds circling other stars.
Blue flames spurted from the engine compartment as a hydrogen line began to burn.
I've looked in the twenty-one centimeter hydrogen line, which everybody says is the obvious beacon frequency -- because hydrogen is the most abundant atom in the universe, and so on.
I've looked in the twenty-one centimeter hydrogen line, which everybody says is the obvious beacon frequency--because hydrogen is the most abundant atom in the universe, and so on.
At one end of the microwave window, at a wavelength of 21 centimeters, is the neutral hydrogen line.
They sent messages by modulating a hydrogen line in the spectrum of the nearest star.
He played a four-hundred-year-old Fender Stratocaster with all the switchgear jammed full on, through a stack of Luthos amps - each one with a guaranteed output of half a kilowatt - and a battery of Hydrogen Line thirty-inch speakers.