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A hydrogen atom with proton and electron spins aligned (top) undergoes a flip of the electron spin, resulting in emission of a photon with a 21 cm wavelength (bottom) The hydrogen line, 21 centimeter line, or H I line [a] is a spectral line that is created by a change in the energy state of solitary, electrically neutral hydrogen atoms.
Detecting the 21 cm emission from this time, all the way through to the end of reionization, has been proposed as a powerful way of studying early structure formation. [9] This period of the Universe's history corresponds to redshifts of z ≈ 30 {\displaystyle z\approx 30} to z ≈ 6 − 12 {\displaystyle z\approx 6-12} , implying a frequency ...
For example, the 2 → 1 line is called "Lyman-alpha" (Ly-α), while the 7 → 3 line is called "Paschen-delta" (Pa-δ). Energy level diagram of electrons in hydrogen atom. There are emission lines from hydrogen that fall outside of these series, such as the 21 cm line.
The hydrogen line, 21 centimeter 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 1420.40575177 MHz, which is equivalent to the vacuum wavelength of 21.10611405413 cm in free space.
The waterhole, or water hole, is an especially quiet band of the electromagnetic spectrum between 1420 and 1662 megahertz, corresponding to wavelengths of 18–21 centimeters. It is a popular observing frequency used by radio telescopes in radio astronomy. [1]
Electromagnetic – the hyperfine transition of hydrogen, also known as the hydrogen line or 21 cm line 2.4 GHz: Electromagnetic – microwave ovens, wireless LANs and cordless phones (starting in 1998) 2.6–3.8 GHz: A common desktop CPU speed as of 2014 5.8 GHz: Electromagnetic – cordless telephone frequency introduced in 2003 10 10: 10 GHz
(H is the chemical symbol for hydrogen, and "I" is the Roman numeral. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionized—HII is H + in other sciences—III for doubly-ionized, e.g. OIII is O ++ , etc. [ 1 ] ) These regions do not emit detectable visible light (except in spectral lines from ...
Hyperfine structure gives the 21 cm line observed in H I regions in interstellar medium. Carl Sagan and Frank Drake considered the hyperfine transition of hydrogen to be a sufficiently universal phenomenon so as to be used as a base unit of time and length on the Pioneer plaque and later Voyager Golden Record.
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