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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.
In its most common variant, 21 cm intensity mapping, the 21cm emission line of neutral hydrogen is used to trace the gas. The hydrogen follows fluctuations in the underlying cosmic density field, with regions of higher density giving rise to a higher intensity of emission.
Since a first study was published in 1986, the neutral hydrogen gas of NGC 5238 and its associated 21 cm line have been the subject of many studies. The first study calculated the total HI 21 cm flux from the galaxy to be 4.5 ± 1.0 Jy ·km/s, with a full width at half maximum of 28 km/s and a maximum flux density of 0.25 ± 0.011 Jy. [ 25 ]
Tianlai Arrays. The Tianlai experiment (Chinese: 天籁) is a radio astronomy experiment run by the National Astronomical Observatories, Chinese Academy of Sciences (NAOC). [1] [2] Its aim is to develop the key techniques of intensity mapping observation for the redshifted 21cm line of neutral hydrogen, in order to probe the large-scale structure, and to detect and measure the dark energy ...
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.
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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The "visible" hydrogen emission spectrum lines in the Balmer series. H-alpha is the red line at the right. Four lines (counting from the right) are formally in the visible range. Lines five and six can be seen with the naked eye, but are considered to be ultraviolet as they have wavelengths less than 400 nm.