Besides the well-known hydrogen line at 21 cm wavelength, the spectra of extraterrestrial radio sources may contain sharp lines characteristic of other atoms, ions, and small molecules.

With improved radio telescopes, hydrogen gas could also be traced in other galaxies.

Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H < sub > 2 </ sub > is CO ( carbon monoxide ).

Drake used a radio telescope 26 meters in diameter at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani near the 1. 420 gigahertz marker frequency, a region of the radio spectrum dubbed the " water hole " due to its proximity to the hydrogen and hydroxyl radical spectral lines.

Mariner 5 ( Mariner Venus 1967 ) was a spacecraft of the Mariner program that carried a complement of experiments to probe Venus ' atmosphere by radio occultation, measure the hydrogen Lyman-alpha ( hard ultraviolet ) spectrum, and sample the solar particles and magnetic field fluctuations above the planet.

This stage is best seen in the radio emission from neutral hydrogen atoms.

* the " hydrogen line ", aka the " 21 centimeter line ": 1420. 40575177 MHz, is used by many radio telescopes including The Big Ear in its discovery of the Wow!

For example, by far the most common molecule in the universe is H < sub > 2 </ sub > ( hydrogen gas ), but it does not have a dipole moment, so it is invisible to radio telescopes.

The MRI machine emits an RF ( radio frequency ) pulse that specifically binds only to hydrogen.

The radio included a balloon with water-activated hydrogen inflator and a fold-up / down metal frame box kite for which the flying line was an aerial wire.

Maps of the regions made with radio telescopes show large streams of neutral hydrogen connecting the two galaxies, also as a result of the encounter.

In the course of this solar work, Wild became interested in the radio spectrum of hydrogen and wrote up an internal report related to the potential for spectral lines in the solar bursts.

However, as the total number of atoms of neutral hydrogen in the interstellar medium is very large, this emission line is easily observed by radio telescopes.

Projects to survey the Zone of Avoidance at radio wavelengths, particularly using the 21 cm spin-flip emission line of neutral atomic hydrogen ( known in astronomical parlance as HI ), have detected many galaxies that could not be detected in the infrared.

The Dwingeloo 1 galaxy was discovered in 1994 by the Dwingeloo Obscured Galaxy Survey ( DOGS ), which searched for neutral hydrogen ( HI ) radio emissions at the wavelength of 21 cm from objects in the Zone of Avoidance.

Dwingeloo 2 was first detected at radio wavelengths from the 21 cm emission line of neutral atomic hydrogen ( known to astronomers as HI ) in the course of follow-up observations after the discovery of Dwingeloo 1.

After this line was discovered, he participated, with Jan Oort and Alex Muller, in the effort to use radio astronomy to map out the neutral hydrogen in our galaxy, which first revealed its spiral structure.

The receiver was tuned to wavelengths near 21 cm, which is the wavelength of radiation emitted naturally by interstellar hydrogen ; it was thought that this would be familiar, as a kind of universal standard, to anyone attempting interstellar radio communication.

The HIPASS survey, which was a blind radio survey for hydrogen spectral line emission, found five uncatalogued galaxies in the group and also identified five previously-catalogued galaxies as members.

An additional dwarf galaxy was identified as a group member in the HIDEEP survey, which was a more intensive radio survey for hydrogen emission within a smaller region of the sky.

Such deeply embedded objects can only be observed at infrared or radio wavelengths, usually in the light of hot molecular hydrogen or warm carbon monoxide emission.

Spiral galaxies contain clouds of neutral hydrogen and carbon monoxide which emit radio waves.

A Bohr model of the hydrogen atom, showing an electron jumping between fixed orbits and emitting a photon of energy with a specific frequency

Eventually, it would reach 3000K ( corresponding to a typical photon energy of 0. 3 eV and so a frequency of 7. 5 × 10 < sup > 13 </ sup > Hz ), and the photons would begin to be absorbed by the hydrogen plasma filling most of the universe, rendering outer space opaque.

Among the most stable frequency references in the world are caesium standards, including caesium fountains, and hydrogen masers.

Examples of primary time, ( i. e., frequency standards ) are caesium standards and hydrogen masers.

For example, at the microwave frequency, the microwave field causes the periodic rotation of water molecules, sufficient to break hydrogen bonds.

Proton precession magnetometers, also known as proton magnetometers, PPMs or simply mags, measure the resonance frequency of protons ( hydrogen nuclei ) in the magnetic field to be measured, due to nuclear magnetic resonance ( NMR ).

They were observed at the emission frequency of neutral hydrogen, 1, 420 MHz.

This spin-flip transition of a hydrogen atom from electron state spin up to electron state spin down can specify a unit of length ( wavelength, 21 cm ) as well as a unit of time ( frequency, 1420 MHz ).

Fourteen of the lines have corresponding long binary numbers, which stand for the periods of pulsars, using the hydrogen spin-flip transition frequency as the unit.

Briefly, replacing normal hydrogen ( protons ) by deuterium within a molecule causes the molecular vibrational frequency of X-H ( for example C-H, N-H and O-H ) bonds to decrease, which leads to a decrease in vibrational zero-point energy.

* Proton precession magnetometer, which measures the strength and / or direction of a magnetic field by measuring the resonance frequency of protons ( hydrogen nuclei ) in the magnetic field

In hydrogen, its wavelength of 1215. 668 angstroms ( 121. 6 nm or 1. 216 × 10 < sup >− 7 </ sup > m ), corresponding to a frequency of 2. 47 × 10 < sup > 15 </ sup > hertz, places the Lyman-alpha line in the vacuum ultraviolet part of ultraviolet in the electromagnetic spectrum.

Multilayer film filters can provide very precise control of the frequencies transmitted and blocked, so that, for example, objects can be viewed at a particular frequency emitted only by excited hydrogen atoms.

It sent a hydrogen maser, a highly accurate frequency standard, into space to measure the rate change of a clock in lower gravity with high precision.

The Lyman-alpha forest is an important probe of the intergalactic medium and can be used to determine the frequency and density of clouds containing neutral hydrogen, as well as their temperature.

He referred this to Dr Hendrik van de Hulst who, in 1944, predicted that neutral hydrogen could produce radiation at a frequency of 1420. 4058 MHz due to two closely spaced energy levels in the ground state of the hydrogen atom.

Similarly the frequency of the hydrogen spin-flip transition was used for a unit of time in a map to Earth included on the Pioneer plaques and also the Voyager 1 and Voyager 2 probes.

On this map, the position of the Sun is portrayed relative to 14 pulsars whose rotation period circa 1977 is given as a multiple of the frequency of the hydrogen spin-flip transition.

The 21 cm hydrogen line is considered a favorable frequency by the SETI program in their search for signals from potential extraterrestrial civilizations.

The frequency 1420 MHz is significant for SETI searchers because, it is reasoned, hydrogen is the most common element in the universe, and hydrogen resonates at about 1420 MHz, thus extraterrestrials might use that frequency on which to transmit a strong signal.