NationMaster - Encyclopedia: Hydrogen line

The hydrogen line refers to the spectral line created by changes in the energy state of neutral hydrogen and occurs at 1420.40575 MHz, or a wavelength of around 21cm. Also known as 21 centimetre radiation, the line is used extensively in astronomy, particularly radio astronomy, as the line falls well within the radio spectrum. General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... MegaHertz (MHz) is the name given to one million (106) Hertz, a measure of frequency. ... A centimetre (American spelling centimeter, symbol cm) is a unit of length that is equal to one hundredth of a metre, the current SI base unit of length. ... A centimetre (American spelling centimeter, symbol cm) is a unit of length that is equal to one hundredth of a metre, the current SI base unit of length. ... Radio astronomy is the study of celestialphenomena through measurement of radio waves emitted by physical processes occurring in space. ...

Cause of the hydrogen line

Neutral hydrogen consists of a single proton orbited by a single electron. As well as their orbital motion, the proton and electron also have spin. Classically, this is analogous to rotational motion (like the Earth rotating as it orbits the Sun), but as they are quantum particles the concept has a slightly different meaning. The spin of the electron and proton can be in either direction - in the classical analogy they are rotating clockwise or anticlockwise around a given axis. They may both have their spin oriented in the same direction or in opposite directions. Because of magnetic interactions between the particles, a hydrogen atom that has the spins of the electron and proton aligned in the same direction (parallel) has slightly more energy than one where the spins of the electron and proton are in opposite directions (anti-parallel). The lowest orbital energy state of atomic hydrogen has hyperfine splitting arising from the spins of the proton and electron changing from a parallel to antiparallel configuration. This transition is highly forbidden with an extremely small probability of 2.9×10⁻¹⁵ s⁻¹. This means that the time for a single isolated atom of neutral hydrogen to undergo this transition is around 10 million (10⁷) years and so is unlikely to be seen in a laboratory on Earth. 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. Also, the lifetime can be considerably shortened by collisions with other hydrogen atoms and interaction with the cosmic microwave background. General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... // Properties [1][2] In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... e- redirects here. ... // Properties [1][2] In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... e- redirects here. ... In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. ... Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... The Sun is the star at the center of the Solar System. ... Fig. ... Thousands of particles explode from the collision point of two relativistic (100 GeV per ion) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... e- redirects here. ... // Properties [1][2] In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... Magnetic lines of force of a bar magnet shown by iron filings on paper In physics, magnetism is one of the phenomena by which materials exert an attractive or repulsive force on other materials. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... e- redirects here. ... // Properties [1][2] In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... e- redirects here. ... // Properties [1][2] In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... In atomic physics, hyperfine structure is a small perturbation in the energy levels (or spectrum) of atoms or molecules due to the magnetic dipole-dipole interaction, arising from the interaction of the nuclear magnetic dipole with the magnetic field of the electron. ... In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. ... // Properties [1][2] In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... e- redirects here. ... Parallel is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. ... A forbidden line is a spectral line emitted by atoms undergoing energy transitions not normally allowed by the selection rules of quantum mechanics. ... The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. ... The 64 meter radio telescope at Parkes Observatory A radio telescope is a form of radio receiver used in astronomy. ... WMAP image of the CMB anisotropy,Cosmic microwave background radiation(June 2003) The cosmic microwave background radiation (CMB) is a form of electromagnetic radiation that fills the whole of the universe. ...

The line has an extremely small natural width because of its long lifetime, so most broadening is due to doppler shifts caused by the motion of the emitting regions relative to the observer. The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. ...

Discovery

During the 1930s, it was noticed that there was a radio 'hiss' that varied on a daily cycle and appeared to be extraterrestrial in origin. After initial suggestions that this was due to the Sun, it was observed that the radio waves seemed to be coming from the centre of the Galaxy. These discoveries were published in 1940 and were seen by Professor J.H. Oort who knew that significant advances could be made in astronomy if there were emission lines in the radio part of the spectrum. He referred this to Dr Hendrik van de Hulst who, in 1944, discovered 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. The Sun is the star at the center of the Solar System. ... It has been suggested that Andromeda-Milky Way collision be merged into this article or section. ... Jan Hendrik Oort (April 28, 1900 – November 5, 1992) was an internationally famous Dutch astronomer. ... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ... Hendrik Christoffel van de Hulst (November 19, 1918 – July 31, 2000) was a Dutch astronomer. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ... In physics, the ground state of a quantum mechanical system is its lowest-energy state. ... Depiction of a hydrogen atom showing the diameter as about twice the Bohr model radius. ...

The 21cm line (1420.4 MHz) was first detected in 1951 by Ewen and Purcell at Harvard University. This discovery was quickly confirmed by Muller and Oort in the Netherlands, and by Christiansen and Hindman in Australia. After 1952 the first maps of the neutral hydrogen in the Galaxy were made and revealed, for the first time, the spiral structure of the Milky Way. Edward Mills Purcell (August 30, 1912 - March 7, 1997) was an American physicist who shared the 1952 Nobel Prize for Physics for his independent discovery (1946) of nuclear magnetic resonance in liquids and in solids. ... Harvard University (incorporated as The President and Fellows of Harvard College) is a private university in Cambridge, Massachusetts, USA. Founded in 1636,[2] Harvard is the oldest institution of higher learning still operating in the United States. ... It has been suggested that Andromeda-Milky Way collision be merged into this article or section. ...

Uses in radio astronomy

Luckily, the spectral line appears within the radio spectrum (in the microwave window to be exact). Light in this range can easily pass through the Earth's atmosphere and be observed from the Earth with little interference. Radio frequency, or RF, refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna. ...

Assuming that the hydrogen atoms are uniformly distributed throughout the galaxy, each line of sight through the galaxy will reveal a Hydrogen Line. The only difference between each of these lines is the doppler shift that each of these lines have. Hence, one can calculate the relative speed of each arm of our galaxy. The rotation curve of our galaxy has also been calculated using the 21-cm Hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy. The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ... The rotation curve of a galaxy; that is, the orbital velocity as a function of distance from the centre of the galaxy. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ... The rotation curve of a galaxy; that is, the orbital velocity as a function of distance from the centre of the galaxy. ... The velocity of an object is its speed in a particular direction. ...

Hydrogen line observations have also been used indirectly to calculate the mass of galaxies, to put limits on any changes over time of the universal gravitational constant and to study dynamics of individual galaxies. According to the law of universal gravitation, the attractive force between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. ...

Uses in cosmology

The line is of great interest in big bang cosmology because it is the only known way to probe the "dark ages" from recombination to reionization. Including the redshift, this line will be observed at frequencies from 200 MHz to about 9 MHz on Earth. It potentially has two applications. First, by mapping redshifted 21 centimeter radiation it can, in principle, provide a very precise picture of the matter power spectrum in the period after recombination. Second, it can provide a picture of how the universe was reionized, as neutral hydrogen which has been ionized by radiation from stars or quasars will appear as holes in the 21 centimeter background. According to the Big Bang theory, the universe emerged from an extremely dense and hot state (singularity). ... Physical cosmology, as a branch of astrophysics, is the study of the large-scale structure of the universe and is concerned with fundamental questions about its formation and evolution. ... Recombination usually refers to the biological process of genetic recombination and meiosis, a genetic event that occurs during the formation of sperm and egg cells. ... In Big Bang cosmology, Reionization is the process that reionized the matter in the universe after the epoch of galaxy formation. ... Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared with that of the Sun (left). ... MegaHertz (MHz) is the name given to one million (106) Hertz, a measure of frequency. ...

However, 21 centimeter experiments are very difficult. Ground based experiments to observe the faint signal are plagued by interference from television transmitters and the ionosphere, so they must be very secluded and careful about eliminating interference if they are to succeed. Space based experiments, even on the far side of the moon (which should not receive interference from terrestrial radio signals), have been proposed to compensate for this. Little is known about other effects, such as synchrotron emission and free-free emission on the galaxy. Despite these problems, 21 centimeter observations, along with space-based gravity wave observations, are generally viewed as the next great frontier in observational cosmology, after the cosmic microwave background polarization. Relationship of the atmosphere and ionosphere The ionosphere is the part of the atmosphere that is ionized by solar radiation. ... Synchrotron radiation is electromagnetic radiation, similar to cyclotron radiation, but generated by the acceleration of ultrarelativistic (i. ...

Other uses

The Pioneer plaque, attached to the Pioneer 10 and Pioneer 11 spacecraft, portrays the hyperfine transition of neutral hydrogen and used the wavelength as a standard scale of measurement. For example the height of the man and woman in the image are displayed as having a height eight times 21 cm, or 168 cm. Similarly the frequency of the hydrogen spin-flip transition was used for a unit of time. The position of the Sun is portrayed relative to 14 pulsars, with the rotation period of these objects being listed as a multiple of this frequency. It is theorized by the plaque's creators that an advanced civilization would then be able to use the locations of these pulsars to locate the Solar System at the time the spacecraft were launched. The illustration on the Pioneer plaque The Pioneer plaques are a pair of aluminum plaques which were placed on board the Pioneer 10 and Pioneer 11 spacecraft, featuring a pictorial message from humanity, in case either the Pioneer 10 or 11 are intercepted by extraterrestrial beings. ... Pioneer 10 was the first spacecraft to travel through the asteroid belt, and was the first spacecraft to make direct observations of Jupiter. ... Position of Pioneer 10 and 11 Pioneer 11 was the second mission to investigate Jupiter and the outer solar system and the first to explore the planet Saturn and its main rings. ... The Space Shuttle Discovery as seen from the International Space Station. ... Composite Optical/X-ray image of the Crab Nebula pulsar, showing surrounding nebular gases stirred by the pulsars magnetic field and radiation. ... Major features of the Solar System (not to scale; from left to right): Pluto, Neptune, Uranus, Saturn, Jupiter, the asteroid belt, the Sun, Mercury, Venus, Earth and its Moon, and Mars. ...

The 21 cm Hydrogen line is considered a favorable frequency to search for signals from another civilization, as part of the SETI program. The original paper by Guiseppe Cocconi and Philip Morrison proposed just such a search in their paper, Search for Extra-Terrestrial Intelligence.[1] This article is about the search for extra-terrestrial intelligence. ...

References

Cosmology

arXiv (pronounced archive, as if the X were the Greek letter Ï‡) is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science and biology which can be accessed via the internet. ... arXiv (pronounced archive, as if the X were the Greek letter Ï‡) is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science and biology which can be accessed via the internet. ... arXiv (pronounced archive, as if the X were the Greek letter Ï‡) is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science and biology which can be accessed via the internet. ... arXiv (pronounced archive, as if the X were the Greek letter Ï‡) is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science and biology which can be accessed via the internet. ... arXiv (pronounced archive, as if the X were the Greek letter Ï‡) is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science and biology which can be accessed via the internet. ... arXiv (pronounced archive, as if the X were the Greek letter Ï‡) is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science and biology which can be accessed via the internet. ... arXiv (pronounced archive, as if the X were the Greek letter Ï‡) is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science and biology which can be accessed via the internet. ...

External Links

Contents

Cause of the hydrogen line

Discovery

Uses in radio astronomy

Uses in cosmology

Other uses

References

Cosmology

External Links

See also

Contents

Cause of the hydrogen line var ACE_AR = {Site: '756743', Size: '300250'};

Discovery

Uses in radio astronomy

Uses in cosmology

Other uses

References

Cosmology

External Links

See also

Cause of the hydrogen line