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	21 cm line
		In physics and astronomy, the 21 cm line is the name for a highly forbidden emission line of hydrogen.
		The 21cm line (1420.40058 MHz) was first observed in 1951 by Ewen and Purcell who were closely followed by Muller and Oort and Christiansen and Hindman.
		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.
	publicliterature.org /en/wikipedia/2/21/21_cm_line.html (408 words)

	Hydrogen line - Wikipedia, the free encyclopedia
		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 21 cm.
		The line is used extensively in astronomy, particularly radio astronomy, as the line falls well within the radio spectrum.
		The 21 cm line can be used to determine structure within the denser sections of molecular clouds.
	en.wikipedia.org /wiki/21_cm_line (788 words)

	21-centimeter line
		An emission line in the radio region of the spectrum that is due to the "spin-flip" transition in neutral hydrogen atoms.
		As the atom flips from the parallel to the antiparallel state, it emits a radio-wave photon with a wavelength of 21.1 cm, equivalent to a frequency of 1,420 MHz.
		Most of what is known about the distribution of cold gas in the Galaxy, including the mapping of the nearby spiral arms, has come from detailed studies of the variation of 21-cm emission across the sky.
	www.daviddarling.info /encyclopedia/A/21cmline.html (209 words)

	Ask An Astronomer > Galactic Astronomy
		The astronomical study of the interstellar medium at the radio wavelength of 21 cm.
		line is considered as probably the most important achievement in the astronomical study of the interstellar medium.
		The higher energy level is due to parallel spins of the proton and electron (equivalent to bar magnets with opposite polarities in a single line) while the lower energy level is due to anti-parallel spins of the proton and electron (equivalent to bar magnets with same polarities on a single line).
	www.iiap.res.in /answers/galactic.html (616 words)

	The Hydrogen 21-cm Line (Site not responding. Last check: 2007-10-12)
		It is a description of the behavior of quantum mechanical angular momentum and does not have a definitive classical analogy.
		The observation of the 21cm line of hydrogen marked the birth of spectral-line radio astronomy.
		The prediction that the 21 cm line should be observable in emission was made in 1944 by Dutch astronomer H. van de Hulst.
	hyperphysics.phy-astr.gsu.edu /hbase/quantum/h21.html (297 words)

	The HI 21 cm Line
		The ground state of HI is split by the interaction of the proton and electron spins and this causes a hyperfine transition with a wavelength of 21.1061 cm (e.g.
		Garwood and Dickey (1989) observed the 21 cm line in absorption close to the Sgr B2 (M) source using the VLA.
		A value of 150 K was adopted for all the components along the line of sight and this probably represents an upper limit.
	www.mpifr-bonn.mpg.de /staff/epolehampton/thesis/node95.html (682 words)

	The Maryland-Green Bank Galactic 21-cm line survey (Site not responding. Last check: 2007-10-12)
		It was observed in 1971-72 with the newly resurfaced 92-m (300-foot) telescope of the National Radio Astronomy Observatory (NRAO) in Green Bank, W.V. The spatial resolution is 0.22 degrees (13 arcmin), and the velocity resolution is 2 km/s.
		The line profiles have 260 values of Brightness Temperature at 1 km/s intervals, and at spacings of 0.1 degree in longitude and 0.1 degree in latitude.
		At the edges of the survey, near l=11 to 12.5 degrees and l=233.5 to 235 degrees, not all Galactic longitudes for a given latitude were measured, so that cross sections at constant b have gaps in l.
	www.cs.wisc.edu /niagara/data/nasa/8047.xml (371 words)

	Reports Submitted to FAMSI - Roberto Lunagómez Reyes
		Excavation continued and at 44 cm the rim of another large vessel was found; therefore, an extension to the east of 1×1 m was undertaken.
		A N-S and E-W axis was established for the drawing of Vessel 2, and the excavation continued in quadrants of 44 cm; the northeastern quadrant was liberated and there, bone remains were found (Burial 1); the corresponding drawing was made, measures were taken, and the second vessel was covered with plastic.
		We began to descend to level IV (61-80 cm) where we observed a charcoal stain that proceeded to the bottom of the tomb.
	www.famsi.org /reports/03084/section03b.htm (1378 words)

	Rosalie Ewald - M.S. Roberts: 21 cm Line Widths of Galaxies (Site not responding. Last check: 2007-10-12)
		Rosalie Ewald - M.S. Roberts: 21 cm Line Widths of Galaxies
		M.S. Roberts: 21 cm Line Widths of Galaxies
		The upper six panels are representative of isolated spiral galaxies.
	spider.ipac.caltech.edu /staff/rosalie/HIR.html (38 words)

	Hydrogen
		Hα is the C line, Hβ the F line, and Hγ the G line.
		This discussion of the 21-cm line is closely related to the general phenomenon of magnetic resonance, which is of more general importance, as in modern medical imaging.
		Alternate lines were missing in the rotational spectrum of parahydrogen, as if only states of even total angular momentum J were present.
	www.du.edu /~jcalvert/phys/hydrogen.htm (7686 words)

	[No title] (Site not responding. Last check: 2007-10-12)
		The 21 cm line of H I is an example of a radio frequency spectral line that is suitable for Zeeman effect measurements.
		The splitting between the highest and the lowest levels is much smaller than the typical line width in most cases; the observed splitting is then proportional only to the parallel component of the magnetic field.
		This difference is the frequency derivative of the line profile with an amplitude proportional to (Bpar/del nu), where del nu is the line width.
	www.pa.uky.edu /~aips/PAGELINKS/mag.txt (344 words)

	radio astronomy: Galactic Sources of Radio Waves
		The first of these “radio lines” to be discovered was the line at a wavelength of 21 cm produced by the hydrogen atom (as opposed to the hydrogen molecule, which is composed of two atoms).
		The intensity of this line in the radiation from a given region is a direct measure of the amount of hydrogen there.
		The visible light is blocked off by the same interstellar material in which the hydrogen giving rise to a 21-cm line lies, so that the view of the galaxy is obscured in certain directions, particularly in the direction of the center of the galaxy.
	www.factmonster.com /ce6/sci/A0860624.html (380 words)

	Replies
		For example, the rotation curve of a disk is a line with a positive slope, because as you move outward on the disk, the angular velocity remains constant.
		This line is produced by a “flip” in the spin of an electron.
		If the line height at a given radius is linear velocity, that would mean that the outer parts of a galaxy (with its flat rotation curve) are indeed rotating much slower (angular velocity) than the inner parts after all, which doesn't sound like the problem befuddling astronomers at all.
	www.freerepublic.com /focus/fr/608580/replies?c=39 (407 words)

	Clark, Barry Gillespie (1964-04-04) The twenty-one centimeter hydrogen line in absorption. ...
		Clark, Barry Gillespie (1964-04-04) The twenty-one centimeter hydrogen line in absorption.
		In some cases there has been found a variation of the absorption across the face of a source, which, with the kinematical distance to the cloud in question, allowed a typical length to be estimated, which in turn allows estimations of densities and masses.
		This material is briefly analysed statistically, from which it is concluded that the distribution law for central optical depths is approximately [...] and that a line of sight, on the average, intersects 4.1 clouds per kiloparsec in the galactic plane.
	etd.caltech.edu /etd/available/etd-09112002-180017 (233 words)

	Physics Today June 2001
		He was the first person at the radiophysics lab to specifically study astronomy, even though the group was already one of the world's leading radio astronomy organizations.
		This was pioneering work--the first detection of a radio spectral line in an external galaxy.
		Frank was heavily involved in conceptual studies for the Parkes 210-foot radio telescope and pushed for an accurate (to 10-cm wavelength) surface for his planned 21-cm line research program.
	physicstoday.org /pt/vol-54/iss-6/p75.html (852 words)

	21-cm line observations of galaxies in the zone of avoidance. (Site not responding. Last check: 2007-10-12)
		21-cm line observations of galaxies in the zone of avoidance.
		We present H I 21-cm line observations of 543 galaxies at low galactic latitude in the Galactic Anticenter region of the Zone of Avoidance (4h<={alpha}<=8h, 0{deg}=<{delta}=<37{deg}).
		Line width measured at 50% of the peak level on each side of the profile
	www.cs.wisc.edu /niagara/data/nasa/AJ_113_905.xml (475 words)

	Prediction of 21 cm radiation. (Site not responding. Last check: 2007-10-12)
		The importance of a spectral line is that the frequency of the line will be shifted by the Doppler effect which means that the velocity of the gas can be measured.
		Oort assigned his student, H.C. Van de Hulst, the job of figuring out what radio spectral lines might exist and what their frequencies would be.
		Building a receiver to detect the predicted line proved to be a daunting task.
	www.gb.nrao.edu /fgdocs/HI21cm/21cm.html (441 words)

	Interstellar Medium and the Milky Way
		Among the broad lines that shift as the two stars orbit each other, you see narrow lines that do not move.
		The narrow lines are from much colder gas in the interstellar medium between us and the binary system.
		The intensity of the 21-cm emission line depends on the density of the neutral atomic hydrogen along the line of sight.
	www.astronomynotes.com /ismnotes/s3.htm (2498 words)

	VIRGOHI 21
		VIRGOHI 21 is the structure in the centre, connected by a bridge to NGC 4254 (near the top of the image).
		VIRGOHI 21 was first observed in the 21-cm line of neutral hydrogen with the Lovell telescope at Jodrell Bank and the detection was confirmed using the Arecibo telescope.
		Isaac Newton Telescope overlaid with contours of neutral hydrogen density (green) from observations with the
	www.naic.edu /~rminchin/virgohi21.html (355 words)

	Jonathan Pritchard : Radiation backgrounds from the first sources and the redshifted 21 cm signal (Site not responding. Last check: 2007-10-12)
		Observations of the redshifted 21 cm line offer a promising probe of early radiation backgrounds.
		Anisotropies in the 21 cm signal arise from fluctuations in the IGM temperature, density, and neutral fraction and through the Lyman alpha flux.
		Later, as the IGM is heated by X-rays, inhomogeneous heating leads to gas temperature fluctuations whose detection could constrain the luminosity and spectrum of the first X-ray sources.
	www.cfa.harvard.edu /cfa/tad/events/s_abstracts/19.html (156 words)

	GSH-138-01-94 (Site not responding. Last check: 2007-10-12)
		Each image corresponds to a wavelength that is 0.000058 cm shorter than that of the preceding image.
		According to the Doppler effect, a shift in wavelength of 0.000058 cm corresponds with a change of 0.824 km/s in velocity along the line of sight.
		A 3-dimensional spherical shell appears as a ring of emission on the sky because a line of sight near the outer perimeter intersects more of the shell than a line of sight through the centre.
	www.ras.ucalgary.ca /~stil/Shellmovie.html (427 words)

	SETHI@Berkeley-- A Piggyback 21-cm Sky Survey at Arecibo
		This feed sits on Carriage House 1, which means that during normal astronomical observations with the new Gregorian dome the feed scans across the sky at twice the sidereal rate.
		The feed used by this project is the line feed extending downward from the carriage house.
		This spectrum was taken along a line of sight near 3C192 (l,b)=(197.7,24.4).
	setiathome.ssl.berkeley.edu /~korpela/sethi_poster (804 words)

	[No title] (Site not responding. Last check: 2007-10-12)
		The mechanism responsible for the 21 cm HI line is the hyperfine splitting of the ground state of the neutral hydrogen atom.
		However, this process does not result in the 21 cm line, but, rather, ensures a ratio of spin-parallel to spin anti-parallel of about 3 to 1 [1].
		In summary, the APOD above shows the 21 cm line due to radiative transitions between hyperfine states in the ground state of the hydrogen atom [2].
	www.astro.indiana.edu /a520/apod3.doc (542 words)

	No Title (via CobWeb/3.1 planetlab2.cs.virginia.edu) (Site not responding. Last check: 2007-10-12)
		) relative to Earth, the central frequency of the H emission line is shifted by 4.74v kHz.
		The prediction of this radiation at 21 cm was made in 1944 by a Dutch Astronomy PhD student, Henk van der Hulst, and detected first in 1951 by Ewen Purcell from Harvard in 1952.
		Over the next 2 decades, the 21 cm line was used to map out the spiral structure in our and other galaxies.
	jupiter.phy.umist.ac.uk.cob-web.org:8888 /~tjm/ISPhys/l7/ispl7.html (547 words)

	Radio Telescopes (Site not responding. Last check: 2007-10-12)
		Radio telescopes are used for many purposes, but let us describe two here explicitly: the mapping of neutral hydrogen concentrations using the 21 "spin-flip" transition line of emission from hydrogen, and the use of multiple radio telescopes operated as if they were a single telescope of very high resolution.
		This neutral hydrogen forms clouds in which stars are born and in fact corresponds to about 90% of the atoms in the Galaxy.
		Here is a map of neutral hydrogen in the galaxy obtained by observation of the 21 cm line with radio telescopes.
	www.pas.rochester.edu /~blackman/ast104/radio.html (522 words)

	[40.03] Discovery and First Observations of the 21-cm Hydrogen Line (Site not responding. Last check: 2007-10-12)
		Unlike most of the great discoveries in the first decade of radio astronomy after World War II, the 21 cm hydrogen line was first predicted theoretically and then purposely sought.
		The story is familiar of graduate student Henk van de Hulst's prediction in occupied Holland in 1944 and the nearly simultaneous detection of the line by teams at Harvard, Leiden, and Sydney in 1951.
		The scientific and technical styles of the three groups will also be discussed as results of the vastly differing environments in which they operated.
	www.aas.org /publications/baas/v37n3/dps2005/799.htm (132 words)

	University of California Berkeley Astronomy Department (Site not responding. Last check: 2007-10-12)
		The nearest of these is the Eridanus superbubble, which was formed by the explosions of previous generations of Orion cluster stars.
		I am currently embarked on a very detailed and comprehensive study of this superbubble---the X-ray emission from the hot interior gas, the H-alpha and 21-cm line emission from the cold gas in the surrounding shell, and the molecular line emission from the denser portions of this shell.
		Zeeman splitting is revealed by exceedingly weak polarization in the spectral lines.
	astro.berkeley.edu /faculty_pgs/heiles.html (259 words)

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