Masers in the Sky

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Science  01 Jul 2005:
Vol. 309, Issue 5731, pp. 71-72
DOI: 10.1126/science.1114855

In 1963, radio emission from interstellar OH (the hydroxyl radical [HN1]) was discovered. The emission patterns in the astronomical sources deviated considerably from expectations based on laboratory conditions. Two years later, researchers realized that some of the most peculiar emission properties of interstellar OH could only be explained in terms of maser amplification. (Maser is an acronym for microwave amplification by stimulated emission of radiation; masers operate on the same principles as lasers, except that they involve microwave radiation instead of visible light.)

Maser emission has now been detected from many different molecules in a variety of astronomical sources [HN2], from nearby comets to faraway galaxies. But the evidence for amplification is indirect in most cases. Observations from 18 pulsars [HN3], reported by Weisberg et al. [HN4] on page 106 of this issue (1), provide direct evidence for an interstellar amplifier in the direction of one of these pulsars, B1641-45. [HN5]

Every 0.455 seconds, B1641-45 emits a pulse of radio radiation toward Earth that passes through an OH cloud. Spectra [HN6] of the four OH ground-state lines detected from the cloud display absorption features in three lines and an emission feature in the fourth [see figure 3 in (1)]. When the pulsar is on, passage of its radiation through the cloud deepens the absorption features, just like the shadow cast by an object in front of a bright light. But the emission feature in the fourth line becomes stronger after passing through the intervening screen, the equivalent of an object amplifying a background light instead of casting a shadow.

Almost 30 years ago, Rieu et al. observed a similar effect for an OH cloud in front of a distant radio galaxy when they switched the telescope between pointing directly at the source and pointing away from it (2). [HN7] In both cases (1, 2), the input signals are amplified by only a few percent, but to date, these weak masers are the only unambiguous direct evidence for amplification. The pulsar method (1) increases confidence in the results, thanks to the repeated detection of signals that arrive more than twice every second.

The maser effect clearly occurs easily in interstellar space, even though it requires special effort on Earth. This disparity reflects fundamental requisites for laser and maser operation. Consider two states, A and B, of a system containing electromagnetic radiation (such as light or radio emission) and microscopic particles (such as atoms or molecules) (see the figure). The total energy is the same in both states; A contains an extra photon, whereas in B, one particle has moved from the lower to the upper level. The process that takes the system from A to B is called absorption and is the reason why the intensity of radiation is attenuated when it passes through matter. The process that takes the system from B to A is called stimulated emission—the interaction of radiation with particles leads to the emission of an additional photon. This process is the essence of the maser effect. As the reverse of absorption, stimulated emission amplifies the radiation. [HN8]

In any given source, the net balance of absorptions and stimulated emissions depends on the particle population distribution between the two levels. Under most circumstances, this distribution follows the rules of thermodynamic equilibrium, and the population of the upper level is so small that stimulated emissions can be ignored; the material attenuates radiation. But when the number of particles in the upper level exceeds that in the lower level, a situation called population inversion, [HN9] stimulated emissions outnumber absorptions and the radiation is amplified. Such an extreme distribution can occur only at low matter densities, and interstellar space thus provides an ideal setting; its density is so low that its densest regions are comparable to the best laboratory vacuum. Thus, thermodynamic equilibrium is the rule in terrestrial circumstances but is the exception in interstellar space, making the latter a natural environment for maser operation.

Maser radiation can be extremely bright; the temperature equivalent of brightness sometimes exceeds 1015 K. These intense beacons enable radio imaging with an angular resolution of 0.0003 arc sec; if the human eye had this resolving power, these words could be read from a distance of ~3000 miles. Separations between neighboring maser spots are measured with the even higher accuracy of 0.00001 arc sec.

Absorption versus stimulated emission.

Sketched are two states, A and B. Each state has two energy levels, with some particles populating each level. The frequency of the radiation (represented by waves) is matched to the energy separation between the levels, such that it can interact with the particles. The number of waves represents the radiation intensity, that is, the number of photons.

It was thanks to this resolving power that the existence of supermassive black holes [HN10] at the center of galaxies was established. Miyoshi et al. (3) found that the galaxy NGC 4258 [HN11] harbors a disk-like structure, with masers of water vapor revolving around the center just like the planets revolve around the Sun. From the orbital rotations, the authors determined that a central mass 7 × 107 times that of the Sun is contained in a region no larger than the solar system. No object other than a black hole can have such a high mass density.

Amplified radiation has distinct properties markedly different from those of ordinary radiation. As the pulsar observations of Weisberg et al. (1) demonstrate, even weak amplification can produce astounding effects. Comets [HN12] provide another example. When a comet approaches the Sun, the rising rate of heating causes ice evaporation and the production of OH, whose population distribution is controlled by interaction with solar ultraviolet radiation. Because the heliocentric velocity of the comet varies during its motion around the Sun, the Doppler effect shifts different solar lines into and out of match with the OH transition frequencies. At some locations the net outcome is population inversion, at others the opposite. As a result, the detected OH lines can oscillate between emission and absorption as the comet moves around the Sun. Since 1973, these striking oscillations have been observed in many comets (4).

Weak amplification can even offer advantages. Weak OH maser emission at 1720 MHz is always accompanied by absorption at the 1612 MHz transition, and the sum of these conjugate features is zero. When such cancellation is observed, the two detected features must originate from the same region, removing a critical uncertainty for sources at cosmological distances (distances of billions of light years). This approach has recently been used to place limits on the possible variation of fundamental constants, such as the electron-proton mass ratio, during the evolution of our universe (5). [HN13]

As demonstrated by galaxy NGC 4258, maser radiation can provide unique information about small details in the structure of the emitting astronomical sources. Strong maser radiation is emitted during both the very early and very late stages of the life of a star, providing invaluable information on stellar evolution. For example, recent analysis of methanol maser observations established the existence of a circumstellar disk around a newly formed star (6) [HN14]. The disk structure is extremely smooth, providing a glimpse of the state of our own solar system before the planets condensed out of its protoplanetary disk.

To fully understand the structure of an astronomical source, one needs data at different regions of the electromagnetic spectrum. Thanks to masers, data at radio wavelengths are much more detailed than at any other wavelength. Facilities are under construction and in the planning stages to extend high-resolution astronomy from radio to infrared and even visible wavelengths. As these regions of the electromagnetic spectrum are combined with maser observations, we will gain valuable insight into the detailed structure and inner workings of a wide range of astronomical objects.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

Dictionaries and Glossaries

A glossary of astronomical terms is provided by Astronomy Unbound, an online textbook provided by the Department of Physical Sciences, University of Hertfordshire, UK.

An astronomy glossary is provided by HubbleSite.

A glossary of physics and astronomy is provided by J. Schombert, Department of Physics, University of Oregon.

A glossary of astronomical terms is provided by LEVEL 5, a reference resource offered by the NASA/IPAC Extragalactic Database.

Web Collections, References, and Resource Lists

The Yahoo Directory provides a collection of links to Internet astronomy resources.

The Google Directory offers links to Internet resources on astronomy and astrophysics.

Academic Info provides links to astronomy and astrophysics resources.

AstroWeb is a database of astronomy-related Internet resources maintained by the members of the AstroWeb Consortium.

D. Meeks, Pima Community College, Tucson, provides links to educational astronomy resources.

The NASA Astrophysics Data System (ADS) makes available abstracts of astronomy and astrophysics journal articles and other resources.

The astrophysics section of the e-Print archive offers a searchable collection of preprints of astronomy and astrophysics papers.

Online Texts and Lecture Notes

HyperPhysics, a reference provided by C. R. Nave, Department of Physics and Astronomy, Georgia State University, includes a section of astrophysics concepts.

The Encyclopedia of Astrobiology, Astronomy, and Spaceflight is maintained by D. J. Darling.

The Fact Guru Astronomy Knowledgebase provides information on astronomy topics, arranged in a concept hierarchy.

Imagine the Universe from the High Energy Astrophysics Science Archive Research Center at the NASA Goddard Space Flight Center provides basic and more advanced educational presentations on astrophysical topics. A dictionary and links to Internet astronomy education resources are provided.

The NASA/IPAC Extragalactic Database (NED) includes Level 5, a knowledge base for extragalactic astronomy and cosmology.

The National Radio Astronomy Observatory (NRAO) offers an introduction to radio astronomy, a radio astronomy glossary, and an image gallery.

The Westerbork Observatory, Hooghalen, Netherlands, offers a presentation on radio astronomy.

Astronomy Notes is an online textbook by N. Strobel, Physical Science Department, Bakersfield College, CA. A glossary is provided.

Gene Smith's Astronomy Tutorial is made available by the Center for Astrophysics and Space Sciences, University of California, San Diego.

Stars and Galaxies is a hypertext astronomy text by R. McCray, Department of Astrophysical and Planetary Sciences, University of Colorado.

T. Herter, Department of Astronomy, Cornell University, offers lecture notes for an astronomy course.

J. H. Black, Chalmers Centre for Astrophysics and Space Science, Onsala Space Observatory, Sweden, provides lecture notes in PDF format for a course on the interstellar medium.

P. Armitage, Department of Astrophysical and Planetary Sciences, University of Colorado, provides lecture notes (in PDF format) for astrophysics 1 and astrophysics 2 courses.

General Reports and Articles

The 8 October 2004 issue of Science had an Enhanced Perspective by M. Claussen titled “Astronomical masers” and an Enhanced Perspective by R. L. Walsworth titled “The maser at 50.”

The 28 June 2002 issue of Science had a review by M. Wardle and F. Yusef-Zadeh titled “Supernova remnant OH masers: Signposts of cosmic collision.”

NED makes available an essay by S. Deguchi titled “Masers, interstellar and circumstellar” and an essay by M. Elitzur titled “Masers, interstellar and circumstellar: Theory.”

E. Herbst, Department of Physics, Ohio State University, makes available a 15 July 2002 C&EN article by R. L. Rawls titled “Interstellar chemistry,” as well as a 2001 review (PDF format) titled “The chemistry of interstellar space.”

Numbered Hypernotes

1. Hydroxyl in the interstellar medium. D. J. Darling's Encyclopedia of Astrobiology, Astronomy, and Spaceflight has an entry on the hydroxyl radical, as well as an overview of interstellar molecules. The Fact Guru Astronomy Knowledgebase includes a section on the hydroxyl radical. R. W. Pogge, Department of Astronomy, Ohio State University, provides lecture notes in PDF format on interstellar molecules for a course on the physics of the interstellar medium. The Department of Physics, University of Queensland, Australia, makes available lecture notes by J. E. Ross on the interstellar medium for a course on stellar astronomy. An 1981 article by D. K. Lynch titled “Masers and lasers” discusses the 1963 discovery of radio emissions from interstellar hydroxyl.

2. Astronomical masers. Maser is defined in the NRAO glossary. The Columbia Encyclopedia has an entry on masers. D. J. Darling's Encyclopedia of Astrobiology, Astronomy, and Spaceflight has entries for maser and interstellar maser. The Maser Group at Jodrell Bank Observatory provides an introduction to cosmic masers and links to related Web sites. A presentation on astrophysical masers is provided by P. Ghavamian. Department of Physics and Astronomy, Johns Hopkins University. Nan Dieter Conklin: A Life in Science, presented by NRAO, includes an account of the 1965 discovery of astronomical masers as reported in Nature, vol. 208, p. 29 (“Observations of a strong unidentified microwave line and of emission from the OH molecule” by H. Weaver, D. R. Williams, N. H. Dieter, and W. T. Lum).

3. Pulsars. Pulsars are defined in HyperPhysics. D. J. Darling's Encyclopedia of Astrobiology, Astronomy, and Spaceflight has an entry on pulsars. The National Maritime Museum, Greenwich, UK, makes available a presentation on pulsars from the Royal Observatory. Imagine the Universe has a presentation on pulsars. offers a tutorial on pulsars. The Pulsar Group at the Australian Telescope National Facility (ATNF) provides a pulsar tutorial. ATNF makes available a presentation on pulsar astronomy by M. Hobbs. The Italian Pulsar Group at the Cagliari Astronomical Observatory provides a pulsar tutorial. The 23 April 2004 issue of Science was a special issue on pulsars. The Pulsars Research Group at Jodrell Bank Observatory offers resources and Internet links on pulsars.

4. Joel M. Weisberg is at the Australia Telescope National Facility, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Epping, NSW, Australia, in the School of Physics, University of Sydney, and in the Department of Physics and Astronomy, Carleton College, Northfield, MN. Simon Johnston is at the School of Physics, University of Sydney, and at the Australia Telescope National Facility. Bärbel Koribalski is at the Australia Telescope National Facility. Snezana Stanimirovic is in the Department of Astronomy, University of California, Berkeley.

5. Pulsar B1641-45. The SIMBAD Astronomical Database has an entry for B1641-45. ATNF issued a 25 May 2005 press release about Weisberg et al.'s research titled “Discovery of pulsed OH maser emission stimulated by a pulsar.”

6. Spectra. D. J. Darling's Encyclopedia of Astrobiology, Astronomy, and Spaceflight includes entries on the spectrum and spectral lines. Imagine the Universe defines spectral line and related concepts and offers a presentation titled “Spectra and what scientists can learn from them.” The Hartebeesthoek Radio Astronomy Observatory, South Africa, provides an introduction to the spectra of masers.

7. Article by Rieu et al. The February 1976 issue of Astronomy and Astrophysics had an article (full text available from ADS in PDF format) by Nguyen-Q-Rieu et al. titled “OH radiation from the interstellar cloud medium.”

8. Stimulated emission. Wikipedia defines absorption (in optics) and has an article on stimulated emission. A discussion of stimulated emission is included in a presentation on masers on the Gravity Probe B Web site. Laser Fundamentals, a section of Molecular Expression's Optical Spectroscopy Primer, includes information on stimulated emissions.

9. Population inversion is defined in the Photonics Dictionary. HyperPhysics includes entries on population inversion and the interaction of radiation with matter. Wikipedia has an article on population inversion with a section titled “The interaction of light with matter.” Physics 2000 offers a presentation on population inversion.

10. Supermassive black holes. The Chandra X-Ray Observatory offers a presentation on supermassive black holes. Supermassive Black Holes is a student project prepared for a course on astronomy and space physics taught by S. G. Alexander, Department of Physics, Miami University, Oxford, OH. M. Camenzind, Landessternwarte, Königstuhl, Germany, offers a presentation on supermassive black holes. D. Merritt, Department of Physics and Astronomy, Rutgers State University of New Jersey, makes available in PDF format a June 2002 Physics World article by L. Ferrarese and D. Merritt titled “Supermassive black holes.” A. Brunthaler, Joint Institute for VLBI in Europe, Dwingeloo, Netherlands, makes available in PDF format a 2004 review by A. Brunthaler and H. Falcke titled “Supermassive black holes in the Universe.”

11. The discovery in galaxy NGC 4258. The Messier Catalog includes an entry for M 106 (NGC 4258). NED has an entry for NGC 4258. The 12 January 1995 issue of Nature had an article by M. Miyoshi, J. Moran, J. Herrnstein, L. Greenhill, N. Nakai, P. Diamond, and M. Inoue titled “Evidence for a black-hole from high rotation velocities in a sub-parsec region of NGC4258” (8). NRAO provides an image gallery entry for NGC 4258 and a 11 January 1995 press release titled “VLBA observations reveal tremendous mass concentration at the heart of strange galaxy.” The Harvard-Smithsonian Center for Astrophysics provides a February 1995 news release about the research titled “A black hole of Brobdingnagian proportions revealed by VLBA” and a presentation titled “The disk-jet system in heart of the active nucleus of NGC 4258.” L. Greenhill, Harvard-Smithsonian Center for Astrophysics, also offers a presentation about NGC 4258 and this research.

12. Comets. The University of Leicester's Educational Guide to Space & Astronomy includes a presentation on comets. Wikipedia provides an article on comets. N. Strobel's Astronomy Notes has a section on comets. D. Jewitt, Institute for Astronomy, Honolulu, offers a presentation on comets. J. Crovisier, Observatoire de Paris-Meudon, makes available a preprint of the October 2002 article by J. Crovisier et al. titled “Observations at Nançay of the OH 18-cm lines in comets. The data base. Observations made from 1982 to 1999” (4); also available is a preprint of a book chapter by Crovisier titled “The molecular complexity of comets.”

13. A recent use of a maser feature. The e-Print archive includes a preprint of the July 2004 article by N. Kanekar, J. N. Chengalur, and T. Ghosh titled “Conjugate 18 cm OH satellite lines at a cosmological distance” (5).

14. A circumpolar disk found around a new star. The e-Print archive includes a preprint of the 10 March 2004 article by M. R. Pestalozzi, M. Elitzur, J. E. Conway, and R. S. Booth titled “A circumstellar disk in a high-mass star-forming region” (6).

15. Moshe Elitzur is in the Department of Physics and Astronomy, University of Kentucky.


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