much more accurate than other distance determinations, and gives us information about the galactic distance scale (e.g. the distance of spiral arms). Open issues Unlike terrestrial lasers and masers for which the excitation mechanism is known and engineered, the reverse is true for astrophysical masers. In general, astrophysical masers are discovered empirically then studied further in order to develop plausible suggestions about possible pumping schemes. Quantification of the transverse size, spatial and temporal variations, and polarisation state (typically requiring VLBI telemetry) are all useful in the development of a pump theory. Galactic formaldehyde masing is one such example that remains problematic. On the other hand, some masers have been predicted to occur theoretically but have yet to be observed in nature. For example, the magnetic dipole transitions of the OH molecule near 53 MHz are expected to occur but have yet to be observed, perhaps due to a lack of sensitive equipment. See also References Weaver H., Dieter N.H., Williams D.R.W., Lum W.T. 1965 Nature 208 29–31 Davis R.D., Rowson B., Booth R.S., Cooper A.J., Gent H., Adgie R.L., Crowther J.H. 1967 Nature 213 1109–10 Cheung A.C., Rank D.M., Townes C.H., Thornton D.D., Welch W.J., Crowther J.H. 1969 Nature 221 626–8 Snyder L.E., Buhl D. 1974 Astrophys. J. 189 L31–33 Ball J.A., Gottlieb C.A., Lilley A.E., Radford H.E. 1970 Astrophys. J. 162 L203–10 Wilson W.J., Darrett A.H. 1968 Science 161 778–9 Knowles S.H., Mayer C.H., Cheung A.E., Rank D.M., Townes C.H. 1969 Science 163 1055–57 Buhl D., Snyder L.E., Lovas F.J., Johnson D.R. 1974 Astrophys. J. 192 L97–100 Whiteoak J.B., Gardner F.F. 1973 Astrophys. Lett. 15 211–5 Baan W.A., Wood P.A.D., Haschick A.D. 1982 Astrophys. J. 260 L49–52 Cohen R.J. Rep. Prog. Phys. 1989 52 881–943 Elitzur M. Annu. Rev. Astron. Astrophys. 1992 30 75–112 Footnotes Radio astronomy maser Astrochemistry