418 Nature Vol. 273 8 June 1978 production rate of comet Bennett (197011) from observations of its dust Quasar measure directly, as the normally ob­ tail and their results were found to be served part of the spectrum is shifted in 'excellent agreement with those luminosity out of the visible range. A very pro­ derived from ultraviolet observations... mising method for inferring lumin­ from F. Graham Smith osity from the intensity of emission Again, in these cases the icy cong~o~er­ lines has now been developed by ate model has given rise to predictions OBSERVATIONAL cosmology, although which have been later confirmed. Baldwin, Burke, Gaskell and Wampler by now a very respectable subject, is (see page 431, this issue of Nature) Such large produotion rates are still short of hard evidence about the seemingly incompatible with the not~on which may remove the difficulty. way in which the Universe is evolv­ They show that the luminosity of that the gas is produced by desorption ing. A particular difficulty is that from dust grains. For example, if the certain quasar emission lines in­ evolution affects the constituent creases as the third power of the con­ solar wind is the source of the de­ bodies as well as the smoothed densi­ sorbed gas it is difficult to account tinuum luminosity. They use the C IV ties of matter and energy, so that we line with rest wavelength 1,549 A for quantitatively for the production rates cannot easily test cosmological of hydrogen" and, more importantly, redshifts between 1.1 and 2.5, and models by simple comparisons of dis­ they show that the Mg II line at for the roughly equally strong produc­ tant and nearby galaxies. This is the tion of OH (ref. 13), as well as for the 2,800 A can be used in the same way main difficulty with tests involving for quasars with lower redshifts. In existence of such parent molecules as counting populations of galaxies at HCN, CHaCN, and H20 which have this way, and with few assumptions, different distances: a change in popu­ they can construct a 'Hubble been directly detected by radio tech­ lation at larger distances might be an niques15-'7. J.t should be noted in this diagram' for quasars covering most indication of youth rather than a of the available range of redshifts. respect that ice-coated grains cannot measure of the general expansion of survive a close perihelion passage; The results favour a high value of the Universe. the deceleration parameter q., which indeed, even a single large nucleus Counts of distant quasars do in fact must lose a layer of ice at least a excludes low-density models of the show very large evolutionary effects. Universe. This agrees with previous metre thick due to sublimation during A recent comparison between counts the perihelion passage with rm;n=0.5 but less reliable work on the relation of bright and faint quasars (Green & between redshift and luminosity, and a.u. The questions raised by these Schmidt Astrophys. J. Lett. 220, L1; modern observations seem to us to also with the evidence on populations. 1978) shows a population increasing But there are still some important constitute a serious, if not impossible, with distance, in all directions, as difficulty for the dust swarm hypothesis. provisos; it is conceivable that the distance to the power 1.6. This could luminosity is itself affected by the It is certa1nly not the case that the be due to an evolution in the average icy nucleus model is inconsistent with local mean density of the Universe, luminosity of quasars, or it could be or that it varies with cosmological observations of the contraction of due to a general cosmological evolu­ cometary comas as the comets approach epoch for some other reason. These tion of density. In either case it is provisos may be overcome eventually the Sun. The apparent contraction is further evidence against any hypothe­ a very obvious consequence of the when the physics of quasars become sis that quasars are 'local' objects. fact that the increase in gas production better understood. There is, however, a serious problem and in excitation of the coma of a This new approach, in which the in allowing for the evolution in lumin­ comet approaching the Sun is counter­ ·luminosity of a quasar can be osity if this observation is to be used balanced by a decrease in the lifetime measured from the intensity of for discriminating between olosed of the molecules and radicals due to emission lines, is important for the and open models of the Universe. dissociation and/or ionisation. For tentative support it gives for a closed A second test of cosmological example, a very roug~ an.alysis Universe; it also shows that ground­ indicates that, because the mtensity of models is obtained from a com­ based optical observations have a parison of the redshift and the solar ultraviolet and of the solar wind new and powerful way into critical 2 luminosity of quasars. This is par­ ftux both vary as 1/r , the characteristic cosmological questions. scale of the coma should vary ap­ ticularly useful if measurements can be made at large distances, that is, at proximately as rn, where n is about 2 F. Graham Smith is Director of the large redshifts. Unfortunately the (ref. 18). The observations are con­ Royal Greenwich Ob~·ervatory, Herst­ sistent with such a variation••-•• and luminosity is then very difficult to monceaux. arc not easily explained by the swarm 5. Code, A. P., Houck, T. E. & Lillie, C. F. hypothesis. It is also interesting to note 'building blocks' for the Solar System IA U Circ. 2201 (1970). that, although a dust swarm contracts is now widely acknowledged and has 6. Keller, H. U. Space Sci.. Rev. 18, 64I (I976). 7. Delsemme, A. H. & Swmgs, P. Ann. Astro­ in terms of its lateral dimensions, its stimulated efforts to perform in situ phys. IS, I (1952}. measurements from a space probe. A 8. Dclsemme, A. H. & Miller, D. C. Planet length actually increases with decreas­ Space Sci. I8, 717 (1970}. ing heliocentric distance. However, as rendezvous mission, even if it involves 9 Keller, H. U. Mitt. Astr, Ges. 30, 143 (1971}. w: Keller, H. U. & Thomas, G. E. Astron. pointed out by Delsemme'', isophotes a fast flyby, should be capable of pro­ Astrophy•·· 39, 7 (1975}. . of monochromatic brightness profiles viding most of the information we need 11. Drake, J. F. '" al. Astrophys. I. (s!'bmJtted}. 12. Sekanina, z. & Miller, F. D. Sctence 179, corresponding to neutral radicals arc to settle existing arguments and should 565 (1973). also give us quite new insights into the 13 Keller, H. U. & Lillie, C. F. Astron. A•tro- always nearly circular, barring rare . phys. 34, 187 (1974}. 165 cataclysmic events. It is difficult to nature of comets. Let us hope we do 14. Mendis, D. A. Astrophy•. Space Sci. 20, (1973}. understand how such results could be not have .to wait too long for the IS. Huebner, W. F., Snyder, L. F. & Buhl, D. realisation of such a mission, which is Icarus 23, 580 (1974}. explained by desorption of gas from 16. Ulrich, B. L & Conklin, B. K, Nature 248, an elongated swarm. certainly feasible and possibly relatively 121 (1974}. 17 Jackson, W. M., Clark, T. & Donn, B. The progress which has been made inexpensive. • · Proc. of 1 AU Col/. No. 25, 272, Greenbelt, during the past decade has been as­ Maryland, 1974 (1976}. d I Whipple F.-A.

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