![]() The data on the extinction curves of starburst galaxies, however, show no sign of any significant 2175 Å ultraviolet absorption. The Spitzer telescope data (top frame) clearly show strong PAH features very similar in strengths and wavelengths to similar bands that occur in sources within the Milky Way. This is illustrated in the bottom frame of Fig. 1.4. The argument that these bands are independent of the 2175 Å dust feature is strengthened by observations of these bands in starburst galaxies, in which no 2175 Å band is seen ( Catzetti et al., 2000). ![]() They are present in galactic as well as extragalactic sources extending to redshifts of up to at least z = 2. The infrared data are mainly in the form of absorptions at 3.3, 3.4, 6.2, 6.7, 8.6, 11.2, and 12.7 μ m. In addition to data on the ubiquitous 2175 Å ultraviolet absorption band discussed in Section 4, infrared, microwave and radio observations have shown the ubiquitous presence of organic molecules including PAHs. Infrared, microwave and radio observations are used to detect the presence of such molecules and the current list of positive detections is likely to be constrained only by limitations of techniques. This, in our view, is extremely improbable and poses a problem for abiotic explanations of the data.ĭetections of interstellar organic molecules of ever-increasing complexity have continued in the last two decades following the deployment of newer and better instruments and telescopes. More recent studies of other comets have yielded generally similar results.Īny alternative nonbiological explanation of the points in Fig. 1.3 would involve abiotically formed organic molecules possessing functional groups that fortuitously matched biology. The solid curve is for nonirradiated bacteria the dashed curve is for X-ray irradiated bacteria. The midinfrared spectrum ( Fig. 1.3: right panel) of the comets dust coma following a major outburst on Ma( Wickramasinghe and Allen, 1986) showed unambiguous evidence of aromatic-aliphatic linkages (C–H stretching modes) that were uncannily consistent with desiccated E. coli ( Wickramasinghe et al., 1986). ![]() The discovery of organic dust in comets came with the last perihelion passage of Comet Halley in 1986 ( Wickramasinghe and Allen, 1986). Right panel: Emission by dust coma of Comet Halley observed by Dayal Wickramasinghe and David Allen on Ma(points) ( Wickramasinghe and Allen, 1986) compared with normalized fluxes for desiccated E. coli at an emission temperature of 320 K( Wickramasinghe et al., 1986). Left panel: Comparision of the normalized flux from GC-IRS7 obtained in 1980 ( Allen and Wickramasinghe, 1981) with the laboratory spectrum of dessicated Escherichia coli ( Hoyle et al., 1982a,b). The distance from the Sun of a body with a parallax angle of 1 arcsecond was thus defined as a unit and, thanks to Turner, named the parsec.Figure 1.3. Once the parallax angle is established you can calculate the distance to a star using trigonometry, because we know Earth’s distance from the Sun. The parallax angle is obtained by halving the angular difference in measurements. The parallax angle is found by measuring the parallax motion (or apparent movement of a star relative to stable, more distant stars) when the star is observed from opposite sides of the Sun (an interval of six months on Earth). Astronomers used trigonometry to calculate the distance to stars long before the term parsec was coined, but the new unit made it easier to conceptualise unfathomable distances.Ī parsec is the distance from the sun to an astronomical object which has a parallax angle of one arcsecond (1/3600 of a degree).
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