Astronomical radiation

If we pass white light through a vapor composed of the atoms of an element, we see its absorption spectrum, a series of dark lines on an otherwise continuous spectrum (Fig 1.11). The absorption lines have the same frequencies as the lines in the emission spectrum and suggest that an atom can absorb radiation only of those same frequencies. Absorption spectra are used by astronomers to identify elements in the outer layers of stars. 

What Is Interferometry (1.3) Interferometry deals with the physical phenomena which result from the superposition of electromagnetic (e.m.) waves. Practically, interferometry is used throughout the electromagnetic spectrum astronomers use predominantly the spectral regime from radio to the near UV. Essential to interferometry is that the radiation emerges from a single source and travels along different paths to the point where it is detected. The spatio-temporal coherence characteristics of the radiation is studied with the interferometer to obtain information about the physical nature of the source. 

We still need to consider the coherence properties of astronomical sources. The vast majority of sources in the optical spectral regime are thermal radiators. Here, the emission processes are uncorrelated at the atomic level, and the source can be assumed incoherent, i. e., J12 = A /tt T(ri) (r2 — ri), where ()(r) denotes the Dirac distribution. In short, the general source can be decomposed into a set of incoherent point sources, each of which produces a fringe pattern in the Young s interferometer, weighted by its intensity, and shifted to a position according to its position in the sky. Since the sources are incoherent. 

It is still unclear what kind of radiation sources can lead to asymmetric reactions. Jeremy Bailey from the Anglo-Australian Observatory in Epping, Australia, investigated which astronomical objects could be considered radiation sources (Bailey et al., 1998 Bailey, 2001). It was possible in laboratory experiments to generate a small enantiomeric excess of some amino acids by using circularly polarized UV light (Norden, 1977). This asymmetric photolysis involves photochemical decomposition of both d- and L- enantiomers, but at different rates, so the more stable form tends to survive. This process must be subject to autocatalytic multiplication. 

On the last three decades, several space experiments with parts at very low temperatures have been flown. Among these, we mention IRAS (Infrared Astronomical Satellite) launched in 1983 (see Fig. 14.1), COBE (Cosmic Background Explorer) launched in 1989, ISO (Infrared Space Observatory) launched in 1995 and Astro-E (X-ray Observatory), launched in 2000 with instrumentation at 65 mK [35], Some cryogenic space missions are in the preparation or in final phase in Europe, USA and Japan. For example, ESA is going to fly Planck (for the mapping of the cosmic background radiation) and Herschel (called before FIRST Far Infrared and Submillimetre Telescope ) [36], These missions will carry experiments at 0.1 and 0.3 K respectively. 

The astronomical calorimeters for the detection of the infrared radiation (usually called bolometers) do not conceptually differ from the cryogenic detectors used in nuclear physics as those just described for CUORICINO. 

Much of the electromagnetic spectrum has been used to investigate the structure of matter in the laboratory but the atmospheric windows restrict astronomical observations from Earth. Irritating as this is for astronomers on the ground, the chemical structure of the atmosphere and the radiation that it traps is important to the origins of life on Earth. The light that does get through the atmosphere, however, when analysed with all of the tools of spectroscopy, tells the molecular story of chemistry in distant places around the Universe. 

Visible astronomy does, however, provide most of the atomic and black body spectra of stars and astronomical objects and is of course appealing to us because the human eye is uniquely adapted to detection in the wavelength range 300-800 nm. The appeal of colour pictures has lead to the development of false colour scales used routinely by astronomers to visualise the intensity of radiation at other wavelengths. The concepts of temperature and colour are linked by the black body radiation and it... 

When a Rydberg atom reduces its principal quantum number by one unit, when emitting a photon, the light is in the microwave region of the electromagnetic spectrum. With this radiation isolated Rydberg atoms can be observed in interstellar space, where interatomic collisions are rare. Atoms with n up to 350 have been observed by radio astronomical methods. 

Today we can no longer ignore the idea that the extreme specialisation of the human eye is a severe handicap when surveying the Universe as a whole. For many stars are much colder or much hotter than our own. Not to mention sources of non-thermal radiation, whose spectra fall a long way outside the frequency bands in which stars tend to radiate. If we wish somehow to perceive this luminous otherness, then we must invent new tools, both mental and experimental, so that we may call ourselves astronomers of the invisible. 

The air renders us insensitive to radiation from space, some forms of which are lethal, rather like a skin-protecting cream. At the same time it acts as a censor with regard to the astronomical information carried by that radiation. There is no choice therefore but to break out from the cocoon, to rise above the atmosphere by means of stratospheric balloons, rockets and satellites. With its airborne and space-borne telescopes, the whole planet Earth is turned towards the Universe, its eye emerging from the air like a bather from the sea. However, the sky is... 

The choice of visible or invisible colours, i.e. the range of wavelengths, in which an object or class of objects will be observed, is carefully premeditated. Pointing an infrared telescope towards an interstellar cloud, seeking out this gentle radiation, so red that it cannot be seen, the astronomer becomes sensitive to star birth, or emissions from newborn stars letting out their first cry of light from a dusty and cloudy placenta. 

Until 1968, astronomers had always assumed that the interstellar medium was essentially made up of atomic hydrogen. Indeed, this ubiquitous element leaves its trace in every quarter in the form of a specific radiation line at wavelength... 

By comparing calculated values with the actual content of these various elements in the oldest astronomical objects, we deduce that the density of nuclear matter cannot exceed 5% of the critical density. Now it so happens that the best cosmological theory to date, the theory of cosmological inflation, predicts that the Universe has exactly the critical density. This conclusion is supported by recent observations of remote supernovas and the relic background radiation. 

For most astronomers, the solution to these cosmological problems resides in a combination of various methods. The luminosity-redshift test must be combined with independent techniques, such as anisotropies in the cosmic background radiation and statistical study of gravitational lenses. 

Several different types of this dust are distinguished by astronomers. On average, interstellar dust resides in widely separated diffuse clouds. But there are also dense regions of gas and dust into which little ultraviolet radiation can penetrate, thereby providing an environment for the formation of complex molecules these are referred to as molecular clouds. Clouds of particles expelled by cooler stars into the regions around them are called circumstellar... 

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