Planetarium Bochum

The emission measure is defined as EM \(= \int n_{\mathrm {e}}^2 \mathrm {d}r\). Thus\(\mathrm {EM} = \int _{r_0}^{\infty } \left [ 1.55 \left ( \frac {r}{r_0} \right ) ^{-6} + 2.99\left ( \frac {r}{r_0} \right )^{-16} \right ]^2\, 10^{16} \mathrm {d}r = 1.65 \times 10^{8}\) cm−6 pc.

Iν = 2kTB∕λ2 and \(S_\nu =\int (2kT_{\mathrm {B}}/\lambda ^2) \mathrm {d}\varOmega =(2kT_{\mathrm {B}}/\lambda ^2) \varDelta \varOmega \).

In local thermodynamic equilibrium (LTE) the intensities of emitted and absorbed radiation are not independent but are related by Kirchhoff’s law (1.14). This applies to both continuous radiation and line radiation. The Einstein coefficients give a convenient means to describe the interaction of radiation with matter by the emission and absorption of photons.

Incoherent radiometers do not preserve phase; these operate as direct detection systems. The most common type of incoherent radiometers is the bolometer, used at millimeter wavelengths. Bolometers are basically very sensitive thermometers, so have no frequency or polarization specific response, wide bandwidths and are sensitive to both polarizations.

Abstract We begin with a summary of the relevant aspects of molecular line physics. In the next chapter we will present an overview of molecular line astronomy. For more complete accounts of molecular structure, consult the extensive treatments by Herzberg and Herzberg (1960), Bingel (1969), Townes and Schawlow (1975), Gordy and Cook (1970), Flygare (1978), and Kroto (1992).KeywordsDipole MomentPartition FunctionOptical DepthMolecular CloudColumn DensityThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

In the early days of radio astronomy the receiver sensitivities restricted measurements to the few hundred megahertz range. At such relatively low frequencies the resolving power of the available radio telescopes was low. Initially only very few of the discrete sources could be identified with objects known from the optical region of the spectrum. Further investigations showed an increase in the number of sources with decreasing source flux density and gave the distribution of sources in the sky. It was then concluded that there are two different families of sources: galactic sources, concentrated towards the galactic plane and extragalactic sources distributed more or less uniformly in space. The unresolved, spatially continuous radiation belongs to the galactic component. In addition, there is the 2.7K thermal background radiation which is cosmological in origin.