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Thermal Emission Nebulae

There are many emission nebulae where vast clouds of hydrogen are ionised by hot O and B type stars forming within them. The intense UV radiation from the stars ionises the gas at temperatures of around 10,000°K and this causes the clouds to emit broad spectrum or 'continuum' radiation with thermal spectral characteristics as described in section 2.2.1. One of the best examples of a thermal emission nebula is the Orion nebula depicted in Figure 9.1.

Figure 9.1 The Orion Nebula (HST image)

The whole nebula is about 0.5° across (1) and the ionisation energy comes from the Trapezium cluster of bright stars containing two O stars and several B stars. There is also a large amount of obscuring gas and dust which is clear in the optical image. The professional radio image from the NARO / AUI telescopes in Figure 9.2 a & b was generated at a frequency of 8.4GHz (3.6cm). Figure 9.2a is centred on RA: 05:35:17.40, Dec: -5:23:28.00 and has a field of view of 0.66° square. It shows the hydrogen nebula without the obscuring dust as this is transparent at GHz frequencies.

Figure 9.2 a Orion nebula @ 8.4GHz
Figure 9.2b Central region of nebula


The thermal emission spectrum of the nebula is shown in Figure 9.3 where the measured values agree with a theoretical prediction based an electron temperature of 10,000°K. It is clear that the amateur radio astronomer would have the best chance of detecting the Orion nebula at wavelengths around 10cm or smaller. A C band satellite TV antenna and feed, such as that shown in Figure 5.1 could be used to make observations at around 4 GHz (7.5cm). The expected signal strength would be of the order of 500Jy.

Figure 9.3 Thermal spectrum of the Orion nebula

© Dr David Morgan 2011