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.
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