HII regions, on-the-spot approximations

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On-the-spot approximations refer to rough estimates used in scientific calculations, where their accuracy is context-dependent. In HII regions, the temperature is primarily influenced by the strength of the radiation field rather than the density of the gas. This is because the heating mechanisms in these regions are dominated by radiation processes. The discussion highlights the importance of understanding the underlying physics when considering these approximations. Overall, the relationship between temperature and density in HII regions is clarified through the dominance of radiation.
sarahrose
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Does anybody know what on- the- spot approximations are and why they are accurate?

Also, I cannot figure out why the temperature of an HII region does not depend on its density. any hints??
 
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sarahrose said:
Does anybody know what on- the- spot approximations are and why they are accurate?

An on the spot approximation, to me, is just another phrase for rough approximation. Each approximation's accuracy needs to be considered in its own right.

Also, I cannot figure out why the temperature of an HII region does not depend on its density. any hints??

The heating of HII regions is dominated by radiation, and hence the temperature is defined by the radiation field strength
 

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