[AZ-Observing] Re: NGC 1788 status

>...the brightest and also earliest-type (i.e. hottest) stars involved 
> have spectral types of B9V. This means they are not hot enough to 
> fluoresce the gas in the cloud, and thus you have a reflection nebula.

How hot do they have to be? Where's the line drawn between hot enough to
flouresce gas and not hot enough. B5?, B3?

BJW
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