>...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 --- This message is from the AZ-Observing mailing list. If you wish to be removed from this list, send E-mail to: AZ-Observing-request@xxxxxxxxxxxxx, with the subject: unsubscribe. The list's archive is at: //www.freelists.org/archives/az-observing This is a discussion list. Please send personal inquiries directly to the message author. In other words, do not use "reply" for personal messages. Thanks.