Beyond wrote:One word, Bruce... inflation. It's everywhere!!
But, apparently, this nuebula has experienced de
flation, since it has gotten smaller. Something just dosen't add up here.
When I checked the assumed, spectral-class derived distances to hot blue stars in Sky Catalogue 2000.0 and compared them with their distances as measured by Hipparcos, I found that the brightness of blue stars based on their spectral classes had been systematically overstimated by Sky Catalogue 2000.0. Oh, there were a few exceptions, of course, some monstrously bright blue stars. By and large, however, the blue stars turned out to be fainter than astronomers had expected. A good example of a young and very blue O star that is fainter than astronomers would have thought is S Monocerotis
, also known as S Mon
or 15 Mon.
I'm going to guess that we are seeing a somewhat similar effect when it comes to the Swan Nebula, or M17. Astronomers found a large number of O stars in Messier 17, and so they may have just assumed that the nebula was titanic. I'm going to guess, by the way, that the very young O stars of M17 are fainter than astronomers first thought. Intrinsically brighter O-type stars might have called for a larger nebula, but if the O-type stars turned out to be fainter than expected, the nebula might have "shrunk" as well.
Why are O-stars fainter than expected, by the way? I'm going to guess here. I think it is the very young O-stars that are faint, and that very young stars simply "do less fusion" than older stars, and so generate less energy. The reason might possibly be that a smaller part of the star is involved in doing fusion in very young stars. As the star ages, more and more of it becomes involved in doing fusion, as the fusion zone moves outward in the star, sort of "covering more" of the existing material there. Also, I think that young main sequence stars are typically smaller in size than somewhat older main sequence stars, and unexpectedly small stars are typically going to be unexpectedly faint. As an aside, I suspect that most (and just possibly all) unexpectedly faint stars of any spectral class are unexpectedly small in size.