Starship Asterisk*

APOD Robot wrote:
Explanation: Sculpted by stellar winds and radiation, the star factory known as Messier 17 lies some 5,500 light-years away in the nebula-rich constellation Sagittarius. At that distance, this degree wide field of view spans almost 100 light-years.

• The cut & paste approach of M17 APOD explanations over the years
  has caused the size of this nebula to be greatly exaggerated as of late:
  
  . http://apod.nasa.gov/apod/ap130418.html
  . http://apod.nasa.gov/apod/ap110630.html
  . http://apod.nasa.gov/apod/ap070810.html

http://en.wikipedia.org/wiki/Messier_17 wrote:
<<The Omega Nebula is between 5,000 and 6,000 light-years from Earth and it spans some 15 light-years (~10 arcmins) in diameter.
The cloud of interstellar matter of which this nebula is a part is roughly 40 light-years in diameter.>>

neufer wrote:

APOD Robot wrote:
Explanation: Sculpted by stellar winds and radiation, the star factory known as Messier 17 lies some 5,500 light-years away in the nebula-rich constellation Sagittarius. At that distance, this degree wide field of view spans almost 100 light-years.

• The cut & paste approach of M17 APOD explanations over the years
  has caused the size of this nebula to be greatly exaggerated as of late:
  
  . http://apod.nasa.gov/apod/ap130418.html
  . http://apod.nasa.gov/apod/ap110630.html
  . http://apod.nasa.gov/apod/ap070810.html

http://en.wikipedia.org/wiki/Messier_17 wrote:
<<The Omega Nebula is between 5,000 and 6,000 light-years from Earth and it spans some 15 light-years (~10 arcmins) in diameter.
The cloud of interstellar matter of which this nebula is a part is roughly 40 light-years in diameter.>>

Beyond wrote:One word, Bruce... inflation. It's everywhere!!

Psnarf wrote:
Approximately how many stars are generating the stellar winds and radiation that energize the visible nebula? 44-Trillion miles is quite a distance for energized particles to travel in order to knock electrons off of some interstellar clouds. (assuming a 7.5 light-year radius)

http://en.wikipedia.org/wiki/Messier_17 wrote:
<<An open cluster of 35 stars lies embedded in the nebulosity and causes the gases of the nebula to shine due to radiation from these hot, young stars; however the actual number of stars in the nebula is much higher - up to 800, 100 of spectral type earlier than B9, and 9 of spectral type O, plus >1000 stars in formation on its outer regions. It is considered one of the brightest and most massive star-forming regions of our galaxy. Its local geometry is similar to the Orion Nebula except that it is viewed edge-on rather than face-on. It's also one of the youngest clusters known, with an age of just 1 million years.>>

BDanielMayfield wrote:

Beyond wrote:One word, Bruce... inflation. It's everywhere!!

But, apparently, this nuebula has experienced deflation, since it has gotten smaller. Something just dosen't add up here.

BDanielMayfield wrote:

Beyond wrote:One word, Bruce... inflation. It's everywhere!!

But, apparently, this nebula has experienced deflation, since it has gotten smaller. Something just dosen't add up here.

BDanielMayfield wrote:

Beyond wrote:One word, Bruce... inflation. It's everywhere!!

But, apparently, this nuebula has experienced deflation, since it has gotten smaller. Something just dosen't add up here.

neufer wrote:

Psnarf wrote:
Approximately how many stars are generating the stellar winds and radiation that energize the visible nebula? 44-Trillion miles is quite a distance for energized particles to travel in order to knock electrons off of some interstellar clouds. (assuming a 7.5 light-year radius)

http://en.wikipedia.org/wiki/Messier_17 wrote:
<<An open cluster of 35 stars lies embedded in the nebulosity and causes the gases of the nebula to shine due to radiation from these hot, young stars; however the actual number of stars in the nebula is much higher - up to 800, 100 of spectral type earlier than B9, and 9 of spectral type O, plus >1000 stars in formation on its outer regions. It is considered one of the brightest and most massive star-forming regions of our galaxy. Its local geometry is similar to the Orion Nebula except that it is viewed edge-on rather than face-on. It's also one of the youngest clusters known, with an age of just 1 million years.>>

BDanielMayfield wrote:
To be clear, I don't mean that literally, I was just joking with Beyond. The nuebula's true size wouldn't have changed all that much, but reports about it's size have. Or, could nuefer's point be that old errors about its size keep getting re-copied?

neufer wrote:

BDanielMayfield wrote:
To be clear, I don't mean that literally, I was just joking with Beyond. The nuebula's true size wouldn't have changed all that much, but reports about it's size have. Or, could nuefer's point be that old errors about its size keep getting re-copied?

Specifics about the size of an old frame [~ 1º ~ 100 ly] keep getting re-copied even though the pictures themselves change.

Ann wrote:

BDanielMayfield wrote:

Beyond wrote:One word, Bruce... inflation. It's everywhere!!

But, apparently, this nuebula has experienced deflation, 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.

Ann

BDanielMayfield wrote:It must be very challenging to keep up to date with every astronomical fact and figure, but that's why fact checkers, [spellcheckers], proofreaders and editors are needed.

BDanielMayfield wrote:

Ann wrote:

BDanielMayfield wrote:But, apparently, this nebula has experienced deflation, 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.

Ann

No real shrinkage has occurred though, since this has been an Artfully contrived tempest in an APOT APOD.