APOD 2005/11/04 colours of nebulae

Comments and questions about the APOD on the main view screen.
Ric
Asternaut
Posts: 5
Joined: Tue Jun 21, 2005 1:05 am
Location: Toronto, Canada

APOD 2005/11/04 colours of nebulae

Post by Ric » Mon Nov 07, 2005 10:56 pm

"... M78, is just below centre in this sharp wide field view, covering an area north of Orion's belt. At a distance of about 1,500 light-years, the bluish nebula itself is about 5 light-years across. Its blue tint is due to dust preferentially reflecting the blue light of hot, young stars in the region. Dark dust lanes and other nebulae can easily be traced through this gorgeous skyscraper that also includes the remarkable McNeil's Nebula -- a newly recognized nebula associated with the formation of a sun-like star."
If dust is "preferentially reflecting the blue light", then no blue light would arrive here on earth because it was reflected rather than transmitted, so this is a poor explanation for seeing a blue colour!
It seems to me that the explanation offered in APOD is in error because the phenomenon-explained differs significantly from the phenomenon-observed.
Scattering, rather than reflection, is likely the mechanism, and particle size in the dust cloud is critical. When blue light is scattered we see a red sunset. When orange to red light is scattered we see a blue moon, which is as rare as the expression "once in a blue moon" indicates because of the requirement for interceding particles of the right size.
I suggest that the blue colour we see is in light left over after Mie scattering removes the orange to red wavelengths, which indicates that a significant proportion of the particles in the cloud have at least one dimension in the order of twice the wavelength of 'visible wavelength's.
From my understanding, I suggest that "once in a blue moon" might be a (twice) daily event on mars, because of its dusty atmosphere (see RCartier.com).

User avatar
Orca
Commander
Posts: 502
Joined: Thu Jan 06, 2005 6:58 pm
Location: Portland, OR

Re: APOD 2005/11/04 colours of nebulae

Post by Orca » Mon Nov 07, 2005 11:16 pm

Ric wrote:" "preferentially reflecting the blue light"
Yes, scattering is the correct term. That's not a clear statement at all. I would think that someone who didn't understand the phenomenon would be even more confused by that statement.

But as usual, the accompanying links help clear things up. 8)

S. Bilderback
Science Officer
Posts: 235
Joined: Tue Jan 25, 2005 12:12 am
Location: The Enchanted Forests of N. Central USA

Post by S. Bilderback » Tue Nov 08, 2005 12:46 am

The actual definition of a "Blue Moon" is the second full moon in a given month, it happens once ever 18 months or so, it has nothing to do with the Moon's color ("colour" to our Canadian and European friends).

Ric
Asternaut
Posts: 5
Joined: Tue Jun 21, 2005 1:05 am
Location: Toronto, Canada

colours of nebulae

Post by Ric » Tue Nov 08, 2005 3:01 am

For those who haven't seen the optical phenomenon the "second full moon in a month" might qualify as a "blue moon" but, in optics, there is a real phenomenon that actually produces a moon that is coloured blue. This phenominal blue moon is much rarer than the "calendar" type. In September 1950 smoke from a Canadian forest fire drifted accross the Atlantic and Europeans were able to see a blue moon!

Ric
Asternaut
Posts: 5
Joined: Tue Jun 21, 2005 1:05 am
Location: Toronto, Canada

colours of nebulae

Post by Ric » Tue Nov 08, 2005 3:17 am

I'm not so sure "accompanying links" clear the issue up, in fact I see a continuation of misunderstanding with citing Rayleigh scattering (which produces blue sky). If blue light is either scattered or reflected, we could not see the nebulae as blue. It must be the other end of the spectrum that is scattered for the residual light to be seen as blue. To my understanding this is only achieved by Mie scattering from larger particles!
It seems to me that the differing colours in the above-centre and the below-centre brightest stars of that APOD could reasonably indicate a statistical difference in the particle size in the dust between us and those particular stars!

craterchains
Commander
Posts: 807
Joined: Thu Jul 21, 2005 2:57 pm
Location: On a boat near Tacoma, WA, usa

Post by craterchains » Tue Nov 08, 2005 4:26 am

Ric, your idea hasn't fallen on deaf ears, as I see your point. It is possible intervening dust is tampering with our images. If indeed that is your point?

Norval
"It's not what you know, or don't know, but what you know that isn't so that will hurt you." Will Rodgers 1938