I can't help noticing the fact that I picked this very picture
just a few days ago to illustrate the concept that "white dwarfs" aren't usually white! Check out the fourth post from the bottom in the thread I linked to. In that post, I wrote:
Check out
this picture of a planetary nebula. Note the very blue color of the central star.
The reason why I picked this picture was, of course, that it so beautifully demonstrates the blue color of basically all central stars of planetary nebulae. Of course I also agree with the caption of today's APOD, which said:
This particularly photogenic planetary
It is certainly very photogenic! And the actual image is wonderful. There is a treasure trove of details in it, like the intricate structure of the nebula itself, with its multiple shells, as well as all those background galaxies, one of which is actually seen in the central "emptiness" of the planetary nebula!
The seemingly "empty" central part of the planetary is indeed
almost empty. Most of the gas has been cleared away from this space by the fierce radiation of the super-hot central star. The faint blue-green light which is seen inside this "cavity" is so-called "forbidden" OIII emission. A near-vacuum close to a super-hot star is the perfect place for a few scattered oxygen ions to emit this blue-green light.
I must say, finally, that I think it's a joy to see such a richly-hued "true-color" RGB image of a planetary nebula.
When I picked this nebula for my post a few days ago, I searched for pictures of planetaries which would actually show that the central star is blue. This one was the first one I found.
Ann
I can't help noticing the fact that I picked this very picture [url=http://asterisk.apod.com/viewtopic.php?f=9&t=29865]just a few days ago[/url] to illustrate the concept that "white dwarfs" aren't usually white! Check out the fourth post from the bottom in the thread I linked to. In that post, I wrote:
[quote]Check out [url=http://astrofoto.es/Galeria/2012/PK164/PK164_CAHA_en.html]this picture[/url] of a planetary nebula. Note the very blue color of the central star.[/quote]
The reason why I picked this picture was, of course, that it so beautifully demonstrates the blue color of basically all central stars of planetary nebulae. Of course I also agree with the caption of today's APOD, which said:
[quote]This particularly photogenic planetary [/quote]
It is certainly very photogenic! And the actual image is wonderful. There is a treasure trove of details in it, like the intricate structure of the nebula itself, with its multiple shells, as well as all those background galaxies, one of which is actually seen in the central "emptiness" of the planetary nebula!
The seemingly "empty" central part of the planetary is indeed [i]almost[/i] empty. Most of the gas has been cleared away from this space by the fierce radiation of the super-hot central star. The faint blue-green light which is seen inside this "cavity" is so-called "forbidden" OIII emission. A near-vacuum close to a super-hot star is the perfect place for a few scattered oxygen ions to emit this blue-green light.
I must say, finally, that I think it's a joy to see such a richly-hued "true-color" RGB image of a planetary nebula. :clap: :clap: :clap: When I picked this nebula for my post a few days ago, I searched for pictures of planetaries which would actually show that the central star is blue. This one was the first one I found.
Ann