by zloq » Mon Dec 05, 2011 8:45 am
My reason for saying the true growth rate is diametrically 2 ly/yr is to remind people that the only thing that needs to move in a light echo situation is light itself - and the true diametric rate is 2 ly/yr rather than 1 ly/yr. I sure hope this basic point is understood. If its diameter had gone from 4 to 6 ly in two years, that would have been subluminal growth and less amazing - but it went from 4 to 7 in a few months. The only thing that needs to move, whether it hits dust or not, is a light pulse itself - and the diameter of its sphere of influence is physically increasing at a solid 2 ly/yr, in contrast to its *apparent* growth rate, which could be much bigger.
As for not knowing the actual dimensions of the structure, the time delay and the subtended angle carry good spatial information, as indicated on the Hubble page where they describe mapping out the 3D structure like a CAT scan. Conversely, you can use the time delay and assumed nebulosity geometry (a uniform blob) to help refine the distance to the source, as in the case of the Cepheid distance determined to 1%:
http://www.eso.org/public/news/eso0805/
If a flat patch of nebulosity were between us and the star and hit by a pulse of light, it would appear as a point of light in the center that expanded into a superluminal ring shooting outward much faster than c. That would be a simple example of the superluminal illusion. Even if the patch were spherical, it would be illuminated relatively simultaneously and present an illusion of rapid outward growth due to light travel time to the observer.
For actual moving matter like a galactic jet, the effect depends on the speed and angle of motion toward the observer. It's all classical and there is no need for relativity - just high speed and a small angle of motion toward the observer.
Either way - yes - it looks like the APOD caption is wrong and I hadn't realized it until the possibility was pointed out - and I looked at the actual scale of the image. I'm not sure what the caption should be - but maybe they meant to say it expanded in diameter 6 ly in 2 years. I think it is more like an apparent expansion of 8 ly in under two years.
I don't know what it looks like now. The Hubble page cites the lead astronomer, Bond, estimating it would run out of material to illuminate by 2010 - as you can see start happening with the hole in the front. The back section will probably disappear last as seen by us - but viewed from the star itself the volume around it just glowed for a while and suddenly faded out all at once - assuming a roughly spherical dust volume. And it would have seen this happen about 20,000 years ago.
zloq
My reason for saying the true growth rate is diametrically 2 ly/yr is to remind people that the only thing that needs to move in a light echo situation is light itself - and the true diametric rate is 2 ly/yr rather than 1 ly/yr. I sure hope this basic point is understood. If its diameter had gone from 4 to 6 ly in two years, that would have been subluminal growth and less amazing - but it went from 4 to 7 in a few months. The only thing that needs to move, whether it hits dust or not, is a light pulse itself - and the diameter of its sphere of influence is physically increasing at a solid 2 ly/yr, in contrast to its *apparent* growth rate, which could be much bigger.
As for not knowing the actual dimensions of the structure, the time delay and the subtended angle carry good spatial information, as indicated on the Hubble page where they describe mapping out the 3D structure like a CAT scan. Conversely, you can use the time delay and assumed nebulosity geometry (a uniform blob) to help refine the distance to the source, as in the case of the Cepheid distance determined to 1%:
http://www.eso.org/public/news/eso0805/
If a flat patch of nebulosity were between us and the star and hit by a pulse of light, it would appear as a point of light in the center that expanded into a superluminal ring shooting outward much faster than c. That would be a simple example of the superluminal illusion. Even if the patch were spherical, it would be illuminated relatively simultaneously and present an illusion of rapid outward growth due to light travel time to the observer.
For actual moving matter like a galactic jet, the effect depends on the speed and angle of motion toward the observer. It's all classical and there is no need for relativity - just high speed and a small angle of motion toward the observer.
Either way - yes - it looks like the APOD caption is wrong and I hadn't realized it until the possibility was pointed out - and I looked at the actual scale of the image. I'm not sure what the caption should be - but maybe they meant to say it expanded in diameter 6 ly in 2 years. I think it is more like an apparent expansion of 8 ly in under two years.
I don't know what it looks like now. The Hubble page cites the lead astronomer, Bond, estimating it would run out of material to illuminate by 2010 - as you can see start happening with the hole in the front. The back section will probably disappear last as seen by us - but viewed from the star itself the volume around it just glowed for a while and suddenly faded out all at once - assuming a roughly spherical dust volume. And it would have seen this happen about 20,000 years ago.
zloq