APOD of Nov 27, 2005: Light Echoes from V838 Mon

Comments and questions about the APOD on the main view screen.
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APOD of Nov 27, 2005: Light Echoes from V838 Mon

Post by SkyWatcher » Sun Nov 27, 2005 10:40 pm

It is said that the echo"spans about 6 light years".
But the flash of light occurred at the beginning of 2002.
The picture shown has been taken on February 8, 2004 (written on Hubble site) that is to say after a little more than 2 years.
Since (I suppose... :? ) the light echo travels at the speed of light, the radius of echo should be a little more than 2 years, and the whole light echoes should span a little more than 4 (not 6) light years.
Where is my reasoning wrong ? :wink:
Thanks for the answer.
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orin stepanek
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Post by orin stepanek » Mon Nov 28, 2005 1:44 am

Check this link http://hubblesite.org/newscenter/newsde ... 10/image/a
It looks like 6 light years might be about right.
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Post by orin stepanek » Mon Nov 28, 2005 1:49 am

[File not found] Sorry but I took that link from APOD site. Anyway it showed 3 year progression of light echos which would have been enough to explain six light year cross section.
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Post by orin stepanek » Mon Nov 28, 2005 2:16 am


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Post by SkyWatcher » Mon Nov 28, 2005 4:28 pm

In the link mentioned (Hubble site) it is said that "the stellar outburst in Jan 2002".
The sequence of 4 pictures shown starts from May 20, 2002 until December 17, 2002.
The picture in APOD is dated Feb 8, 2004.
So it is 2 years later than the outburst.
If the star is in the center of the nebula, the light echo has traveled for 2 years outward.
But perhaps the star is not in the center of the nebula but far away in the background.
If so the echo can be wider because it would be traveling from the back toward the nebula.
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Post by William Roeder » Fri Dec 02, 2005 9:03 pm

A previous APOD on light echos stated that it can appear to enlarge FASTER than the speed of light.

If the matter was a perfect sphere centered on the pulsar, the echo would appear instantaniously - infinite speed.

If the mater was a plane the speed would be initially infinite as the light intersects the plane, and would slow based on the tangent of the angle.

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Post by SkyWatcher » Fri Dec 02, 2005 11:36 pm

William Roeder wrote:A previous APOD on light echos stated that it can appear to enlarge FASTER than the speed of light.

If the matter was a perfect sphere centered on the pulsar, the echo would appear instantaniously - infinite speed.

If the mater was a plane the speed would be initially infinite as the light intersects the plane, and would slow based on the tangent of the angle.
Thanks: your explanation is 100% clear and exact.
After having posted the first message, I also had such idea. :idea:
My mistake was to think as if the matter had been a plane with the pulsar on the same plane.
Best regards.
Silvano - Milano - Italy