APOD: Light Echoes from V838 Mon (2011 Dec 04)

[DonMannino]

Thanks for all the excellent replies. The simplest answer seems to be that the first observation in 2002 was not the first bright event - the photo sequence clearly shows that the light echo had already expanded to (I am guessing) 1.5 light years at that time, so the 2004 photo would be greater than 6 light years in width.

Thanks for all the interesting reading and references! I learned a lot.

Don Mannino

[alter-ego]

But, to be clear, the circle need not expand (diametrically) at two light years per year.It cou ld be less than that- if, for example, the nebula being illuminated was actually between us and the star, and not all around the star, the apparent expansion rate would be lower.

Chris, I think you have this reversed. I was re-reading an interesting 1991 S&T article discussing the evolution of light-echo understanding which starting in 1901 with Nova Persei. Paul Couderc's model of a dust sheet in front of the nova (between us and the nova) resolved the superluminal paradox in 1939 that arose when the distance to the nova was finally determined to be ≈5x further than previously thought. Although I think he did not introduce the "kinematically accessable paraboloid" (or eplipsoid), the reasoning is still correct. The following blurb is from http://www.aavso.org/vsots_gkper:

And to quote the S&T article:
"... Although a variety of behaviors is possible, depending on dust cloud's shape, position, and orientation, such a far-foreground echo can be highly superluminal. In contrast, echoes from dust behind the nova will usually be subluminal, while those from clouds to the sides of the explosion will move roughly at the speed of light"

For Nova Persei, the fastest expanding ring was ~4.7c and determined to be a dust layer located ~25lyr in front of the nova, while the inner, and brighter ring, moved at ~2.6c and estimated to be located ~7lyr in front.

I appologize if this seems picky. I think this is fundamental to understanding light echoes. To be honest, I was a bit confused in the details and I happened to remember this article. It just so happened that the article above gave the issue information. I dug it up and enjoyed it again.

[Chris Peterson]

But, to be clear, the circle need not expand (diametrically) at two light years per year.It cou ld be less than that- if, for example, the nebula being illuminated was actually between us and the star, and not all around the star, the apparent expansion rate would be lower.

Chris, I think you have this reversed.

No, I don't think so. But I don't mean you are wrong, either. It is clear how a sheet of dust can create the superluminal illusion. I was suggesting a volume of dust, close to the star but on one side (not likely, but possible). The point was only that the geometry of the star and the dust cloud could result in the light echo appearing to move at something other than c- either faster or slower is possible.

[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

[neufer]

In addition, you are seeing a 3D medium projected in two dimensions. So you don't really know how far away from the central star any particular point really is.

Well...you do actually.

At any given time the primary reflections all take place on a 2D paraboloid shell whose focus is the variable star and whose Latus rectum is 2cδt so everything is well defined (provided, of course, that the distance to the variable star is known).

Yes, but the solution for any point is only unambiguous if you assume an instantaneous flash from the star. In fact, that did not occur. The star flared at least three times over more than three months. The resulting illumination field, convolved with the 3D structure of the scattering medium, results in a physical projection that can't be unambiguously deconstructed. Approximately, yes... but a fair degree of uncertainty remains, and contributes to the non-spherical (non-circular in projection) appearance of the light echo.

Fair enough.

[flash]

Regarding superluminal:

Consider swinging a laser pointer in an arc such that its beam sweeps across a surface (such as the inside of a cylinder) The light travels at 186000 miles/second, but the beam traverses the surface at a rate dependent only on the angular velocity of the laser pointer! If the pointer is rotated faster and faster, the point where the laser hits the cylinder will move faster and faster too. The leading edge of the light so emitted will move out from the pointer in a spiral, but given the fact that the surface of the cylinder is the same distance from the source, the arrival times of the light at the beginning and end of one rotation will be exactly the time to make the rotation. It is easy to see that as you increase the radius of the cylinder, the speed of the point of light increases without limit.

Given the diameter of the moon as roughly 3000 miles, and the speed of light as 186000 miles / sec, to cause the point to sweep across the face of the moon at a speed of 2 times the speed of light would require you (on the surface of the Earth) to traverse the moon with the laser pointer in roughly a mere 1/124 of a second (~8 ms). (time = (1sec / (2x186000 miles)) x 3000 miles So if you could see the point of light moving across the moon you would see it move at 2 times the speed of light. (But there is no information traveling at that speed.)

[keithsaunders]

I, too, am puzzled by the statement that the light echo, which presumably started from the detonating star in January 2002, reached a diameter of 6 light years in just over 2 years. According to Einstein this is about 50% beyond the possible! Careful verification of this (diameter) measurement would seem imperative. If confirmed perhaps we have supporting evidence for the recently claimed 'violation' by the nutrino of the absolute nature of the speed of light in vacuo. Or perhaps it is an enigmatic quality of enigmatic 'dark matter' that allows this apparent aberration - i.e. When is a vacuum not a vacuum . . . .??

Keith Saunders

[neufer]

Click to play embedded YouTube video.

[c]"When is a vacuum not a vacuum" optical illusion.[/c]

I, too, am puzzled by the statement that the light echo, which presumably started from the detonating star in January 2002, reached a diameter of 6 light years in just over 2 years. According to Einstein this is about 50% beyond the possible! Careful verification of this (diameter) measurement would seem imperative. If confirmed perhaps we have supporting evidence for the recently claimed 'violation' by the nutrino of the absolute nature of the speed of light in vacuo. Or perhaps it is an enigmatic quality of enigmatic 'dark matter' that allows this apparent aberration

- i.e. When is a vacuum not a vacuum . . . .??

[c]It's an optical illusion [/c]
You are assuming that the star and the light echos are on the same synchronized time...but they aren't

The most distant parts of the light echo are light years closer to you
and hence their clocks/calendars are set years later than the star clock/calendar.

You must take into account the unsynchronized time of the things you are observing.

[NoelC]

All the confusion / discussion really illuminates the fact that it is our basic nature to assume we perceive things as they "are" - instantaneously... This of course was true (for all intents and purposes) for anything important (nearby) that our ancestors saw all through the multi-billion year course of our evolution.

We imagine ourselves to be Godlike, when we look at astroimages, in that we construct mental models of reality in which we imagine all things we see existing at a given "point" in time, when only a being outside the 4 dimensions of our universe could possibly actually perceive such a thing.

NOTHING we see out there was really ever just as we're seeing it; we see things through time as well as space. Sometimes it seems the only thing that keeps us sane is that objects in the grandest scales don't change quickly. Just look at how the particular (rapidly changing) view of this APOD sparks our imaginations.

Optical Illusion

It's mind blowing when the higher brain functions arrive at different answers than the primitive parts. And humbling.

-Noel