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

[APOD Robot]

Light Echoes from V838 Mon

Explanation: For reasons unknown, star V838 Mon's outer surface suddenly greatly expanded with the result that it became the brightest star in the entire Milky Way Galaxy in January 2002. Then, just as suddenly, it faded. A stellar flash like this has never been seen before. It's true that supernovae and novae expel matter out into space. But while the V838 Mon flash appears to expel material into space, what is seen here is actually an outwardly moving light echo of the bright flash. In a light echo, light from the flash is reflected by successively more distant rings in the ambient interstellar dust that already surrounded the star. V838 Mon lies about 20,000 light years away toward the constellation of Monoceros the unicorn. In this Hubble Space Telescope image from February 2004, the light echo is about six light years in diameter.

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[DonMannino]

I am puzzled by the explanation for this photo. If the star became bright in 2002, and the picture was taken in 2004, shouldn't the size of the light echo ring be 4 light years (radius of 2 light years)? The picture explanation says the reflected light ring is 6 light years in diameter.

[Beyond]

I think the constellation link wll be somewhat familiar to owlice. Reasons unknown It should be obvious The star coughed up a giant fur-ball. And a very nice fuzzy one it is!

[AlienVisitor]

I am puzzled by the explanation for this photo. If the star became bright in 2002, and the picture was taken in 2004, shouldn't the size of the light echo ring be 4 light years (radius of 2 light years)? The picture explanation says the reflected light ring is 6 light years in diameter.

This is puzzling me also.

The light would have to travel at 1.5x c. This would suggest\prove the speed of light is not constant. Also, this is in interstellar dust so not in a vacuum.

This has led me to this forum as I feel I had to comment.

[K1NS]

Has anyone made a time lapse video of the expanding light echo? If so, is it on the Internet somewhere? I would think that such a video would be fascinating. I'd like to see it.

[Case]

Timelapse

Click to play embedded YouTube video.

[neufer]

I am puzzled by the explanation for this photo. If the star became bright in 2002, and the picture was taken in 2004, shouldn't the size of the light echo ring be 4 light years (radius of 2 light years)? The picture explanation says the reflected light ring is 6 light years in diameter.

The outer edges of the reflected light echo lie between us and the explosion.
The important factors are the size of the preexisting nebula of gas & dust
and the total travel time of (20,000+δt) years for the light echo at δt years

[b]Observed tomographic slice of the preexisting nebula of gas & dust at time δt.[/b] [color=#00AF00][size=125]The Latus rectum is 2cδt in diameter.[/size][/color]

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[zloq]

I am puzzled by the explanation for this photo. If the star became bright in 2002, and the picture was taken in 2004, shouldn't the size of the light echo ring be 4 light years (radius of 2 light years)? The picture explanation says the reflected light ring is 6 light years in diameter.

A quick answer is - yes - if you measure the apparent speed at which the signal is travelling perpendicular to the view direction, that apparent speed can be greater than the speed of light. This is a so-called superluminal illusion - of which there are many in astronomy - and this is a classic case.

A good write up of this object and the superluminal effect is here:

http://en.wikipedia.org/wiki/Light_echo

A separate write up on more general superluminal effects is here:

http://en.wikipedia.org/wiki/Superluminal_motion

An important point is that with a light echo - the only thing that has to move is light itself - which of course is moving at c and lights up things in its path as it races through stuff, illuminating a shell of said stuff to make it fleetingly visible from Earth. Therefore, light itself can appear, by this illusion, to be moving faster than c. But nothing crazy is going on - it's just an illusion caused by geometry and the finite speed of light.

More surprising is that actual matter, i.e. high speed jets, can be moving so fast they create this illusion also. That isn't just a case of light moving at the speed of light - but matter moving at high relativistic speeds comparable to the speed of light. Such jets have been observed for decades, as described in the second link above. That is pretty impressive - but still nothing is exceeding c.

A light echo could be seen as an expanding, superluminal shell from any direction - but for a jet to appear superluminal it would need to be pointed more directly at you - so as time goes by it gets a lot closer each time you measure its lateral change in position - causing the superluminal illusion by magnifying the apparent rate of lateral motion.

zloq

[orin stepanek]

This APOD is very beautiful! I like it whenever it is shown! I liked the time lapse int the explanation; it is very neat! Case's insert is very neat also!

Click to play embedded YouTube video.

[neufer]

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<<On January 6, 2002, an unknown star was seen to brighten up in Monoceros, the Unicorn. Being a new variable star, it was designated V838 Monocerotis, the 838th variable star of Monoceros.

According to some evidence, V838 Monocerotis may be a very massive supergiant. If that is the case, the outburst may have been a so-called helium flash, a thermonuclear event where a shell in the star containing helium suddenly ignites and starts to fuse carbon. Very massive stars survive multiple such events, however they experience heavy mass loss (about half of the original mass is lost while in the main sequence) before settling as extremely hot Wolf-Rayet stars. This theory may also explain the apparent dust shells around the star. V838 Monoceros is located in the approximate direction of the Galactic anticenter and off from the disk of the Milky Way. Stellar birth is less active in outer galactic regions, and it is not clear how such a massive star can form there. However, there are very young clusters like Ruprecht 44 and the 4 million years old NGC 1893 at a distance of ca. 7 kpc and 6 kpc, respectively.

The outburst may have been the result of a so-called mergeburst, the merger of two main sequence stars (or an 8 M☉ main sequence star and a 0.3 M☉ pre-main sequence star). This model is strengthened by the apparent youth of the system and the fact that multiple stellar systems may be unstable. The less massive component may have been in a very eccentric orbit or deflected towards the massive one. Computer simulations have shown the merger model to be plausible. The simulations also show that the inflated envelope would have come almost entirely from the smaller component. In addition, the merger model explains the multiple peaks in the light curve observed during the outburst.

Another possibility is that V838 Monocerotis may have swallowed its giant planets. If one of the planets entered into the atmosphere of the star, the stellar atmosphere would have begun slowing down the planet. As the planet penetrated deeper into the atmosphere, friction would become stronger and kinetic energy would be released into the star more rapidly. The star's envelope would then warm up enough to trigger deuterium fusion, which would lead to rapid expansion. The later peaks may then have occurred when two other planets entered into the expanded envelope. The authors of this model calculate that every year about 0.4 planetary capture events occur in Sun-like stars in the Milky Way galaxy, whereas for massive stars like V838 Monocerotis the rate is approximately 0.5–2.5 events per year.>>

[OverlordE]

Thank you Neufer and Zlog for the explanation of the apparent speed at which the echo expanded.

[What's In A Name?]

I don't quite buy in to this 'light echo' thing. The outer edge of the expanding pulse of light has passed our position in space and is over 20, 000 light years in diameter.....

[moonstruck]

The Lord worketh in strange ways

[nstahl]

This is a great APOD. Striking and it's engendered curiosity, teaching and hopefully learning.

[Chris Peterson]

I don't quite buy in to this 'light echo' thing. The outer edge of the expanding pulse of light has passed our position in space and is over 20, 000 light years in diameter.....

The first light that reached us (and it doesn't matter how far away the star and nebula are) was that light that traveled in a straight line. So the first thing we saw was the brightening of the star. That light (which illuminates the nebula) spread out in all directions, and the path taken by a photon from the star, to a scattering molecule in the nebula, and then to us is obviously longer than the direct path from the star. So we must, by necessity, see it later. The nebula is a few light years across, so we see the wave of light spread across it over a period of a few years.

[pmurphy]

I thought that looked familiar (if you rotate it 180 degrees):

http://www.wired.com/gadgetlab/2008/04/firefox-logo-sp/

[Tom B]

I am puzzled by the explanation for this photo. If the star became bright in 2002, and the picture was taken in 2004, shouldn't the size of the light echo ring be 4 light years (radius of 2 light years)? The picture explanation says the reflected light ring is 6 light years in diameter.

DonMannino is correct. The six Light Year diameter was a typo. The actual diameter of the initial light echo was four LY. The apparent superluminal velocity here was due to multiple light echoes, not from geometry or relativistic effects. Read the Wikipedia entry on light echoes more carefully. (http://en.wikipedia.org/wiki/Light_echo).

[zloq]

DonMannino is correct. The six Light Year diameter was a typo. The actual diameter of the initial light echo was four LY. The apparent superluminal velocity here was due to multiple light echoes, not from geometry or relativistic effects. Read the Wikipedia entry on light echoes more carefully. (http://en.wikipedia.org/wiki/Light_echo).

Here is a captioned image from the Hubble site that indicates the date of the image, and shows the scale. I think you are right that the APOD caption is slightly wrong - but it was actually much larger than 6 light-years in 2004. The wiki page says it appeared to expand from 4 to 7 ly in a matter of months - back in 2002. Now, in 2004 when the apod image was taken, it appears to be more like 10-12 ly across. The true rate of expansion, by diameter, is 2 ly/ year, though, since light is going radially from the star. So the true rate of expansion in 2 years would be a change of 4 ly, taking it from 4 to 8 ly, and the apparent change looks to be more than that due to the superluminal illusion.



Its growth *appears* faster than c due to the superluminal illusion of the light-echo effect. If you study the diagram on the wiki light-echo page, you can see that ray B, which corresponds to a very large shell and subtends a wide angle from the star, arrives very shortly after light from the central star, making it appear to expand rapidly. There was just one pulse of light in this case - and it is gradually encountering material farther from the star and illuminating it.

zloq

[Sinan İpek]

Why the edge of the light echo isn't a perfect circle? It must be, because since it is kind of like a ripple of water in a pool. Am I wrong?

[Chris Peterson]

Why the edge of the light echo isn't a perfect circle? It must be, because since it is kind of like a ripple of water in a pool. Am I wrong?

No, because the medium isn't uniform. You only see light where there is something to scatter it. 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.

[islader2]

@ CHRIS PETERSON As one of the best contributors to this site==Why did you not comment on the fact that the light circle would expand two {2} light years in a year's time? It seems to ne that your explanations are superb, even about such mundane concepts as a radius being half a diameter.

[saturn2]

Star V 838 Mon isn´t supernovae, isn´t novae, isn´t cefeida.
This star is other class of variable star.

[neufer]

Why the edge of the light echo isn't a perfect circle? It must be, because since it is kind of like a ripple of water in a pool. Am I wrong?

No, because the medium isn't uniform. You only see light where there is something to scatter it.

Agreed.

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).

http://asterisk.apod.com/viewtopic.php? ... 61#p163918

However, if multiple reflections are important then things become quite messy.

[Chris Peterson]

@ CHRIS PETERSON As one of the best contributors to this site==

Thanks!

Why did you not comment on the fact that the light circle would expand two {2} light years in a year's time? It seems to ne that your explanations are superb, even about such mundane concepts as a radius being half a diameter. :) :)

Well, I thought that some of the other comments covered that well- and that it should be fairly evident to most people in any case, that the expansion rate, being radial, was referencing the radius.

But, to be clear, the circle need not expand (diametrically) at two light years per year. It could 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 Peterson]

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.