Starship Asterisk*

DonMannino wrote: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 wrote:
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 wrote: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.

http://en.wikipedia.org/wiki/V838_Monocerotis wrote:

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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.>>

What's In A Name? wrote: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.....

DonMannino wrote: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.

Tom B wrote: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).

Sinan İpek wrote: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 wrote:

Sinan İpek wrote:
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.

Chris Peterson wrote:

Sinan İpek wrote:
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 wrote:@ 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. :) :)

neufer wrote:

Chris Peterson wrote: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).