I just read a little blurb in Science News about a light echo observed over the past year in Monoceros. Sorry don't have the exact reference, I left it at home but want to post NOW! Since it's the first galactic light echo seen since 1937, maybe you'll know which one I'm talking about.

My question is this, couldn't a light echo be used to measure distance very accurately? (or at least get an accurate lower bound).

Follow up: what advantages would the light-echo yardstick have over the cepheid yardstick? (if any)

The light echo in Monoceros has now grown to a certain size in one year's time. If I understand the object correctly, that means the echo is at most one light year across, in actual, physical, local space. (it could appear to be less than one light year across, if by chance the dust distribution was not spherically symmetric and more or less aligned with the line of sight to earth). Since we can measure it's angular size on the sky, we can thus observe it's distance directly from knowing its real size.

(What we would measure is the greatest extent of the echo from the star, if the echo does not appear perfectly round or uniform, as is likely. But I imagine chances are good that some part of the echo is formed at a right angle to the star - as seen from earth - and that part would have the greatest extent.)

Does that sound reasonable and has anyone proposed or used a light echo for this purpose.

(Another light echo I recall is the one for SN1987A in the LMC. What other light echoes have been observed?)

Thanks...

Boris

Here is the APOD article refering to it:
http://antwrp.gsfc.nasa.gov/apod/ap030402.html

According the APOD info the largest frame in the sequence depicts a span of about 6 light years!

What you say makes sense to me, too. I would think this would be a very accurate celestial yardstick because we are not left to assume anything from 'intrinsic brightness'. We know quite accurately how much time elapsed between frames and we know the speed of light therefore we should be able to put an exact linear measurement on the angular distance between echo images.

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Heisenberg was here (probably).

Wait a sec...

The light is reflecting off of material that is (presumable) moving outwards, too, though at a much slower rate. Wouldn't this add an unknown to the calculation?

__________________
Heisenberg was here (probably).

Wasn't a similar approach used to calculate the distance to the Large Magellanic Cloud after supernova 1987A?

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Everything I need to know I learned through Googling.

Unless the dust is moving out at a significant fraction of c, I don't think this would affect the yardstick at all. The point is we can SEE the light moving outward almost like the windows in a train at night. So we can see an apparent angular speed to the light - which is presumed constant throughout the universe, certainly the nearby universe - and from this apparent speed we have a very direct indicator of distance.

The confounding variables are reflections off of dust that is relatively in front of or behind the star, which is why we should measure only the greatest extent of the echo.

The other confounding variable is the lightcurve of the initial burst, which will affect how different parts of the echo change in brightness. Perhaps there are too many confounding variables to make this an accurate yardstick?

Boris

PS
and the big bangers will say that time dilation has to be taken into account ;-). (Or I might suggest this would be an interesting way to TEST time dilation and the BBT. But this would be for the "against the mainstream" section). But I don't think this will work as a yardstick on any kind of cosmological scale. I think the objects are way too small, right?