View Full Version : Light Echo distances.

2011-May-09, 12:56 AM
It occured recently that a light echo
in an astronomical object allows quite
a precise distance to be found. The speed
of light being known and angular separation
being accurately measured. And the first
example was some 90 years ago! I would
like to know if it was possible to do
some calibration of other distance
deductions when the first example was

And the Crab Nebula has been showing
echos from the central pulsar. This
fixes the distance I presume.

2011-May-09, 02:31 AM
As long as it is certain that we are seeing light echos rather than some change in the synchrotron structure of the Crab (such as bursts of newly accelerated electrons physically moving in the nebula). There are two unknowns for a light echo - time delay from the illuminating event and how far in front of the illuminating source the scattering material in that direction lies. The corresponding observables are projected separation from the illuminating source and projected expansion velocity of the echo (which may be superluminal).

2011-May-09, 05:45 PM
Thanks. So my vague thoughts of the illuminated
stuff at right angles to the illuminator is
too simplistic. Still...I wonder if in a few
instances it has allowed a better handle on
the distance.