Re: speed of light

[Ron Ebert <ebert@CITRUS.UCR.EDU>]

At 05:11 PM 10/6/99 -0700, Leigh Palmer wrote:
> > Has anyone looked for a reflected supernova flash off adjacent
> > stellar objects? (It would have to be a glancing reflection,
> > I imagine if one had the ..er.. 1988 event in mind?)
>
> As the light from the SN1987A event expands in a spherical shell
> it illuminates the surrounding interstellar medium. By doing the
> geometry* (and trigonometry) on what should be seen at Earth of
> this reflection nebula you will see that an expanding circle of
> illumination should surround the star. It does (it has been
> observed) and it expands in time as expected. There is a real
> bonus to this. By measuring the apparent angular diameter of the
> illuminated circle one can get a geometric distance to the
> object directly without resorting to any "distance ladder" or
> "standard candle" arguments. The determination of distance is as
> good as the accuracy that can be obtained in measuring the size
> of the circle.
>
> Neat, eh?

Things aren't this simple, Leigh. Here is from a recent article in Science:

Take the ring of gas around the supernova that exploded in the LMC over 10
years ago, called 1987A. In principle, the true ring size should have
popped out when astronomers measured the time delay between the burgeoning
flash of the supernova and the later reflection off the ring. The speed of
light times the delay would give the size. Then, Hubble or ground-based
measurements of the ring's angular size would yield the LMC distance.
"That's pretty comforting," says Gould.

But it's not that easy. "The first real snag comes from the fact that light
is not reflected --it's fluorescing," says Gould, and fluorescence takes
time to develop, adding a lag that has to be calculated. Worse, the ring is
not face-on but tilted, meaning that the secondary bursts of light are
further spread out in time because of the varying distances they have to
travel from the ring to Earth. Although initial estimates were all over the
map, several recent calculations, including one by Gould, come up with
similar distances to the LMC, about 47 kpc.

------------------

The article is about the uncertainty of calibrating the "standard candles"
that astronomers use for distance measurements.


Ron Ebert
ron.ebert@ucr.edu
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"Science is best defined as a careful, disciplined, logical search for
knowledge about any and all aspects of the universe, obtained by
examination of the best available evidence and always subject to
correction and improvement upon discovery of better evidence.  What's
left is magic. And it doesn't work."
                                           -- James Randi