Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: speed of light



I just performed the calculation again and I now realize that the
result of the reflection of light from a diffuse three-dimensional
interstellar medium would not be a ring, but a disc. (When I said
"circle" I was really thinking "circular ring".) That disc
should now appear to be ten times larger in diameter than the
multiple ring structure associated with SN1987A. It is not seen,
as the pictures cited below will clearly (not) show. I don't know
how I misremembered that. When I first did the calculation I got
a ring, but the disc is a set of rings corresponding to reflection
at successive times after the event.

Sorry about that, folks.

At 8:08 AM -0700 10/7/99, Ron Ebert wrote:
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:

That's why I put in that last sentence, Ron.

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.

There are at least three luminous rings associated with SN1987a.
See http://ecf.hq.eso.org/stecf-pr-images/sn87auv.gif for the
spectroscopy on the smallest one*. No one understands those outer
rings, but they are not causally connected to the SN1987A
*event*. It would be surprising to find such an unusual object
to be unrelated to the precursor star, though SN 1987A itself
is somewhat eccentric from the ring structure. Proper motion in
prehistory could explain this, of course. The outer rings are
thought to be about 30,000 years old.

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.

The fluorescence is associated with the interaction of the
light burst from SN1987A with a ring of material surrounding
the object rather than with a three dimensional dust cloud
in the ISM (which would have been ideal).

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.

That's the smallest distance I've heard quoted.

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

I looked for the article but a scan of Science TOCs did not
find it for me. May I please have the citation? I usually don't
read Science weekly. I prefer Nature, which points me to the
Science articles I should read.

Leigh

*There are better pictures (e.g.
http://oposite.stsci.edu/pubinfo/pr/1998/08/a.html )
but this one identifies the spectral components.