Re: supernova rings

Glenn R. Morton (
Wed, 06 May 1998 19:04:54 -0500


Are you sure that you or the investigator actually understood what Bill was
asking? I would suggest that they didn't. There is a distinct difference
between the .6 ly radius ring and the light echos. They are quite
different phenomenon. I have read a bunch about this supernova because of
its proof of a universe that is at least 169,000 years old and its proof
that the rates of radioactive decay haven't changed in 169,000 years. It
would appear to me that Chris replied about the .6 ly radius ring, not the
light echos.

At 12:05 PM 5/6/98 -0400, Paul Arveson wrote:
>I sent our question to one of the Principal Investigators and received
>this response (at bottom). Sorry about the confusion of my answer:
>>We are having difficulty interpreting the geometry of the SN1987A
>>supernova rings in regards to the speed of light. Can you relay this
>>to the investigators involved, or give me their email addresses?
>>(Chris Burrows, Peter Garnavich, et al.)
>>Thank you.
>>At 1:19 AM -0400 4/30/98, Bill Payne wrote:
>>>Paul Arveson wrote:
>>>> These captions (also see the pictures) indicate that the rings
>>>> were formed from gas shed by the star thousands of years before
>>>> it exploded in a supernova. The rings are now several
>>>> thousands of light years in radius. Light from the supernova explosion
>>>> lit up the dust in the rings, although the rings are now
>>>> fading because the central source is fading. The pictures show
>>>> this.
>>>Thanks for the info Paul. Now, please help me understand this. SN1987A
>>>exploded some time in the past. Immediately, light from the explosion
>>>radiated out in all directions. Some of the light travelled in a
>>>straight line to the earth and arrived here in 1987. Some of the light
>>>travelled perpendicular to the SN1987A-earth path, hit the rings, and
>>>then bounced off the rings and travelled to the earth, arriving in
>>>February, 1988. If the echo was one light year behind the initial light
>>>that travelled in a direct straight line from the supernova to the
>>>earth, then it would seem to me (and I think Glenn as well) that the
>>>ring radius is one light year. If the ring radius were thousands of
>>>light years, then the echo should be thousands of light years after the
>>>1987 explosion.
>>>What am I missing?
>Date: Tue, 05 May 1998 12:04:13 -0500 (EST)
>From: "CHRIS (410)516-6562" <>
>Subject: Re: CNSHD65217 Re: Re: supernova rings
>Mime-Version: 1.0
>You are absolutely right that the light from the rings is delayed... but the
>rings are only a few light years across not thousands as you seem to
>so there was time for the whole structure to be lit up before our HST
>observations... In fact this time delay has been used to derive the best
>distance yet to the Magellanic clouds...The time it took for the rings to
>illuminate, together with their apparent angular size gives us the
>Hope this answers your question.

Paul Murdin, End in Fire, (Cambridge: Cambridge University Press, 1990) pp

"First reported by Crotts (1988)[IUAC 4561--grm] of McDonald Observatory
from exposures in the first week of march, the light echoes were confirmed
o two exposures taken 1988 February 13 and March 16 at ESO. In retrospect
they were visible on ESO pictures taken as early as 1987 August 16.
Detection of the faint light echoes was technically difficult because of
the brightness of the supernova in the midst. Early in 1988 March the
supernova had just dropped below visual magnitude 7.0--easily visible in a
pair of binoculars and dazzlingly bright in a large telescope. To avoid
flooding their detectors with unwanted scattered light, both the Carnegie
Institute and ESO astronomers obstructed the image of the supernova with a
dark patch at the focus of their telescopes. Because of the similarity of
this device to an eclipse of the Sun by the Moon , which reveals the solar
corona, the apparatus which they each used is called a coronagraph.
"The light echoes from SN1987A did not look like haloes--they were rings,
rather than discs. This was because the dust did not completely surround
the supernova, but lay in two sheets in front of it. The two rings lay at
the intersection of the paraboloidal surface and the two sheets. Since the
nebulae were nearly circular in shape, the sheets lay across the line of
"Suppose that a dust sheet lies a distance d light-years in front of the
supernova, S. Light which has passed directly from the supernova through
the dust at the point P, to the Earth, reached us in 1987 February. Light
which radiated at an angle A has taken an extra time t (one year in this
case) o dogleg via the dust sheet at the point D to us, and is lighting up
a circle on the dust sheet of radius r light-years. D lies on a paraboloid
whose focus is at the supernova--the paraboloidal surface lies a distance
0.5t behind the supernova, so any dust there is lit up at the same time as
D. The dust sheet lies across the line of sight, so DPS is nearly a right
angle triangle and Pythagoras' theorem gives:

(d + t)^2 = d^2+r^2.



With r-30 light-years and t-1 light-year, d-450 light-years. The second
ring has a radius of 45 light-years, and t is still 1 light-year, so it
lies on a second sheet about 1000 light-years in front of the supernova.
As t increases, D moves outwards and r increases: the rings get larger.
"Crotts (1988) calculated that hte two sheets lie 450 and 1000 light-years
in front ofthe supernova. Thus they were nothing particularly to do with
the supernova or the star, Sk-69 202, which exploded; they were in the
foreground of the LMC and lit up by chance." (p. 200-202)

Also the spectra given off by the light echoes resembled the spectrum of
the supernova 11 month previous. p. 202

Now, how big are the light echoes? Bill had used the value of 450 and 1000
light years.This was not the radius and I used his values for my example
rather than looking it up. In 1988 the light echoes were 60 and 90
light-years across. But they expand (due to the geometry) at 1-3 light
years per month (Bill, this is not faster than light travel). Today the
outer light echo easily could have up to 400 light years diameter.

In short, the light echoes are different than the inner .6 ly ring of dust.

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