Starship Asterisk*

by **neufer** » Mon Feb 25, 2019 3:04 pm

JohnD wrote: ↑Mon Feb 25, 2019 8:32 am
That quote made it clear for me - different reflecting dust clouds are at different distances, from the Supernova and Earth, combined, so the echo is seen at different times.

Those echos did, indeed, take place about 830 years apart due to their different separations from SN1987N.

However, the echos were seen (by us) at the same time due to their different separations from us.

If there is yet a third reflecting dust cloud about 500 light years behind SN1987N then we might get to see a third echo in about 1,000 years because the time delays add rather than cancel (...but, personally, I don't have the patience to wait around for that one).

by **JohnD** » Mon Feb 25, 2019 8:32 am

Thank you Art!

That quote made it clear for me - different reflecting dust clouds are at different distances, from the Supernova and Earth, combined, so the echo is seen at different times.

John

by **neufer** » Sun Feb 24, 2019 9:40 pm

NGC3314 wrote: ↑Sun Feb 24, 2019 8:18 pm
JohnD wrote: ↑Sun Feb 24, 2019 3:16 pm
The APOD blurb doesn't say why there are two expanding reflection rings.

Did Supernova 1987A explode twice?
For each value of the time delay (time since we served the direct light of the explosion), there is a locus of possible light echos which is an ellipse with the observer and the supernova at the foci. We will, in principle, see echoes from anywhere on this ellipse where there is enough scattering material. Multiple rings correspond to multiple density enhancements along the line of sight. For SN1987A, they imply fairly narrow sheets of dust in the Large Magellanic Cloud one of which is tilted to the line of sight as shown by its ring being off-center to the SN direction.

Note that the sheets of dust are hundreds of light years in front of SN1987A
whereas the rings are only about one hundred light years wide.

https://images.datacentral.org.au/malin/AAT/066 wrote:
The light echo of SN 1987A, February, 1989
Date created: 1989-02-16

When supernova 1987A was seen to explode in the Large Magellanic Cloud, the Milky Way's nearest companion galaxy, the brilliant flash of light from the self-destructing star had taken about 170,000 years to arrive at the telescope. Some light was deflected by two sheets of dust near the supernova, and is seen after the star has faded away because the reflected light covers a longer path to reach us. The dust responsible for the rings seen here lies in two distinct sheets, about 470 and 1300 light years from the supernova, close to our line of sight to it.

There's an article by David Malin and David Allen describing the phenomenon in Sky and Telescope magazine for January, 1990 (p22) and here is a diagram and caption from the article showing how the light echoes were formed. From our point of view, at the focus of the long ellipse shown in the diagram, the light echoes appear to expand with time and we were able to record this expansion in a series of six images, the last being 1440 days (almost five years) after the appearance of the supernova.

Image width is about 5.5 arc min (272 lys).

Credit: David Malin © Australian Astronomical Observatory

by **NGC3314** » Sun Feb 24, 2019 8:18 pm

JohnD wrote: ↑Sun Feb 24, 2019 3:16 pm The APOD blurb doesn't say why there are two expanding reflection rings. Did Supernova 1987A explode twice?

For each value of the time delay (time since we served the direct light of the explosion), there is a locus of possible light echos which is an ellipse with the observer and the supernova at the foci. We will, in princjple, see echoes from anywhere on this ellipse where there is enough scattering material (in practice, interstellar dust forward-scatters light very strongly and backscatters very weakly, so material near the SN is easier to see). Multiple rings correspond to multiple density enhancements along the line of sight. For SN1987A, they imply fairly narrow sheets of dust in the Large Magellanic Cloud one of which is tilted to the line of sight as shown by its ring being off-center to the SN direction.

Because they do not correspond to moving material objects, it is possible for the projected locations of light echoes to move faster than c - in fact the combination of apparent velocity and time delay can give the distance to the scattering material. Using this property, light echoes have been identified from Cassiopeia A and Tycho's supernova, and the reflected spectra provided classifications of the explosions.

by **Fred the Cat** » Sun Feb 24, 2019 4:34 pm

Light echoes, like a reverberation from the past, where evident in the history of AAT from Fred Hoyle. And takes as long to download.

Reflecting his "bureaucratic" sense of humor, it has worth the wait.

by **JohnD** » Sun Feb 24, 2019 3:16 pm

De58te,

1/ There was no "light aimed at Earth". A superanova radiates in all directions, just as any star does.
2/There are no "light rings headed for Earth". The omnidirectional light from the nova is scattered, randomly, and some heads our way. An observer on the other side, or at our disatance from the nova but at right angles to our line of sight, would see the same.
3/ Can't see a second ring? I can see two rings in the video, what can't you see?
4/ "Two dimensional shot"? You can see this in 3D?

The APOD blurb doesn't say why there are two expanding reflection rings. Did Supernova 1987A explode twice?

JOhn

by **orin stepanek** » Sun Feb 24, 2019 1:03 pm

Does make a nice Bull's eye!

by **De58te** » Sun Feb 24, 2019 11:28 am

Curious question. I suppose the first light ring that was aimed at the Earth wasn't included in the video. So why can't we see a second ring also heading for the Earth. Answer, because the light hasn't got here yet? Then why do we see the other rings in the two dimensional shot?

by **APOD Robot** » Sun Feb 24, 2019 5:09 am

The Expanding Echoes of Supernova 1987A

Explanation: Can you find supernova 1987A? It isn't hard -- it occurred at the center of the expanding bullseye pattern. Although this stellar detonation was first seen in 1987, light from SN 1987A continued to bounce off clumps of interstellar dust and be reflected to us even many years later. Light echoes recorded between 1988 and 1992 by the Anglo Australian Telescope (AAT) in Australia are shown moving out from the position of the supernova in the featured time-lapse sequence. These images were composed by subtracting an LMC image taken before the supernova light arrived from later LMC images that included the supernova echo. Other prominent light echo sequences include those taken by the EROS2 and SuperMACHO sky monitoring projects. Studies of expanding light echo rings around other supernovas have enabled more accurate determinations of the location, date, and symmetry of these tremendous stellar explosions. Yesterday marked the 32nd anniversary of SN 1987A: the last recoded supernova in or around our Milky Way Galaxy, and the last to be visible to the unaided eye.

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