Editing SN 1987A (section)

==Interaction with circumstellar material==
[[File:SN1987a debris evolution animation time scaled.gif|thumb|Sequence of [[Hubble Space Telescope|HST]] images from 1994 to 2009, showing the collision of the expanding [[supernova remnant|remnant]] with a ring of material ejected by the progenitor 20,000 years before the supernova<ref name=Larsson2011>
{{Cite journal
 |last1=Larsson |first1=J.
 |display-authors=etal
 |year=2011
 |title=X-ray illumination of the ejecta of supernova 1987A
 |journal=[[Nature (journal)|Nature]]
 |volume=474 |issue=7352 |pages=484–486
 |arxiv=1106.2300
 |bibcode=2011Natur.474..484L
 |doi=10.1038/nature10090
 |pmid=21654749
 |s2cid=4388495
}}</ref>]]

The three bright rings around SN 1987A that were visible after a few months in images by the Hubble Space Telescope are material from the [[stellar wind]] of the progenitor. These rings were ionized by the ultraviolet flash from the supernova explosion, and consequently began emitting in various emission lines. These rings did not "turn on" until several months after the supernova and the process can be very accurately studied through [[Astronomical spectroscopy|spectroscopy]]. The rings are large enough that their angular size can be measured accurately: the inner ring is 0.808 arcseconds in radius. The time light traveled to light up the inner ring gives its radius of 0.66 (ly) [[light years]]. Using this as the base of a right angle triangle and the angular size as seen from the Earth for the local angle, one can use basic trigonometry to calculate the distance to SN 1987A, which is about 168,000 light-years.<ref>
{{cite journal
 |last1=Panagia |first1=N.
 |year=1998
 |title=New Distance Determination to the LMC
 |journal=[[Memorie della Societa Astronomia Italiana]]
 |volume=69 |page=225
 |bibcode=1998MmSAI..69..225P
}}</ref> The material from the explosion is catching up with the material expelled during both its red and blue supergiant phases and heating it, so we observe ring structures about the star.

Around 2001, the expanding (>7,000&nbsp;km/s) supernova ejecta collided with the inner ring. This caused its heating and the generation of x-rays—the x-ray flux from the ring increased by a factor of three between 2001 and 2009. A part of the x-ray radiation, which is absorbed by the dense ejecta close to the center, is responsible for a comparable increase in the optical flux from the supernova remnant in 2001–2009. This increase of the brightness of the remnant reversed the trend observed before 2001, when the optical flux was decreasing due to the decaying of [[Isotopes of titanium|<sup>44</sup>Ti]] isotope.<ref name="Larsson2011"/>

A study reported in June 2015,<ref name="New Scientist">
{{cite web
 |last1=Kruesi |first1=L.
 |title=Supernova prized by astronomers begins to fade from view
 |url=https://www.newscientist.com/article/mg22630254.500-supernova-prized-by-astronomers-begins-to-fade-from-view.html#.VXt0YOeqdYP
 |work=[[New Scientist]]
 |access-date=June 13, 2015
 |archive-url=https://web.archive.org/web/20150611202711/http://www.newscientist.com/article/mg22630254.500-supernova-prized-by-astronomers-begins-to-fade-from-view.html#.VXt0YOeqdYP
 |archive-date=June 11, 2015
 |url-status=dead
}}</ref> using images from the Hubble Space Telescope and the [[Very Large Telescope]] taken between 1994 and 2014, shows that the emissions from the clumps of matter making up the rings are fading as the clumps are destroyed by the shock wave. It is predicted the ring would fade away between 2020 and 2030. These findings are also supported by the results of a three-dimensional hydrodynamic model which describes the interaction of the blast wave with the circumstellar nebula.<ref name="Template SN1987A"/>
The model also shows that X-ray emission from ejecta heated up by the shock will be dominant very soon, after which the ring would fade away. As the shock wave passes the circumstellar ring it will trace the history of mass loss of the supernova's progenitor and provide useful information for discriminating among various models for the progenitor of SN 1987A.<ref name="Destruction of Ring">
{{Cite journal
 |last1=Fransson |first1=C.
 |display-authors=etal
 |year=2015
 |title=The Destruction of the Circumstellar Ring of SN 1987A
 |journal=[[The Astrophysical Journal]]
 |volume=806 |issue=1|pages=L19
 |arxiv=1505.06669
 |bibcode=2015ApJ...806L..19F
 |doi=10.1088/2041-8205/806/1/L19
 |s2cid=118602808
}}</ref>

In 2018, radio observations from the interaction between the circumstellar ring of dust and the shockwave confirmed that the shockwave has now left the circumstellar material.  It also shows that the speed of the shockwave, which slowed down to 2,300&nbsp;km/s while interacting with the dust in the ring, has now re-accelerated to 3,600&nbsp;km/s.<ref name="The Reacceleration of the Shock Wave in the Radio Remnant of SN 1987A">
{{Cite journal
 |last1=Cendes |first1=Y.
 |display-authors=etal
 |year=2018
 |title=The Reacceleration of the Shock Wave in the Radio Remnant of SN 1987A
 |journal=[[The Astrophysical Journal]]
 |volume=867 |issue=1|pages=65
 |arxiv=1809.02364
 |bibcode=2018ApJ...867...65C
 |doi=10.3847/1538-4357/aae261
 |s2cid=118918613
|doi-access=free
 }}</ref>