Sky and Telescope - February 2017 - 37

Technology has changed our lives since 1987, but it has
changed astronomy more. The instruments we used in those
ﬁrst heady days were primitive compared to the ones we use
now. And a good thing, too: today SN 1987A is ten million
times fainter than at its peak, but better tools let us study it
across the electromagnetic spectrum. For example, in 1987
the space telescope available to our team was the International Ultraviolet Explorer, with an aperture of 45 centimeters (18 inches). I had been using IUE to study extragalactic
supernovae and had mentioned in passing that we would also
like to observe any supernova that might appear in the Local
Group of galaxies. The Large Magellanic Cloud certainly
qualiﬁed, and in a phone call that morning, Yoji Kondo at the
IUE proudly told me that observations had already begun.
Since the Hubble Space Telescope was launched, we've
been able to use its 2.4-meter aperture with a succession of
ever more powerful visible-light, infrared, and ultraviolet
instruments installed by Space Shuttle astronauts. At radio
wavelengths, the Australian Compact Telescope Array (ACTA)
has tracked the rising radio emission from SN 1987A. The
new ALMA observatory is creating millimeter-wave images
of it that are as sharp as our HST images, to tease apart the
ongoing chemistry and physics of the explosion. Far-infrared
observations from the Spitzer and Herschel missions have
probed the cold dust that formed from the expanding debris.
X-ray observatories, such as the durable Chandra and the
recent NUSTAR, measure emission from million-degree gas
where the debris is currently colliding with surrounding
clouds. These tools give us a rich view, as the supernova of 30
years ago morphs into the supernova remnant of the future.

TOP: DAVID M A LIN / AUSTR A LIA N ASTRONO MICA L OBSERVATORY; BOT TO M: N ASA / STSCI / H A RVA RD SINS

The Cry of a Collapsing Star
There were some real surprises in 1987. To start with, the star
that exploded turned out to be number 202 in the -69° band
of Nick Sanduleak's catalog of bright stars in the Large Magellanic Cloud. It had been a 12th-magnitude blue supergiant,

S BEFORE AND AFTER Ten days in, the exploded blue supergiant star
Sanduleak -69° 202 was shining in naked-eye view and still brightening
as it expanded.

spectral type B3Ia. Oops. In giving astronomy exams at Harvard, the answer that I marked correct was that core-collapse
supernovae happen only in red supergiants at a different
stage of development. Nature hadn't read the textbook.
Stars in the Large Magellanic Cloud have less than half
the heavy-element abundance of in stars in the Milky Way's
disk, and people calculated the effect that this chemical difference would have on the life of a massive star. If it began
with 18 times the mass of the Sun, a likely estimate, it would
shed about 4 solar masses as its core stepped through the
T HANGING IN SPACE The three-ring ornament and the supernova
remnant inside it are seen against the fringe of the Tarantula Nebula in
the Large Magellanic Cloud. The two bright stars that appear on the
rings are unrelated chance alignments. This view was assembled in
1998 from Hubble images of the region, by the Supernova Intensive
Study group at the Harvard-Smithsonian Center for Astrophysics.

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* FEBRUARY 2017

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Sky and Telescope - February 2017

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