Sky and Telescope - February 2017 - 38

Supernova 1987A Today

of the wreckage of SN 1987A using the Hubble Space Telescope
has failed to show any sign of a point source in the center,
even with the view clearing during the last 30 years. Is the
neutron star shrouded by dust? Or did infalling debris shove
it over the next gravitational-collapse brink, to become a black
hole? The case is open, and we will keep looking with ever
more sensitive techniques for the presumed hot corpse, as the
remaining light from the explosion continues to fade away.

Ionized gas recombines

Dust formation

Expanding
photosphere

SN 1987A:
The First 12 Years

"Freeze out"
(remaining ionized
gas stays ionized)

Struck By the Light . . .
Ring emission

Radioactive tail
(44Ti)

Ejecta emission

4
5
6
7
8
Years since outburst

S SLOW FADE After the supernova exploded, it brightened rapidly for
many days as its opaque photosphere expanded like that of a gigantic
star. Then as it cooled and thinned, new energy sources took over. Peak
brightness came three months after the explosion, from the energy released by electrons and ionized atoms recombining in the debris. Radioactive heating later kept the remnant glowing as it dimmed, though newformed dust suddenly began hiding some of the light. By 7 years out, the
surrounding hydrogen ring was brighter than the expanding ejecta.

ﬁnal, unstable stages of nuclear burning - fusing helium to
carbon and oxygen, then step by step up to silicon, ending
with a hot and brief episode of fusing silicon into iron. Iron
is nearly the minimum-energy atomic nucleus: it's unburnable, the "ash" at the end of the line for releasing energy by
fusion. As the core of Sanduleak -69° 202 accumulated iron
and could produce no further heat to hold it up, it shrank,
teetered on the edge of gravitational collapse, and ﬁnally fell
over the brink.
Computer models had shown that the headlong collapse of
a star's core would stop only when the inner few solar masses
became as dense as an atomic nucleus - forming an incredibly dense neutron star about the size of a city on Earth. In
those last seconds, as the neutron star forms, the material
crashing down heats to about 100 billion kelvin. It should be
hot and dense enough to produce an immense pulse of neutrinos, amounting to about 10% of the star's rest mass. Most
of the neutrinos ﬂy clean out of the star, carrying off the vast
majority of the energy released in the entire disaster.
One of the great physics events of 1987 was the ﬁrst detection, in two giant underground neutrino detectors, of this
surge of ghostly neutrinos. It began with a burst and trailed
out in the next 13 seconds.
The visible brightening got under way in the subsequent
hours, as the shock wave from the core hit the star's surface,
abruptly heating it to ultraviolet temperatures and blasting it
outward to enormous size.
The neutrino event is widely regarded as the yelp of a newborn neutron star, not a black hole. But our careful scrutiny
38

F E B R U A R Y 2 0 1 7 * SK Y & TELESCOPE

Other surprises have played out over the past three decades.
Our IUE measurements showed a prompt ﬂare of ultraviolet light, denoting the shock wave bursting through the
star's surface. In the next 1.1 years, that ﬂare of ultraviolet brilliance travelled 1.1 light-years outward and reached
something that was already loitering in the neighborhood:
slow-moving, dense gas. We saw new emission as the ultraviolet ﬂash excited it to glow. This was, plausibly, gas that the
star had shed tens of thousands of years earlier, toward the
unstable end of its fuel-burning life.
Then in 1990 Hubble was launched. Even with its compromised resolution due to the notorious error in its primary
mirror, we were surprised and delighted to see that this newly
illuminated material was not a spherical shell as we expected,
but a beautiful, thin ring. Something had previously sculpted
the star's ejecta into this shape. Here was a lesson: nature
doesn't always produce the simplest thing we think of.
That lesson was repeated when we obtained the ﬁrst
images of SN 1987A after astronauts installed corrective
optics into Hubble in 1993. We saw not just one ring but
three, aligned in remarkable symmetry.
The two larger, fainter, outer rings seem to indicate a tilted
hourglass-shaped structure around the exploded star, with
the inner ring as its neck - like the hourglass shapes often
seen among planetary nebulae, where a star with lower mass
blows off material near the end of its life. An artist's concept

S SIDE VIEW This artist's concept from a different perspective illustrates the hourglass-shaped cavity that the three visible rings highlight.

BOT TO M: ESO / L. CA LÇA DA . TOP: S&T / LE A H TISCIONE, DATA FRO M C. FR A NSSON / P. LUNDQVIST / R. A . CHE VA LIER

V magnitude

Radioactive heating
(56Ni, 56Co)

Sky and Telescope - February 2017

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