Sky and Telescope - February 2017 - 40

Supernova 1987A Today

dust, and the gamma-ray photons coming directly from the
decaying nuclei, the total continued to track the decay of the
nickel-56 and its cobalt-56 daughter.
In time, as the cobalt decayed away, a slower time-release
of stored energy became dominant: the decay of titanium-44,
half-life 1,200 days. This was the main power source for the
remnant's dimming emission until about 5,000 days (14
years) after the explosion.
That was more or less what astrophysicists had inferred
from the fading light curve. They were able to test these
ideas directly for the first time by measuring the energies of
gamma-ray photons arriving from the debris. These matched
the laboratory energies from decaying 56Ni and 44Ti. It's an
iron-clad case that these iron-peak elements powered the
remnant's long-lasting glow.

A New Process Takes Over

40

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

S NOT UNIQUE Other blue supergiants - that haven't blown up yet
- show similar three-ring hourglass ejecta as they near the ends of
their lives. This is a Hubble image of SBW 2007 (also known as SBW1),
20,000 light-years distant.

embodied in the three-ring circus of surrounding hydrogen.
Nature made SN 1987A in a more intricate and interesting
way than we imagined. It will be good to remember that lesson of humility.

The Fire Next Time
Tycho had his supernova, Kepler had his. Perhaps there's an
astronomer on Earth today whose name will go on the next
one in the Milky Way itself. More likely, the first galactic
supernova of the telescopic era will be caught by some large
cooperative effort. Perhaps it will be an array of survey
telescopes staring unblinkingly at the whole sky. Perhaps it
will be specks of light produced by neutrinos as they slash
through Antarctic ice, or a flare of ultraviolet light detected
by a satellite, or the jiggle of the gravitational waves that a
messy core collapse should produce. The next Milky Way
supernova could outshine every star in the sky, or it might
remain hidden from view behind thick interstellar dust. But
the statistics are pretty clear: the Milky Way has a supernova
about once a century on average. The next could happen at
any time. Keep looking up.

¢ Harvard professor ROBERT P. KIRSHNER specializes in supernovae. He won the 2015 Wolf Prize in Physics, and the U.S.
National Academy's Watson medal, for his work using supernovae to measure cosmic expansion. He is the chief program
officer for science for the Gordon and Betty Moore Foundation
and author of the popular-level book The Extravagant Universe:
Exploding Stars, Dark Energy, and the Accelerating Cosmos.

ESA HUBBLE / N ASA / NICK ROSE & STE V E BY R NE

But now, the energy source for the supernova remnant has
shifted again. Radioactivity accounted neatly for the decline,
but since about 2001, careful measurement of Hubble images
shows that the expanding debris remnant inside the inner
ring - the keyhole-shaped nebula seen in the top row on
the previous page - is rebrightening. Radioactivity cannot do
that. But the violent collision of the blast's outer wave with
the ring converts some of the blast's kinetic energy into heat.
The gas at the collision sites becomes so hot that it shines in
X-rays, which we can see with the Chandra satellite. Chandra has sharp enough vision to show that the X-ray emission
is indeed coming from near the ring. The surprising thing is
that these X-rays are shining back on the slower debris that
has yet to reach the ring, reheating it from the outside.
The collision with the ring is also accelerating electrons
there to relativistic energies. When the electrons interact
with the magnetic fields in the neighborhood, they emit
at radio wavelengths (synchrotron radiation). We've seen
increasing radio emission over the past decade, again mostly
coming from near the ring.
Nobody alive today will live long enough to see the
complete transition of Supernova 1987A into the supernova
remnant SNR 1987A. But the balance has tipped in the 30
years we've watched. The physical processes that make the
star-debris shine have shifted from the explosion itself, to
radioactive decay of what the explosion created, to its kinetic
crash into the surrounding interstellar gas.
When we look at remnants of historical supernovae, such
as Kepler's of 1604 or Tycho's of 1572 or somewhat older ones
in the Milky Way, we see their completed transitions. Their
conspicuous features now are radio emission from particles
that the shock wave accelerates to nearly the speed of light,
and X-ray emission from gas heated to millions of degrees.
For SN 1987A, we have a much richer and more complete
story that tells us what type of star exploded, what elements
the explosion initially produced, and the detailed sequence
of events that then unfolded. We can watch as the explosion
destroys the evidence of the star's pre-explosion behavior, as



Sky and Telescope - February 2017

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Sky and Telescope - February 2017 - 1
Sky and Telescope - February 2017 - Contents
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