Sky and Telescope - February 2017 - 39

The Blast Hits the Inner Ring
Visible light
2000.9

2003.0

2006.0

2009.3

2011.0

2013.1

2014.5

2005.0

2007.0

2009.0

2011.2

2013.2

2014.7

1˝

X-rays (0.5 - 3 keV)
2001.0

2005.7

2003.0

Radio (9 GHz)

DICK MCCR AY A ND CL A ES FR A NSSON ( VISIBLE: C. FR A NSSON E T AL. / ASTR OPHYSICAL JOUR NAL LE T TERS 2015, X-R AY: K . FR A NK E T AL.
/ ASTR OPHYSICAL JOUR NAL 2016, R A DIO: C.-T. NG E T AL. / ASTR OPHYSICAL JOUR NAL 2013)

2001.9

2003.0

2005.2

2006.9

of the side view is at lower left. More recently, astronomers
have found other superluminous blue-giant stars with similar
three-ring structures. One is pictured on the next page. Are
these, too, about to become core-collapse supernovae?

2009.4

2011.3

2013.3

S THE NECKLACE LIGHTS UP As the leading edge of the supernova
debris reached the inner ring, astronomers tracked developments at
many wavelengths. In the visible-light images at top, from the Hubble
Space Telescope, the brightness of the ring has been reduced by a factor of 20 to make it possible to see the faint emission from the bulk of
the debris: the expanding, oddly shaped nebula inside.

. . . and Then By the Blast
Meanwhile, the shredded remains of the star itself were hurtling outward behind the ultraviolet pulse at up to about 10%
the speed of light. It did not take a supercomputer to calculate that a ring about 1 light-year from the explosion would
get whacked in about 10 years. Sure enough, in 1995 Hubble
began to see this collision.
But, just as the material around the star was not a shell
but a three-ring extravaganza, the collision was more intricate than we expected. Instead of lighting up all at once as
the ﬁrst debris hit, the ring lit up in about 30 "hot spots,"
spaced around its circumference like a string of pearls.
The necklace of spots was presumably the sign of dense
ﬁngers of gas in the surrounding ring pointing toward the
star, a little like stalagmites from the ﬂoor of a cave. As the
shock wave expanded, it hit the tips of the ﬁngers ﬁrst. Now
this preliminary encounter is coming to an end: some of the
hot spots are fading and merging, and the destruction of the
entire ring by the oncoming blast is under way.

Radioactive Time Release
The interaction of the expanding shock with the ring was not
just a spectacle - it represented a fundamental change in the
energy source for the light and other emissions we see.
In the initial supernova explosion itself, the nuclear

alchemy that changes one element into another produces
an array of isotopes. Some of these are stable nuclei of iron
and other elements near iron in the periodic table. The iron
in your blood came from star-core destruction of this sort,
which seeded iron into interstellar clouds before our solar
system formed from them. But not all the nuclei produced
are long-lived. Some are radioactive, having decay times from
moments to years.
The exploding debris was initially heated by the shock
wave that blew the star apart. As it expanded and cooled, the
recombination of ionized atoms with their lost electrons provided most of the light around the time of peak brightness a
few months along. But after that, the debris was kept hot and
glowing by the decay of radioactive nuclei.
In the case of SN 1987A, we infer that the heat and pressure of the core collapse created 0.07 solar mass (2,300
Earth masses) of nickel-56: the isotope of nickel with 28
protons and 28 neutrons. It decays with a half-life of 6 days
to cobalt-56, which then decays with a half-life of 111 days to
iron-56. That was the main energy source lighting the debris
for about the ﬁrst 500 days of its long, slow fade. Along the
way an abrupt episode of dust formation partially veiled the
view, further dimming the visible light. But when you added
up the light, the infrared radiation re-emitted by the warm
s k y a n d t e l e s c o p e .c o m

* FEBRUARY 2017

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Sky and Telescope - February 2017

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