http://www.skyandtelescope.com.au 11was carried away by neutrinos. (As a side
benefit, analysing these measurements
also gave us more information about
these particles. They were thought to
have no mass at all, but now we know
that if they do have any mass, it is less
than one 30,000th the mass of an
electron. Close enough to nothing.)
Almost as soon as SN 1987A was
spotted, astronomers suspected that
the star which had collapsed was a
blue supergiant, one built on the same
principles as our Sun but vastly larger
and more luminous; both much hotter
and much younger. Its formal name was
Sanduleak -69°202; the numbers give
its position in the sky. Once the light of
the supernova had faded, that suspicion
was confirmed by the fact that the
suspect star was no longer there. It had
blown itself to bits.
Something else was missing as
well — the very small but very heavy
neutron star that should have been
there as a remnant of the collapsed star.
The Hubble Space Telescope has being
imaging that region of the sky for more
than 25 years but has found no trace,
nor have ground-based astronomers
using radio telescopes. Perhaps the
neutron star is hidden by dust, or maybe
it collapsed further to become a black
hole. As usual, more
questions than answers.■ DAVID ELLYARD
presented SkyWatch on
ABC TV. He is the author
of Who Discovered What
When, and Who Invented
What When.by David Ellyard DISCOVERIESA star goes off
The first bright supernova in 400 years both answered and posed questions.
T
he Large Magellanic Cloud (LMC)
is one of the many splendid sights
of our southern sky, even though
you need a dark sky to see it best. A
small galaxy lying not far from the
South Pole of the sky, it appear as a
faint, fuzzy patch of light to the naked
eye. The LMC is a neighbour of our own
galaxy, the Milky Way, and sits some
168,000 light years from Earth.
If you’d been lucky enough to be
looking towards the LMC at the right
moment on February 23, 1987, you
would have noticed something new —
a sudden brightening that outshone
all the other stars in the vicinity.
You would have been seeing the first
supernova bright enough to be visible
to the naked eye since the one that
appeared in 1604, commonly called
Kepler’s Star (after the legendary
astronomer Johannes Kepler who
studied it intently).
Supernovae are relatively rare events.
SN 1987A, as it is formally labelled, is
the most recent supernova spotted in
any of the dozen or so galaxies in our
Local Group. In a typical galaxy of 200
billion stars, supernovae occur about
twice per century, suggesting that others
have occurred in our Milky Way since
1604 but have been obscured from our
eyes by interstellar dust.
Supernovae happen for
various reasons. In cases
such SN 1987A (known
as Type II supernovae) a
very large star, maybe a
hundred times the mass
of the Sun, reaches a
point where the outward
radiation pressure (from
the internal nuclear
reactions that generate
its energy) is no longer
able to fight against the
weight of its outer layers,
and the star suddenly andW Light from supernova
1987A reached Earth in
February 1987 (left). An
image taken before the
explosion (right) shows the
progenitor star.catastrophically collapses, in a matter of
minutes or less.
Kepler couldn’t even use a primitive
telescope to make his observations,
as they were not pressed into service
for astronomy until Galileo pioneered
them half a decade later. In contrast,
astronomers observing SN 1987A had
a cupboard full of up-to-the-mark
observing equipment, able to detect
radiation from the event across the
whole range of electromagnetic radiation
from visible light to gamma rays.
But SN 1987A’s initial flash of
light was not the first indication that
something was going on out there. Two
or three hours before the first light
arrived at Earth, three specially equipped
laboratories recorded bursts of neutrinos,
uncharged particles associated with
the sort of nuclear reactions that go on
inside stars. Neutrinos are very elusive,
exceedingly reluctant to interact with
any ordinary matter, such as the material
in the detectors. So capturing any at
all was an achievement. In this case,
only 25 neutrinos from the supernova
were trapped, but given the distance to
the exploding star and the size of the
detectors, the findings were consistent
with models that suggest that 99% of the
energy released in the collapse of the starDAVID MALIN/AAO