http://www.skyandtelescope.com.au 7SThispictureoftheSmallMagellanicCloud
iscomposedoftwoimagesfromtheDigitized
SkySurvey2.LAST YEAR, A TSUNAMIof gravitational
waveswashedoverEarth,heraldingthe
ancientcollisionoftwoneutronstarsin
afar-offgalaxy.Suchcollisionsappear
tobethebirthplaceofmanyofthe
heaviest elements such as gold, platinum
anduranium.Nowanunusual,rapidly
brightening supernova might help
researchers understand just how such
neutronstarduosariseinthefirstplace.
Thesupernova,designatediPTF14gqr,
wentoffintheoutskirtsofagalaxysome
900millionlight-yearsaway.Detected
inOctober2014,thissupernovaseemed
odd right away. Most supernovae take a
few weeks to hit their peak brightness.
Thisonedidsoinlessthan7days,
suggesting it had only a relative paucity
ofdebristoclearoutoftheway.
Kishalay De (Caltech) and colleaguesSThe moments before (left), during (middle),
and after (right) Supernova iPTF14gqr appeared
in the outskirts of a spiral galaxy.In 2012, researchers offered an
answer. The Magellanic Clouds must
have had a close encounter with each
other in the recent past — perhaps even
a direct collision — to pull the bridge’s
stars and gas away so strongly.
In the November 1 issue of
Astrophysical Journal Letters, Oey and
her collaborators present support
for this hypothesis based on stellar
position and velocity data from the
Gaia satellite. They discovered that
many of the stars in the eastward
'wing' of the SMC are moving in
concert, a result that can only be
explained by the LMC’s gravitational
force acting on the galaxy globally.
With further study, astronomers
might be able to use the observed bulk
motion to understand the direct collision
in even more detail: how long ago it
occurred, at what speed, and at what
angle. The better astronomers understand
the past trajectories of these galaxies,
the more precisely they can predict their
future fates, which will likely see them
accumulated into the Milky Way.
■ EMILY SANDFORDestimate that SN iPTF14gqr released
onlyone-fifthofthemassoftheSun
—mostsupernovaeexpelseveralSuns’
worthofgas.What’smore,theteam
deducedthatthestarwasabout1.
timesasmassiveastheSunshortly
beforetheexplosion.Butforastartogo
boom,itsoriginalmassneedstobeat
leasteighttimesasmassiveastheSun.
Somethingmusthavestrippedthestar
of most of its mass before it died. The
mostlikelycandidate,theteamargues
in the October 12 issue of the journal
Science,isacompanionneutronstar.
Here’s how the team thinks the
storyplayedout:Twomassivestars
onceorbitedeachother.Oneofthem
exploded,leavingitscorebehindasa
neutronstar.Thisneutronstarthen
strippeditsmateofmostofitsgas.Bythe time the second star exploded — and
became known as SN iPTF14gqr — there
wasn’t much gas left. The explosion
would also have produced a neutron star,
leaving two neutron stars in a tight orbit.
“This is the first example of a
supernova that produced a compact binary
system that is tight enough to eventually
merge and produce gravitational waves,”
De says. Unfortunately, at nearly 1 billion
light-years away, the high-energy light
from any putative neutron stars is much
too faint to detect.
■ CHRISTOPHER CROCKETTEvidence mounts for mighty Magellanic collision
THE PAST DECADE HAS seen
astronomers’ understanding of the
Magellanic Clouds — two dwarf galaxies
near the Milky Way — completely
overthrown, resulting in new revelations
about the violent and ongoing formation
of our own galaxy. Now, a team led
by Sally Oey (University of Michigan)
has made a discovery that bears out
predictions that these two galaxies
once collided: A big chunk of the Small
Magellanic Cloud (SMC) is moving
toward the Large Magellanic Cloud
(LMC) and the Magellanic Bridge of gas
and stars that joins them.
Astronomical wisdom once held that
the LMC and SMC had been orbiting
the Milky Way for billions of years, but a
landmark study in 2007 showed that the
dwarf galaxies are likely falling toward
the Milky Way for the first time. That
realisation introduced a new puzzle: If
the Magellanic Bridge wasn’t stripped
from the LMC and SMC by the powerful
gravity of the Milky Way, as previously
suspected, how did it get there?Low-key blast marks possible
birth of neutron star duo
SUPERNOVA: SDSS / CALTECH / KECK; MAGELLANIC CLOUD: DAVIDE DE MARTIN (ESA / HUBBLE) / DSS