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TYIMAGESPLUS“STANDARD” CANDLES by Shannon Hall
14 JUNE 2019 • SKY & TELESCOPE
Type Ia supernovae — the cosmic mileposts that helped prove the universe is
accelerating — are not as uniform as astronomers once thought they were.puPINWHEEL LIGHTS UP In August 2011 a white dwarf exploded in
M101 as a Type Ia supernova.Cosmıc Fuse
P
eter Nugent knew he had to act
fast. It was a little past noon
on August 24, 2011, and he was
sifting through images of the previ-
ous night’s sky when he discovered a
new point of light: a young supernova
explosion. While the explosion was
only 12½ hours old, it was already so
bright that it outshone the nearby stars
in its galaxy and so large that its bal-
looning debris cloud would easily fi ll
the orbit of Saturn.
Nugent checked the time of the
image and saw that another 16 hours
had already passed. He dashed to the
phone.
Some 20 minutes later, a colleague
swung a telescope in the Canary Islands toward the blast and
shot a spectrum. The speedy observations told Nugent (Uni-
versity of California, Berkeley) that the supernova had surged
in brightness: It was now fi ve times more luminous than it
was in the discovery images. It had also doubled in size, the
debris cloud now large enough to fi ll the orbit of Uranus.
But perhaps the most exciting characteristic of the “new
star” was that it sat within the spiral arms of the Pinwheel
Galaxy, a mere 21 million light-years away. Catching a super-
nova so soon after the explosion was rare, but catching an
early supernova so close was the chance of a lifetime.
Nugent spent the next 36 hours awake, convincing those
running space- and ground-based telescopes to observe the
supernova. As night’s shadow swept from the Atlantic across
North America, Lick Observatory and the CARMA radio array
peered at the new star. Then, as darkness continued farther
west and hit the Pacifi c Ocean, the Gemini North Observa-
tory and the Keck Observatory in Hawai‘i swung their mirrors
in its direction. In all, seven observatories imaged the super-
nova that fi rst night. In the days and weeks that followed,SN 2011fe became the most studied
supernova yet.
And it wasn’t just any supernova: SN
2011fe was a special kind of supernova,
called Type Ia. These outbursts each
explode with a near-identical luminos-
ity, brightening and fading in a predict-
able pattern that allows astronomers to
calculate their cosmic distance — mak-
ing them a crucial tool in cosmology.
In an advance that secured the 2011
Nobel Prize in Physics, for example,
they helped prove that the universe is
expanding at an ever-increasing rate.
But just what causes these identical
fl ares? Astronomers have long thought
that Type Ia supernovae are like fi re-
works built in a cosmic assembly line, each set off by the cata-
clysmic death of a white dwarf (the stellar remnants that cram
the mass of the Sun into the volume of Earth). Indeed, the
immense amount of data enabled Nugent’s team to confi rm
that the star that went bang was the size of a white dwarf.
But there’s one problem: White dwarfs can’t explode on
their own. They are remarkably stable, so something else
has to trigger their eruptions — and astronomers don’t agree
on what that something is.
“It’s crazy that these are some of our fundamental cos-
mological probes and we don’t know what causes them,” says
Ryan Foley (University of California, Santa Cruz).
But thanks to that fateful observation in 2011 and others
since, many astronomers have come to accept what they have
long denied: that there might be more than one way to create
a Type Ia supernova. That means that the standard candles
cosmologists depend on are anything but standard.Lighting a