http://www.skyandtelescope.com.au 37PETER 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 previous 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 ballooning debris cloud would easily fill a volume
equal to 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 (University of California,
Berkeley) that the supernova had surged in brightness: It was
now five times more luminous than it was in the discovery
images. It had also doubled in size, the debris cloud now large
enough to fill 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
supernova 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 Pacific Ocean, the Gemini North
Observatory and the Keck Observatory in Hawai‘i swung their
mirrors in its direction. In all, seven observatories imaged
the supernova that first 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 luminosity, brightening and
fading in a predictable pattern that enables astronomers to
calculate their cosmic distance — making them a crucial tool
in cosmology. In an advance that secured the 2011 Nobel
Prize in Physics, for example, Type Ias helped prove that the
universe is expanding at an ever-increasing rate.
But just what causes these identical flares? Astronomers
have long thought that Type Ia supernovae are like fireworks
built in a cosmic assembly line, each set off by the cataclysmic
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 confirm
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
cosmological probes and we don’t know what causes them,”
says Ryan Foley (University of California, Santa Cruz).
But thanks to that fate 2011 and others
since, many astronomers ept what they have
long denied: that there mi one way to create
a Type Ia supernova. That tandard candles
cosmologists depend on a tandard.A tale of two supernova
A Type Ia supernova is easy to pick wd.It
increases in brightness in a matter
over the course of hundreds of da
silicon, calcium and iron, but no
Because Type Ia explosions look so similar to one another,
astronomers long assumed that they originated from an
identical physical process, one in which white dwarfs play a
starring role.
The problem is, something has to push them over the edge.
“You have a mysterious agent — some kind of hidden assassin
— who came along and caused this white dwarf to explode but
did so in such a way that it left very little evidence that it was
ever there,” says Stuart Sim (Queen’s University Belfast, UK).
That assassin is likely a companion star, but astronomers are
unsure if it’s another white dwarf, a star like the Sun, or a
giant bloated star that ran out of hydrogen fuel in its core
long ago.
It’s a crucial question because the nature of the
companion determines the exact cause of death. If the
companion is a white dwarf, then the two stars will spiral
in toward each other and collide in a violent explosion. But
if the companion is a larger star, either like the Sun or a red
giant, then the white dwarf will siphon matter from it until
it ignites a runaway thermonuclear reaction in the core and
blows itself to smithereens.
Researchers have long argued over which scenario is
true. The thinking throughout most of the 20th century
was that the ‘hidden assassin’ is a comparatively larger star,
which could feed the dwarf until it reaches a critical limit.
The dwarf actually scrunches down in size as it siphons
material from its companion star, which causes its density
and temperature to skyrocket. Eventually, conditions
become so extreme that there is no longer space for the
atoms’ electrons, which are forced into the nuclei, igniting
a runaway thermonuclear reaction that forces the star to
explode. Because that reaction always occurs when the star
hits the same density and temperature, it explodes with an
identical brightness — explaining why all Type Ia supernovae
look alike.
Or so we thought. Then in 1991, two supernovae
were discovered that did not explode at their expected
luminosities — one was fainter and one was brighter. “That
meant they were not standard candles and you really had
to worry about this,” says Alexei Filippenko (University of
California, Berkeley). Luckily, astronomers soon discovered
that the brightest supernovae fade more slowly than their
dimmer kin, meaning that astronomers could correct for the