1048 5 JUNE 2020 • VOL 368 ISSUE 6495 sciencemag.org SCIENCE
more dramatically. Just short of the Chan-
drasekhar limit, at about 1.4 solar masses,
the density and temperature of the core
shoot up. Flares of carbon fusion break
out. After smoldering for 100 or more
years, a runaway reaction detonates the
star and blows it apart in a matter of sec-
onds. The resulting fireball, 5 billion times
as bright as the Sun, forges a suite of met-
als from chromium to nickel in the peri-
odic table. The radioactive decay of nickel
to cobalt and then iron powers a brilliant
afterglow that peaks in a couple of weeks
and fades over years. And because, in this
picture, every type Ia explodes with the
same mass, they should all have the same
peak brightness.
THAT SCENARIO satisfied astronomers for
decades. But they have yet to find definitive
evidence for it. After a supernova disperses
enough to be transparent, astronomers
routinely search for a surviving red giant
companion but have never found one.
Models also suggest a flash of blue light
should appear, hours after the supernova
begins, as the expanding fireball slams into
the tenuous hydrogen atmosphere of the
red giant and heats it enough to glow in the
ultraviolet. But when astronomers spotted
a supernova in the nearby Pinwheel galaxy
within hours of its ignition in 2011, they saw
no blue flash. “SN 2011fe was a paradigm
changer,” says Stan Woosley, a supernova
expert at UC Santa Cruz.A second predicted signal—weaker but
more persistent—has also been elusive.
Astronomers would expect some hydro-
gen from the red giant to be swept along
with the other explosion debris, creating
an absorption line in the spectrum of the
supernova’s light as the remnant cools. But
a 2019 study of 227 type Ia supernova rem-
nants found no hint of this hydrogen.
The numbers are against the classic sce-
nario, Maoz says. In a galaxy like the Milky
Way, a type Ia supernova occurs once ev-
ery few hundred years. If they all originatefrom a white dwarf and a red giant, the
galaxy would have to host some 10,000 of
these pairs—and there’s little evidence of
them. Because it’s hard to see binary pairs
as separate stars at such great distances,
astronomers have looked for indirect evi-
dence, such as the recurrent novae trig-
gered when hydrogen from a red giant
spills quickly onto a white dwarf compan-
ion, or the soft x-ray glow that can result
from a steady stream of hydrogen. If the
Milky Way harbored 10,
white dwarf-red giant pairs,
we should see many novae and
soft x-ray sources, but only a
handful have been found, Maoz
says. “We’re missing 99.9% of
the systems.”
The theoretical underpin-
nings of the classic scenario
have problems, too. A key as-
sumption is that the explosion
is so violent that the entire star
is blown apart. But theorists
have had a hard time model-
ing how the smoldering fusion
flares would burgeon into an
all-consuming detonation. Mod-
eling that process requires sim-
ulating nuclear reactions with
centimeter resolution across
an object the size of Earth for
100 years or more. Until recently,
computers couldn’t handle that
challenge—and now that they
can, Shen says, only some mod-
els predict the transition. “The
state of the field is not conclu-
s i v e y e t .”ENTER ANOTHER WAY to blow up a
white dwarf. Astronomers know
that two white dwarfs orbiting
each other—a so-called double
degenerate—can gradually spi-
ral inward as their rapid orbits
throw out gravitational waves,
which carry away energy. When
they merge, there would be eas-
ily enough heat and pressure to
kick-start carbon fusion in the
combined stars.
Models suggest a problem with this mech-
anism: The burning would not start in the
core. It might kick off in the hot zone where
the two are actively colliding, leading to a lop-
sided and incomplete explosion that would
leave much of the stars’ mass unburned. Or
it could ignite close to the surface, only blow-
ing off the outer layers of the merged star.
“Getting a detonation in a double degenerate
is by no means trivial,” Woosley says.
Yet Maoz says the odds are irresist-
ible: White dwarf binaries are thought to IMAGE: FLASH CENTER FOR COMPUTATIONAL SCIENCE/UNIVERSITY OF CHICAGOTheorists have struggled to simulate classic type Ia explosions. This is one of their successful models.
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