NEWS1046 5 JUNE 2020 • VOL 368 ISSUE 6495 sciencemag.org SCIENCECREDITS: (X-RAY) NASA/CXC/SAO/D. PATNAUDE; (OPTICAL) DSST
he white dwarf stars were zip-
ping across the Milky Way at
more than 1000 kilometers per
second—thousands of times faster
than a speeding bullet, so fast that
they would eventually escape the
gravitational clutches of the Gal-
axy. “They were not like anything
we had seen before,” says Boris
Gaensicke, an astronomer at the University
of Warwick.
Gaensicke and his colleagues suspected
these burnt-out embers were fleeing
scenes of violence: supernova explo-
sions in which another white dwarf
had detonated like an Earth-size hy-
drogen bomb. In the standard pic-
ture of these explosions, known as
type Ia supernovae, a nearby giant
star lights the fuse. But the extreme
speed of the white dwarfs suggested
a different scenario, in which the flee-
ing dwarfs had delivered the sparks,
from close orbits around the doomed
stars. When they blew up, these part-
ners were flung away like shots from a
biblical sling.
The speeds of the three white dwarfs,
discovered in a 2018 data set from Eu-
rope’s Gaia satellite, were just one clue
to this picture. Subsequent ground-
based observations found traces of
iron and other metals in the stars’ light—ele-
ments that might have been implanted by a
supernova blast. From the color and bright-
ness of the light, the astronomers could de-
duce that the stars were hotter and larger
than typical white dwarfs, as if they had been
puffed up by a sudden energy boost. Most
telling, the researchers rewound the stars’
trajectories and found that one hailed from a
known site: the remnant of a 90,000-year-old
supernova. They are “the very best evidence”
for the twin white dwarf scenario, says teamleader Ken Shen of the University of Califor-
nia (UC), Berkeley.
The evidence that twin white dwarfs
drive most, if not all, type Ia super-
novae, which account for about 20% of the
supernova blasts in the Milky Way, “is
more and more overwhelming,” says Dan
Maoz, director of Tel Aviv University’s Wise
Observatory, which tracks fast-changing
phenomena such as supernovae. He says
the classic scenario of a white dwarf
paired with a large star such as a red giant“doesn’t happen in nature, or quite rarely.”
Which picture prevails has impacts
across astronomy: Type Ia supernovae play
a vital role in cosmic chemical manufac-
turing, forging in their fireballs most of
the iron and other metals that pervade
the universe. The explosions also serve as
“standard candles,” assumed to shine with
a predictable brightness. Their brightness
as seen from Earth provides a cosmic yard-
stick, used among other things to discover
“dark energy,” the unknown force that isaccelerating the expansion of the universe.
If type Ia supernovae originate as paired
white dwarfs, their brightness might not
be as consistent as was thought—and they
might be less reliable as standard candles.
Robert Kirshner, a longtime supernova
watcher at the Gordon and Betty Moore
Foundation, isn’t ready to give up on the
classic scenario, but he acknowledges the
misgivings about it. “It’s plausible that there
is more than one path to a type Ia super-
nova, but the uniformity of the light out-
put is a tiny bit of a paradox,” he says.
Nevertheless, “There’s a nagging
doubt: Do we fully understand the na-
ture of these explosions?”SUPERNOVAE COME in two basic fla-
vors. The most common are called
core-collapse. They occur when a mas-
sive star, far larger than the Sun, runs
out of fuel. The pressure of the core’s
heat can no longer counteract grav-
ity; the outer parts of the star collapse
into the core with such force that a
rebounding shock wave, or sometimes
a renewed fusion burn, blows out the
star’s outer layers, leaving a neutron
star or black hole behind.
The rest are all type Ia—white
dwarf stars somehow reignited into
a runaway fusion reaction. The burst
of new energy happens too fast for the
star to absorb, blowing the entire thing to
smithereens in a blast that is brighter and
longer than a core-collapse supernova.
White dwarfs might seem unlikely can-
didates for fireworks. They are the cinders
of Sun-like stars that have burned up their
hydrogen and helium fuels, leaving—in most
cases—carbon and oxygen, heavier elements
that can’t fuse in such a low-mass star. White
dwarfs shrink to the size of Earth, and only
glow from leftover heat. Theoretically, theyThe brightest supernovae, used as cosmic yardsticks, have an
unexpected trigger: pairs of dead stars
By Daniel Clery
DOUBLE TROUBLE
FEATURES
Kepler’s supernova, a type Ia explosion from 1604, lingers today with
the x-ray glow of hot, leftover debris.Published by AAAS