SCIENCE sciencemag.org 5 JUNE 2020 • VOL 368 ISSUE 6495 1049
be fairly common. In a 2018 study, Maoz
and his colleagues looked for the wobble
of white dwarfs being tugged by a partner,
combining survey results from the Sloan
Digital Sky Survey and Europe’s Very Large
Telescope. They concluded that about half
a billion white dwarf binaries have merged
in the Milky Way since its formation. If just
one-sixth of those mergers led to a type Ia
supernova, that would be one supernova
every 200 years—roughly what is observed
in the Milky Way. Another team, using Gaia
data, came to a similar conclusion.
Maoz is undeterred by the problem of
getting a complete, symmetrical blast.
“Just because we don’t know how it hap-
pens doesn’t mean nature hasn’t found
a way.” In fact, many believe nature has
found a way to blow up one member of a
white dwarf pair—but without a merger.
White dwarfs can have some leftover he-
lium in their atmospheres after the core
stops burning. When an orbiting pair is
on the cusp of merging, the larger of the
two stars can rapidly steal helium from the
smaller one to form a dense helium layer
on its surface. The helium layer can act as
a kind of blasting cap, exploding in a small
thermonuclear blast and sending a shock
wave into the star that can ignite the core.
This scenario is called D6, for dynami-
cally driven double-degenerate double
detonation. The idea was first developed
in 2010 by James Guillochon, a researcher
at the Harvard-Smithsonian Center for
Astrophysics, and his colleagues. It leaves
the smaller white dwarf battered but
thrown free, like those Shen’s group found
in the Gaia data. D6 was originally thought
to require a hefty amount of helium, mak-
ing it a rare event, but more recent model-
ing suggests just a few percent of a solar
mass could be enough, Woosley says.
ONE KEY FEATURE of the D6 scenario is that
the exploding white dwarf can be well be-
low the critical mass, because the spark
comes from a shock wave and not from
gravitational pressure. A less massive ex-
ploding star will produce less nickel and
be less bright.
Recent studies of the metallic elements
supernovae forge suggest the low-mass
type Ia may be the norm. According to
models, the production of manganese in
type Ia supernovae is particularly sensi-
tive to density in the white dwarf ’s core:
If the star is close to the 1.4–solar mass
threshold, its high-density core produces
lots of manganese; if the star is lighter—as
is likely in the D6 mechanism—it produces
one-tenth as much.
As a result, manganese abundances de-
rived from the light of stars today can hint
at the masses of the ancient supernovae
that seeded them with heavier elements.
“Manganese provides an indirect way
to probe previous generations of type Ia
supernovae that went off in that galaxy,” says
Ashley Ruiter of the University of New South
Wales, Canberra.
In a pioneering study from 2013, re-
searchers led by Ivo Seitenzahl, then at the
Julius Maximilian University of Würzburg,
compared manganese abundance in the
Sun with models of how much manganese
would be produced by supernovae of dif-
ferent masses. They found that only half of
the supernovae that exploded in the solar
neighborhood in the past needed to be high
mass to explain the Sun’s manganese con-
tent. “This was the first of a new wave of
results,” says Maria Bergemann of the Max
Planck Institute for Astronomy. This year,
she and her colleagues reported looking
at manganese in 42 stars across the Milky
Way and concluded that the abundances
suggest 75% of the galaxy’s type Ia super-
novae were low mass.The implications of undersize type Ia su-
pernovae extend far beyond the elements
in the present-day universe. They also
raise questions about the explosions’ long-
standing role as “standard candles” for
probing cosmic history.
In 1998, researchers compared a few
dozen distant type Ia supernovae with
closer ones and found that they were dim-
mer than they should have been. They
concluded that the universe’s expansion
is accelerating, driven by some unknown
dark energy—a discovery for which they
were awarded the 2011 Nobel Prize in
Physics. Supernova distances are also at
the heart of a dispute over the value of the
expansion rate itself, known as the Hubble
constant (Science, 10 March 2017, p. 1010).
In the nearby universe the expansion rate
is measured using standard candles such
as type Ia supernovae; in the distant, early
universe, it is derived from clues such as
the cosmic microwave background, the
echo of the big bang. When the effect of
dark energy is taken out, the two values
should agree—but they don’t.
Could a not-so-standard candle jeopardizethose discoveries? “It means something, but
not that dark energy goes away,” Woosley
says. Dark energy has been confirmed us-
ing other methods, so he’s not worried about
that. But he thinks cosmologists will run into
trouble as they put their theories to more
rigorous tests that require more precise stan-
dard candles. “Supernovae could be less use-
ful for precision cosmology,” he says.
Astronomers already knew the peak
brightness of type Ia supernovae isn’t per-
fectly consistent. To cope, they have worked
out an empirical formula, known as the Phil-
lips relation, that links peak brightness to
the rate at which the light fades: Flashes that
decay slowly are overall brighter than those
that fade quickly. But more than 30% of type
Ia supernovae stray far from the Phillips re-
lation. Perhaps low-mass D6 explosions can
explain these oddballs, Shen says. For now,
those who wield the cosmic yardstick will
need to “throw away anything that looks
weird,” Gaensicke says, and hope for the best.
Andy Howell, a supernova watcher at
Las Cumbres Observatory, thinks type Ia
supernovae could still be reliable tools for
cosmology if astronomers could separate the
different varieties of type Ia that are now
lumped together. “If we knew there were two
populations, we could make the measure-
ments even better,” he says.
So far, astronomers can’t say how many of
their favorite cosmic explosions are sparked
by white dwarf pairs rather than a giant and
a dwarf. “It’s too early to say with certainty
what that fraction is,” Ruiter says. But the
coming years could bring more clarity.
Survey telescopes that scan the skies
nightly or even hourly are catching more
and more supernovae. The current frontrun-
ner, the Zwicky Transient Facility in Califor-
nia, spots about 30 supernovae per night.
Its output will be dwarfed in 2022 with the
opening of the Vera C. Rubin Observatory, an
8.4-meter survey telescope in Chile that is
expected to find thousands of supernovae
nightly. Other telescopes able to obtain spec-
tra from thousands of objects simultane-
ously will enable astronomers to study the
explosions for the features—the blue flash,
the hydrogen absorption lines—that could
betray the involvement of a giant star.
Shen and Gaensicke hope the next data re-
lease from Gaia will contain more high-speed
white dwarfs fleeing from D6 explosions. And
the Laser Interferometer Space Antenna, an
orbiting gravitational wave detector due for
launch in 2034, will be able to sense white
dwarf pairs as they spiral in toward merger,
giving astronomers a better idea of how com-
mon they really are. “It’s a real golden age for
supernovae because we’re finding so many,”
Howell says. “We’ve now finally got the tools
to see them in new ways.” j“It’s a real golden age
for supernovae because
we’re finding so many.”
Andy Howell,
Las Cumbres ObservatoryPublished by AAAS