“Standard” Candles
20 JUNE 2019 • SKY & TELESCOPE
they quick-moving, but they possess bizarre characteristics
that can be perfectly explained if they underwent such a cata-
strophic event (S&T: Dec. 2018, p. 30).
Stuart Sim says this model is his personal favorite.
Although other theories also suggest that a white dwarf can
siphon matter from a second, less massive dwarf, the scenar-
ios still require that the primary dwarf reaches critical mass
in order to detonate its core. But because the helium model
relies on an explosion on the surface to squeeze the core, the
explosion can occur well before the star hits that mass. Given
that there should be more low-mass white dwarfs than high-
mass white dwarfs in the universe, which might create duos
that never reach critical mass, that’s an attractive feature.
These scenarios might sound wild, and they are: None
of these ideas is exactly natural. “If you were to talk to a
17th-century physicist about all the things that happen in
the universe, I doubt any of these things would have come
to mind,” Sim says. “They have been thought up by people
who are scratching their heads thinking about how we can
arrange conditions for this rather spectacular but strange
thing to occur.”Blazing Forward
Although scientists do not fully understand the various chan-
nels that lead to Type Ia supernovae, the explosions’ role as
cosmic milestones is on solid ground. “No one needs to give
back their Nobel Prize or anything like that,” Foley says.
That’s because empirically-speaking, astronomers know
the brightness of every Type Ia supernova’s fl ame fairly well.A better understanding of what ignites that fl ame will only
improve the precision of those measurements — especially if
those systems change over time.
Currently, astronomers think that double-degenerate
systems might take longer to explode than single-degenerate
systems, because it can take billions of years after the white
dwarfs form for them to spiral in close enough to each other
for something explosive to happen. That could explain why
so many Type Ia supernovae in today’s universe seem to
come from double-degenerate systems, not single-degenerate
ones. It also suggests that a larger fraction might have come
from single-degenerate systems in the early universe. If one
pairing explodes at a slightly different luminosity than the
other, then it could affect the precision of our cosmological
measurements.
And that precision is crucial in illuminating dark energy,
the poorly understood phenomenon that propels the expan-
sion of the universe (S&T: May 2018, p. 14). A precise
measurement of the expansion rate throughout cosmic
history will help astronomers discover whether or not darkpWIDE-FIELD VIEW Technical lead Roger Smith inspects one of three wide-fi eld optical fi lters in the Zwicky Transient Facility’s camera. The survey
scans the entire visible sky over three nights with a fi eld of view equivalent to 247 full Moons. Astronomers expect it to fi nd tens of thousands of
cosmic explosions of various types each year.Although scientists do not fully understand
the various channels that lead to Type Ia
supernovae, the explosions’ role as cosmic
milestones is on solid ground.
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