Sky & Telescope - June 2019 - 19

survey with the Hubble Space Telescope that took a second
look at the locations where Type Ia supernovae had once
exploded. Because SN 2011kx only displayed hydrogen -
and hence proof of a gaseous companion - roughly 60 days
after its explosion, she worried that astronomers might
have missed that signature in other supernovae that weren't
observed for so long. But after observing 70 different supernovae that exploded one to three years ago, she found only
one that contained those telltale hydrogen emission lines.
Assuming that such lines come from a large star that triggered the Type Ia supernovae, such scenarios occur less than
5% of the time.
Moreover, Graham notes that the ﬁnding only proves a
large star is nearby, not that it pushed the star to explode.
"It's like a guilt-by-association situation," she says, "which
isn't a strong case." It appears that, while possible, the singledegenerate scenario may be rare.

roughly a decade, one that relies on an often-overlooked element within white dwarfs: helium. White dwarfs are mostly
composed of carbon and oxygen, but a thin layer of helium
often overlies the bulk. Shen thinks that one white dwarf may
steal enough of the other's helium to ignite its outer layer.
This detonation squeezes the white dwarf, sending a shock
wave deep below that ignites a second explosion - the Type Ia
detonation - in the star's core.
With the primary white dwarf annihilated, the companion - which until now had been orbiting the dwarf at rapid
speeds - has nothing to hold it in place and shoots off into
space. Shen even published a study in 2018 that shows three
such white dwarfs speeding through the galaxy. Not only are

E X PLOSION GR APHIC: TATIANA Z AE TS /
ISTOCK / G E T T Y IM AG ES PLUS

Twists and Turns
Now that astronomers have determined that both scenarios
are at play, there are still a number of other questions -
mostly about how the explosion occurs. Does it start at the
center of the white dwarf, for example, or does it start at the
interface between the core and the envelope? Could there be
two explosions, one that starts on the surface that sparks an
explosion below?
The possibilities have caused many scientists to argue that
perhaps a number of scenarios - each a variant on the singleand double-degenerate scenarios but with a unique explosion mechanism - produce Type Ia supernovae. "I would be
surprised if there were just two paths, actually," says Saurabh
Jha (Rutgers).
Take a new twist on the double-degenerate scenario. Typically, models suggest that the two white dwarfs spiral toward
each other and merge. But Ken Shen (University of California, Berkeley) has been honing a slightly different idea for

DIFFERENT DWARFS
White dwarfs are the dense, naked cores of dead
stars, exposed when the aging star shed its outer
layers. They're primarily carbon and oxygen. Many
have outer helium envelopes, while others have a thin
layer of hydrogen. Some show no signs of either. But
when they explode, there's usually no sign of hydrogen
or helium in their spectra - that's one of the ways
astronomers distinguish Type Ia supernovae from
other kinds of stellar explosions.

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Sky & Telescope - June 2019

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