skyandtelescope.com • JUNE 2019 17“You have a mysterious agent — some kind
of hidden assassin — who came along and
caused this white dwarf to explode but
did so in such a way that it left very little
evidence that it was ever there.”
—Stuart Sim
kin, meaning that astronomers could correct for the dif-
ference and continue to use the objects in cosmology. As
many scientists say, they’re not standard candles, but they’re
standardizable candles.
Still, scientists did not know why some Type Ia supernovae
reached different brightnesses, causing many to reconsider
an alternate possibility: White dwarfs were instead dancing a
deadly waltz with their own kind.
Because the dwarfs collide in this scenario, the fi nal sys-
tem won’t necessarily hit the same critical conditions every
time. White dwarfs in a binary can vary in weight from 0.
to 1.4 times the mass of the Sun, so the fi nal system couldtheoretically vary from 0.6 to 2.8 Suns (although having two
massive white dwarfs paired up is highly unlikely). Convert
that to energy and you have a wide range of luminosities,
thus explaining the oddities discovered in 1991.
By 2011, the pendulum had swung toward this so-called
double-degenerate scenario, where “degenerate star” refers
here to a white dwarf (as opposed to the single-degenerate
scenario, where there is only one dwarf and a large star). Not
only can low-mass companions explain the range of bright-
nesses, but they also explain why astronomers had failed to
spot bereaved companion stars within the debris of ancient
supernova remnants.
Indeed, when Nugent discovered the nearby supernova,
he was able to scour previous images to search for signs of a
companion. But Hubble images of the Pinwheel Galaxy taken
before the blast revealed no trace of a star at SN 2011fe’s loca-
tion, leaving him to argue that the culprit could be no larger
than a second white dwarf.More Than One Pair Can Tango
The case, however, was far from closed. Consider another
stellar blast also detected by Nugent and his colleagues in
GR 2011. At fi rst, the supernova, known as PTF 11kx, looked like
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