“Standard” Candles
uSCENARIO #1: SINGLE DEGENERATE
A white dwarf paired with a much larger
star — such as an aged red giant — can
siphon gas from the companion star and
skirt itself in a disk (A). As the disk gas falls
onto the white dwarf, the temperature and
density build until carbon fusion ignites and
the white dwarf explodes with a standard
luminosity (B).uSCENARIO #2: DOUBLE DEGENERATE
Over billions of years, two white dwarfs
will inspiral toward each other by emitting
gravitational waves (A). Eventually they
will collide and be destroyed (B). Because
the white dwarfs don’t have to reach a
critical density and temperature in order
to explode, the resulting fl ash could have
a range of luminosities depending on the
white dwarfs’ masses.A Tale of Two Supernovae
A Type Ia supernova is easy to pick out from the crowd. It
increases in brightness in a matter of hours, before fading
over the course of hundreds of days. Its spectrum contains
silicon, calcium, and iron, but no hydrogen or helium.
Because Type Ia explosions look radically similar to one
another, astronomers long assumed that they originated from
an identical physical process, one in which white dwarfs play
a starring role.
The problem is, something has to push them over the edge.
“You have a mysterious agent — some kind of hidden assas-
sin — 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,” says Stuart Sim (Queen’s University Belfast,
UK). That assassin is likely a companion star, but astrono-
mers are unsure if it’s another white dwarf, a star like the
Sun, or a giant bloated star that ran out of hydrogen fuel in
its core long ago.
It’s a crucial question because the nature of the compan-
ion determines the exact cause of death. If the companion is
a white dwarf, then the two stars will spiral in toward each
other and collide in a violent explosion. But if the compan-
ion is a larger star, either like the Sun or a red giant, thenthe white dwarf will siphon matter from it until it ignites a
runaway thermonuclear reaction in the core and blows itself
to smithereens.
Researchers have long argued over which scenario is true.
The thinking throughout most of the 20th century was that
the “hidden assassin” is a comparatively larger star, which
could feed the dwarf until it reaches a critical limit. The
dwarf actually scrunches down in size as it siphons mate-
rial from its companion star, which causes its density and
temperature to skyrocket. Eventually, conditions become so
extreme that there is no longer space for the atoms’ electrons,
which are forced into the nuclei, igniting a runaway thermo-
nuclear reaction that forces the star to explode. Because that
reaction always occurs when the star hits the same density
and temperature, it explodes with an identical brightness —
explaining why all Type Ia supernovae look alike.
Or so we thought. Then in 1991, two supernovae were
discovered that did not explode at their expected luminosi-
ties — one was fainter and one was brighter. “That meant
they were not standard candles and you really had to worry
about this,” says Alexei Filippenko (University of California,
Berkeley). Luckily, astronomers soon discovered that the
brightest supernovae fade more slowly than their dimmer EXPL
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TYIMAGES
PLUSi edTHE MASS LIMIT
Astronomers often speak of Type Ia supernovae as the
explosions of white dwarfs that hit a specifi c mass limit
of 1.4 Suns, called the Chandrasekhar mass. But in fact the
dwarf never reaches that limit: If it did, gravity would force
it to collapse, not explode. Instead, as the dwarf approaches
the Chandrasekhar mass, it contracts, causing the internal
temperature and density to spike. At a certain point — which
is incredibly close to the Chandrasekhar mass — carbon
fusion ignites and blows the white dwarf apart.16 JUNE 2019 • SKY & TELESCOPE