Sky and Telescope - January 2018 - 28

Beyond Exoplanets

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p BEFORE & AFTER These plots compare two light curves, one for a
well-studied supernova in 1999 (top, turquoise dots) and the other for a
supernova detected by Kepler in 2011 (bottom). Each Kepler point is an
average of 3 hours of data. Both supernovae were core-collapse events.

provide the elements crucial in forming life. If cores rotate
slowly, then perhaps it affects how heavier elements mix into
the interstellar medium.
"That's another legacy of Kepler: It left a lot of observational puzzles that will keep the theorists busy for quite some
time to come," Huber says.

The Moment Before a Star Explodes
Although Kepler stared at some 150,000 stars in nearby reaches
of the Milky Way, distant galaxies also lurked in the telescope's
view. And that allowed the telescope to catch the rise of light
emitted during the early death throes of a distant star.
"It was just stunning to me," says Peter Garnavich (University of Notre Dame), who has been working with supernovae for decades. Typically, astronomers catch a supernova
several days or weeks after it explodes. Then they observe it
every night for months. As such, a typical light curve misses
the initial rise of the explosion and then includes data points
taken every 24 hours.
But Kepler took the game to a new level. "It was like a
theorist gave us the light curve," Garnavich says. "We had
the observations before the explosions. We had it rising up
every 30 minutes we had an observation. It was just unbelievably beautiful."
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JA N UA RY 2 018 * SK Y & TELESCOPE

And such beautiful light curves have helped astronomers
distinguish between competing explosion scenarios for Type
Ia supernovae. All scenarios require two culprits: a white
dwarf - the Earth-sized remnants of Sun-like stars - and a
companion star. It's the companion star that's the mystery.
If the companion star is rather large, say a Sun-like star
or a red giant, then the white dwarf takes on material from
that star until the dwarf reaches the so-called Chandrasekhar
limit. The increasing pressure ignites a runaway thermonuclear explosion. But if the companion star is another white
dwarf, the two explode only when they collide. In theory, the
two scenarios are distinguishable if astronomers can glimpse
the early light from the explosion - something that was
impossible before Kepler.
In the ﬁrst scenario, the expanding shell of material from
the white dwarf rams into the companion star, generating
extra heat and light that would show up as a slight bump
in the ﬁrst days of a supernova's brightening. In the second
scenario, there is no bump. As of today, Garnavich and his
colleagues have spotted roughly a dozen Type Ia supernovae
with Kepler and they haven't seen a single bump, suggesting
that all companion objects are reasonably tiny.
But that doesn't mean the case is closed. On March 10,
2017, a supernova appeared in the outskirts of the spiral
galaxy NGC 5643, roughly 55 million light-years away (S&T:
July 2017, p. 11). It looked like a typical Type Ia supernova in
every way except one: There was an early bump in its light
curve. Models suggest that the bump occurred when the
white dwarf's shock wave ran into a rather large companion
star that's roughly 20 times the size of the Sun.
Garnavich hails this as the best evidence yet that a Type Ia
supernova can result from a white dwarf and a larger companion star. That means the explosions can come from an array of
systems - a result that has universe-scale repercussions.

"It was just stunning to me," Garnavich
says. "It was like a theorist gave us the
light curve."
Astronomers use these explosions, which detonate at
roughly the same luminosity, to measure distances. These
cosmic mileposts have provided the best evidence yet that our
universe's expansion is accelerating, caused by something we
call dark energy. But astronomers haven't been able to pin
down exactly what dark energy is. Garnavich thinks that with
these observations, we're one step closer. It could be, he says,
that understanding exactly how these supernovae explode
will enable astronomers to take more precise measurements
of the cosmos and pin down a model for dark energy.

Making Lemonade
In July 2012, one of Kepler's four stabilizing reaction wheels
failed, and then in May 2013 a second one broke, leaving the

BEFORE & A F TER: LE A H TISCIONE / S&T, SOURCE: PE TER G A R N AVICH

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Sky and Telescope - January 2018

Table of Contents for the Digital Edition of Sky and Telescope - January 2018