Sky & Telescope - June 2019 - 21

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

energy has changed over time. If the rate is stable,
then a model that relies on the cosmological constant - which suggests that dark energy arises from
the short-lived virtual particles and antiparticles
within empty space - might be the best explanation. But if it does change, then a model that
relies on quintessence - which suggests that
dark energy results not from the vacuum of
space but from a ﬁeld that pervades spacetime
- might be more appropriate (see page 22).
A better understanding of Type Ia supernovae will help scientists pin down not only dark
energy and our cosmic past but also the fate
of our universe, which is determined by dark
energy's true nature.
Filippenko argues that one way forward is
to study the extreme cases. Consider a lesserknown class dubbed Type Iax. These explosions
look like Type Ia supernovae but move slower, fade
faster, and have less energy overall. You can almost
think of them as a miniature stellar blast.
"They're sort of like this different beast," Foley
afﬁrms. "But they're similar enough that we know some
of the physics has to be shared."
The data scientists have gathered suggest that Type Iax
supernovae occur in single-degenerate systems. But in this
situtation, the companion star has already lost its outer
layer of hydrogen, meaning that the white dwarf accumulates helium instead. Jha speculates that perhaps that's the
difference between normal Type Ia supernovae and Type Iax
supernovae - one accretes hydrogen and the other helium.
Unfortunately, scientists have very few examples of oddball Type Ia supernovae (see sidebar at right for three types).
So they're scouring the skies in search of every system that
might soon explode.
Among those efforts is the Zwicky Transient Facility in
California, which reached ﬁrst light in late 2017 (S&T: Mar.
2018, p. 13). Like its predecessor, the Palomar Transient Factory (which caught both of the game-changing supernovae
in 2011), the ZTF scans the skies nightly and alerts astronomers if any object's brightness has changed. Meanwhile, in
the Southern Hemisphere, the future Large Synoptic Survey
Telescope in Chile is expected to detect at least half a million
supernovae over its lifetime.
But the key isn't simply to ﬁnd new supernovae; it's to
catch them early and then observe them often. In the past,
supernovae were often imaged once every 3 or 4 days - time
gaps that meant astronomers might miss crucial details. But
observatories like the Transiting Exoplanet Survey Satellite
(S&T: Mar. 2018, p. 22) enable astronomers to observe these
explosions every 30 minutes. Follow-up will be done from the
ground, including by those in Andrew Howell's Global Supernova Project, a collaboration of 150 astronomers around the
world who will gather light curves and spectra when supernovae are discovered.

The Weirdos
While most Type Ia supernovae look strikingly similar
to one another, astronomers occasionally stumble
upon one of what they call "the weirdos." Some we've
discussed in the story - such as the super-luminous
and under-luminous supernovae discovered in 1991, and
Type Iax - but here are a few more:
Calcium-Rich Supernovae In 2001, astronomers discovered an odd supernova - it was the ﬁrst in a
class of objects that astronomers later realized had
a huge amount of calcium in their ejecta. These
explosions may occur when a low-mass white dwarf
steals enough helium from its binary companion to
boost its surface temperature and pressure, igniting
a thermonuclear explosion. The blast would blow oﬀ
at least the outer layers of the white dwarf star -
and half of that lost material is calcium.

02es-like Supernovae Similar to Type Iax supernovae,
these objects explode with less energy than normal.
They're also much cooler. "There aren't that many
examples of these, but they are clearly something
weird and diﬀerent, and they almost certainly come
from white dwarfs," Foley says.
06bt-like Supernovae These supernovae are true
weirdos. They look similar to normal Type Ia events
except for some subtle diﬀerences, like the fact
that their temperatures seem inconsistent with
their brightnesses. "Those are the ones that actually trouble me for cosmology," Foley says. That's
because with minimal information, they perfectly
resemble Type Ia supernovae.

With so many surveys online or in the works, astronomers are optimistic that they'll soon have enough data to
pin down what sets these explosions in motion and how
often. "No supernova can hide from us now," Nugent says.
"We're going to ﬁ nd it during the night or the next night
every single time."

¢ SHANNON HALL is an award-winning science journalist based in the Rocky Mountains. Her favorite supernova
remnant to observe is the Crab Nebula, created by a star that
exploded in AD 1054.
sk yandtele scope.com * J U N E 2 019

http://www.skyandtelescope.com

Sky & Telescope - June 2019

Table of Contents for the Digital Edition of Sky & Telescope - June 2019