68 ASTRONOMY • DECEMBER 2018A: When a massive star runs
out of nuclear fuel at the end of
its life, it collapses — the star’s
dense core falls in on itself,
squeezing protons and elec-
trons together to make the
neutrons that form an ultra-
dense neutron star remnant.
The implosion is arrested
when the core reaches the
extreme density of nuclear
matter (i.e., the density found
inside an atom’s nucleus). A
violent rebound follows, eject-
ing matter outward in a super-
nova explosion, which, for
a short while, can shine as
brightly in photons (light)
as an entire galaxy.
The explosion drama starts
in the murky midst of the stel-
lar envelope surrounding the
core. It can take some timeAstronomy’s experts from around the globe answer your cosmic questions.
SUPERNOVA
SIGNALS
through matter like phantoms
through walls, they can escape
the star within a few tens of
seconds. On Earth, we can
capture a burst of them (which
is only a tiny fraction of the
total produced) in huge under-
ground neutrino detectors,
before the supernova’s light
shows up.
You can sign up with the
Supernova Early Warning
System network (https://snews.
bnl.gov) to get an alert when
this happens from the neutrino
detector network.
Kate Scholberg
Professor of Physics, Duke University,
Durham, North CarolinaASKASTR0
Q: IN A SUPERNOVA, WHY DO WE DETECT
NEUTRINOS BEFORE LIGHT?
Charles Johnson, Lake Geneva, Wisconsin— hours or even a good frac-
tion of a day — for a shock
wave to burst out of the dead
star’s corpse and start the
shining of the supernova. Long
before the visible fireworks,
nearly all the energy of the
star’s inward tumble has
already escaped, in the form of
nearly invisible neutrinos.
Neutrinos are elementary
particles that are famously
“ghostly” — they only rarely
bump into matter, so they’re
difficult to detect. On average,
a neutrino will travel through
a light-year of matter before
colliding with an atom! In the
ferociously hot and dense mat-
ter at the heart of the star’s
collapse, neutrinos are pro-
duced in huge quantities.
Because neutrinos just slipQ: WHAT’S THE DIAMETER
OF THE LARGEST
EXOPLANET FOUND SO FAR?
Terrence Gollata
Manitowoc, WisconsinA: The largest planet discov-
ered to date, that astronomers
are sure is a planet and has an
accurately measured diameter,
is HAT-P-67 b. This planet is a
“hot Jupiter” — a gas giant
similar to Jupiter or Saturn, but
orbiting so close to its star that
it takes only a few days (4.8
days in this case) for the planet
to orbit once around its sun.
HAT-P-67 b is 2.08 times
the diameter of Jupiter (which
has an average diameter of
about 88,850 miles [143,000
kilometers]), but HAT-P-67 b is
less than 60 percent of Jupiter’s
mass. This makes it the least
dense planet discovered so far.
The large diameter is likely due
to the planet’s proximity to its
star. Gas giant planets like
HAT-P-67 b experience
extreme levels of radiation.
High-energy X-rays and ultra-
violet light from the star ionize
the outer gaseous atmosphere
of the planet, stripping it of
electrons and causing it to heat
up and expand. The heated
outer atmosphere also begins
to escape. This hot, escaping
gas is called the exoatmosphere.
HAT-P-67 b was discovered
in 2017, so measurements of
the hot, ionized exoatmosphereThe widest known exoplanet, HAT-P-67 b, is a gas giant spanning twice the diameter of Jupiter, but it orbits
so closely to its star that its year lasts less than five Earth days. KEVIN GILLCassiopeia A (left) is the remnant of a star whose supernova light reached
Earth about 300 years ago. Today, huge detectors, such as the one at the
Daya Bay Reactor Neutrino Experiment (right), can capture a small fraction
of the neutrinos produced by supernovae. NASA/JPL C ALT ECH; ROY K ALT SCHMIDT, LBNL