itself apart. On the night of the 10th of April 2024,
the Atlas survey was conducting a
routine sweep of the cosmos. It consists
of four wide field telescopes positioned
around the globe in South Africa, Chile,
and two in Hawaii. Together, they
systematically scan the entire night sky
every 48 hours, imaging each region four
times in that window. This rapid revisit
time helps catch fastmoving events as
they happen. And at 3:21 a.m. on the
11th of April, that's exactly what
happened. Atlas detected something
unusual. A single point in the sky
brightening dramatically over the
previous 5.8 hours. Immediately, this
reading triggered the telescope's
automated alert system. The news of the
unknown transient spread to the
astronomical community across the world,
who quickly got to work deciphering what
it could mean. 23.8 8 million lighty
years away in the spiral galaxy NGC 3621
a star had just died. What Atlas caught
was a type 2 supernova right as it
happened. Yiang from the Chinua
University in Beijing and his colleagues
knew what this meant. It was their
chance to see for the first time the
true shape of a supernova explosion in
real time. But they had no time to
waste. In addition to being impossible
to predict, the initial breakout period
of a supernova explosion is incredibly
short-lived. Once they go off, their
original geometry remains intact for
just hours. After that, the ejected
material crashes into surrounding gas
and dust, warping and obscuring the
blast's original shape. The pristine
fingerprint of the core collapse, the
very thing needed to understand how
massive stars die, would be gone
forever. So to get a true reading, the
researchers had to act fast. While most
of the world slept, Yang and his team
worked through the night, drafting an
emergency proposal to the European
Southern Observatory. They urgently
needed time on one of the most powerful
instruments on Earth, the Very Large
Telescope in Chile. If they were going
to measure the shape of the explosion,
they had to start gathering data right
away.
Luckily, ESO immediately approved the
request, and the VLT swung towards
Galaxy NGC 3621.
Yang and his team waited with their
hearts in their throats and their eyes
locked on their screens. This was the
moment of truth. Had they taken too
long, would they get a pure reading back
or a scrambled mess of inputs from an
already warped explosion?
By the slimmest of margins, they'd
pulled it off. Precious data started
streaming in, painting a picture of true
undistorted supernova geometry. Yang and
his team were witnessing the birth cry
of supernova 2024 GGI and learning at
last how the universe makes its most
violent fireworks.
Supernova come in two main types, but
we'll focus on type two here, the kind
Yang and his team observed. Through a
mix of physical theories, astronomical
observations, and computer simulations,
we've managed to build a pretty
respectable model of how we think stars
die.
daily access to everything Brilliant has to offer. Stars spend their lives fusing
lighter elements into heavier ones by
combining nuclei into more tightly bound
configurations.
This fusion process releases excess
energy, creating an outward pressure
that pushes back against the stars own
gravity. This keeps the star in a stable
spherical shape. Over billions of years,
the stars core works its way up the
periodic table, fusing hydrogen into
helium, helium into carbon, carbon into
oxygen, neon, magnesium, silicon, and
finally iron. But this is where the
chain stops. If you want to fuse iron
into something heavier, that reaction
won't release energy. It will require an
energy input instead. Iron 56 has the
lowest mass per nucleon out of every
possible nuclear configuration. So,
every chain of nuclear reactions will
ultimately turn every other type of
nucleus into iron 56. Eventually, the
star's entire core becomes iron. It's a
dead end. With no more energy releasing
fusion reactions available, the star has
run out of fuel. It stops burning. The
outward pressure that has been holding
it up against its own gravity
disappears. Without that support, the
core collapses on itself at breakneck
speeds. The outer part of the core can
reach velocities of 70,000 km/s. That's
23% the speed of light. The collapse is
so violent that it crushes protons and
electrons in the inner core into
neutrons. It also releases a flood of
nutrinos, nearly massless particles that
carry away enormous amounts of energy.
The collapsing material slams into the
newly formed neutron core and rebounds,
creating a shock wave that propagates
outward at thousands of kilome/s, a
process known as core bounce.
But the shock wave doesn't make it far.
Within milliseconds, it stalls nearly
100 to 200 km from the center as it
plows through the dense outer iron core.
The extreme temperatures and pressures
tear apart the heavy iron nuclei back
into lighter elements like helium, a
process that absorbs enormous amounts of
energy from the shock. This is the
moment where one of two things can
happen. If the shock stays stalled for
more than about a second, the star will
keep accreting mass until it collapses
into a black hole. There's no bang, no
brilliant burst. Everything slips into
the void. A failed supernova.
The more common and mysterious
alternative is that the shock doesn't
stall, it gets revived. something,
nutrinos, jets, magnetic fields, or some
combination we don't yet understand,
transfers enough energy to allow the
shock wave to continue and turn the
implosion into an explosion. Exactly how
this happens remains one of the biggest
unsolved mysteries in astrophysics and
is why events like SN 2024 GGI are so
tantalizing to the scientific community.
The shock wave travels up through
thousands of kilometers of stellar
material. The journey takes several
hours until finally it breaks through
the outermost layer of the dying star.
This is the breakout phase. As it
emerges, the shock wave creates a
brilliant flash of ultraviolet and
optical light, releasing huge amounts of
energy and heating the stars outer
layers to tens of thousands of degrees.
It's only at this point that the
supernova becomes visible for the first
time, reaching peak brightness over the
next week or two. And the beginning of
this rising brightness is exactly what
Atlas saw in the early morning hours of
the 11th of April, 2024.
Atlas catching SN204 GGI as early as it
did was a triumph. But detection was
only the opening move. The real work
began after the alert went out. A race
against the clock to put the right
instruments on target before the
explosion's original geometry vanished
forever. That urgency was exactly why
Yang and his team had fought for time on
the VT. They weren't just trying to see
the supernova. They were after a very
specific signature hidden in its light.
Something only one instrument could
reveal.
Mounted on the VLT is an instrument
called Fors 2, the only one of its kind
in the southern hemisphere. It measures
spectropolarimemetry,
how light is polarized across different
wavelengths of the visible spectrum.
Normally, light waves vibrate in all
directions perpendicular to their
direction of travel. This is unpolarized
light. But polarized light has waves
that only vibrate in one direction. For
example, when sunlight reflects off a
flat surface like water or glass, it
becomes partially polarized. It's also
why when you put on polarized
sunglasses, this glare disappears.
They're designed to filter out that kind
of polarized light. In the context of
SN204 GGI, Yang was interested in
polarized light because of what it could
reveal about the shape of the explosion.
A perfectly spherical explosion scatters
light equally in all directions,
producing no polarization at all. An
asymmetric explosion, on the other hand,
one that is symmetrical along one axis,
like a football or a peanut-shaped
explosion, would polarize light in a
specific pattern. Fors 2 can help
scientists decode this pattern. When
light enters the instrument, it first
passes through a rotating crystal plate
that can shift the orientation of the
polarized light by a precise amount. By
moving this plate to different angles,
the instrument effectively rotates any
polarized light, allowing it to measure
polarization in all directions.
Next, the light hits a Wallist prism, a
unique optical component which
physically separates the incoming beam
of light based on the polarization
direction. Light waves vibrating in one
plane go one way. Waves vibrating
perpendicular to that go the other way.
Each beam is then split into its
component colors, creating two
sidebyside spectra on the detector, one
for each polarization orientation.
By comparing how bright each wavelength
is across the two spectra, astronomers
can determine how strongly the light is
polarized and in which direction.
Knowing how the light from SN2024 GGI is
polarized tells us the shape of its
explosion. From there, we can predict
the mechanism that caused it.
For over 50 years, scientists have tried
to explain the physical processes that
revive a stalled shock wave and trigger
a supernova explosion. From the creative
rubble emerged two leading ideas. The
first says that nutrinos must have
something to do with it. We know the
core collapse sends huge amounts of
nutrinos shooting into space. But in the
incredibly dense region just above the
neutron star, even nutrinos have trouble
getting through. It's thought a tiny
fraction of them deposit their energy
into the material above the core,
heating it from below. This creates a
violent convection. If enough energy
accumulates, the shock surges back to
life and the star literally blows its
top, exploding furiously.
But here's the thing, the convection
doesn't heat the nutrinos evenly.
State-of-the-art 3D simulations led by
Bernard Müller at Mon University show
these explosions are highly asymmetric,
and we've observed real supernova that
fit this model well. The Cassiopia a
supernova remnant, for example, shows an
asymmetric distribution of radioactive
chemicals like titanium 44 and nickel
56, which trace back to asymmetries
present from the onset of the explosion.
The second theory says jets are behind
the boom. Instead, the collapsing core
forms a rapidly rotating neutron star
surrounded by a disc of infalling
material. Magnetic fields threading
through this disc get wound up and
amplified by the rotation. Twisted
tighter and tighter like a rubber band.
These amplified magnetic fields channel
material into two powerful jets shooting
out along the stars rotation axis like
cosmic blowtorrches. They punch through
the stars envelope with tremendous
force, driving a fundamentally different
explosion. In this scenario, the blast
would produce axial symmetry along the
stars spin. The explosion then expands
along this axis, leading to a
football-shaped or olive shaped
geometry. We've seen this kind of
structure in other supernova like
SN203XF,
whose explosion was also organized and
elongated.
So moment of truth. This is what SN 2024 GGI look like. The spectro polar imagery
data collected by the VLT's force 2
revealed a clear well-defined axis of
symmetry and an olive or football-shaped
explosion. Technically a prolate
ellypoid.
At first, it seemed like a clear win for
the jet driven theory. If the explosion
were driven by nutrinos, it would have
been completely asymmetrical.
Finally, we had the conclusive
observational data we'd always been
missing. Case closed, right?
If only it were that simple. These
models are based on simulations,
theoretical physics, and extrapolated
information from supernova we caught
much later after the critical breakout
phase window shut. The nutrinobased
model certainly has its shortcomings. A
2015 article from the Royal Astronomical
Society showed that 3D nutrinodriven
simulations produce explosions an entire
order of magnitude less than what we've
seen observationally even in the most
favorable of conditions. This suggests
nutrino heating alone simply isn't
enough to explain the blast. But the jet
driven theories aren't flawless either.
2D models seemingly work well. But when
astrophysicists simulate jet driven
explosions in 3D, something strange
happens. A team at Caltech introduced a
1% wobble around the axis of symmetry of
a rapidly rotating magnetized stella
core. That tiny change made the jets
unstable. They twisted and kinkedked,
winding around the rotation axis like
water streaming out from a garden hose
left lying on the ground. Instead of
punching cleanly through the star and
driving an explosion, the jets produced
two misshapen lobes of twisted, highly
magnetized material that slowly pushed
outward. The explosion never happened.
There simply wasn't enough energy to
trigger an explosion. Yet SN 2024 GGI
clearly exploded. So, there must be
something else going on. Either our
models are incomplete or we've missed
something else entirely.
The solution might lie in flipping the
old theory on its head. What if the
fatal wobbling and kinking is actually
an essential part of the explosion? If
jets rapidly jitter rather than maintain
a stable axis, they can't drill a clean
channel through the star like in the
classic jet driven model. Instead,
they're forced to deposit their energy
close to the core, roughly a thousand km
from the center through shock waves.
These shock waves create hot pressurized
bubbles that merge and expand, pushing
the stellar material outward and driving
the explosion. The jets themselves are
chaotic and unstable, but the overall
explosion geometry remains actually
symmetric, just like SN204
GGI.
So where does this leave us? Well,
SN2024 GGI definitely showed us that
explosions can have organized axial
symmetry. and how it arises, whether
through narrow jittering jets or a
magneetto rotational mechanism or
something else, remains a mystery. We're
still unsure what role nutrinos play in
explosions like this, if any. It also
raises questions about alternative
mechanisms that could cause asymmetrical
supernova, like Cassiopia A and others
which don't follow axial symmetry. Are
they in fact caused by nutrinos or
something else? Which factors determine
what kind of explosion occurs? This
might come as a shock, but we can't know
without more data. SN204 GGI is the only
breakout phase we've observed in such
detail and measured in real time. If we
could somehow know when and where
supernova were going to explode, we
could make a point to track them all and
look for the patterns. And that's
exactly what researchers plan to do
thanks to new fall sky surveys like
Atlas becoming increasingly popular.
2025 alone saw the inauguration of the
Very Rubin Observatory Legacy Survey of
Space and Time, NASA's Sphere X, and the
latest data release of the Sloan Digital
Sky Survey. Astronomers are optimistic
that with these new tools, they'll be
able to regularly catch supernova within
24 hours of explosion. If they notice an
object is absent from previous night's
images, but then appears within a
galaxy, they'll follow up immediately
with spectroscopy to determine the
object's characteristics as fast as
possible. As more automated survey
systems come online worldwide, our
ability to catch transient events in
their earliest moments will only
continue to improve. SN 2024 GGI showed
us our models aren't quite there yet.
The next generation of observations
should help us find the missing piece
and solidify our understanding of how
stars all over the universe draw their
final breath. And I mean, who wouldn't
want to see more supernova getting
caught in their explosive moments?
by clicking their link in the
description below.