We've Never Seen a Supernova Explode Until Now

0:00 Supernovae are full of mystery.

0:04 We still don't fully understand what makes them explode.

0:09 That's partly because they're impossible to predict.

0:12 We know what type of star is set to detonate,

0:15 but have no way of knowing exactly when.

0:19 In fact, most of the data we have

0:21 about these spectacular events comes from studying their remnants,

0:25 in many cases hundreds, if not thousands of years after they began.

0:31 That or a stroke of dumb luck,

0:34 where we just happen to be looking in the right direction at the right time.

0:39 But earlier last year, this changed.

0:42 We finally captured a supernova as it was exploding, and on purpose.

0:49 Finding it just hours after it began,

0:52 this is one of the earliest detections we've ever made,

0:56 and it's shown us something we've never seen before.

0:59 The shape of the explosion.

1:01 It might sound like an unusual thing to get excited about,

1:05 but this discovery has the power to unlock

1:07 the inner workings of exploding stars and answer centuries-old conundrums.

1:12 What are the mysterious mechanisms that drive a supernova?

1:17 How does a core collapse inward, then suddenly explode outward?

1:22 And why do some supernovae fail?

1:26 Thanks to new data from the ATLAS survey,

1:29 we're edging closer [music] to answers.

1:32 I'm Alex McConaughey, and you're watching Astro.

1:34 Join me today as we watch a star explode in real time,

1:38 decode the shape of its blast,

1:40 and find out what really happens in those critical

1:44 seconds before a star rips itself apart.

1:49 On the night of the 10th of April, 2024,

1:52 the ATLAS survey was conducting a routine sweep of the cosmos.

1:56 It consists of four wide-field telescopes

1:59 positioned around the globe in South Africa,

2:02 Chile, and two in Hawaii.

2:04 Together, they systematically scan the entire night sky every 48 hours,

2:10 imaging each region four times in that window.

2:14 This rapid revisit time helps catch fast-moving events as they happen.

2:19 And at 3:21 a.m.

2:21 on the 11th of April, that's exactly what happened.

2:26 [music] ATLAS detected something unusual,

2:27 a single point in the sky brightening dramatically over the previous 5.8 hours.

2:34 Immediately, this reading triggered the telescope's automated alert system.

2:39 The news of the unknown transient

2:41 spread the astronomical community across the world,

2:44 who quickly got to work deciphering what it could mean.

2:47 23.8 million light-years away, in the spiral galaxy NGC 3621,

2:55 a star had just died.

2:58 What ATLAS caught was a type two supernova right as it happened.

3:04 Yi Yang from the Tsinghua University in Beijing

3:07 and his colleagues knew what this meant.

3:10 It was their [music] chance to see, for the first time,

3:13 the true shape of a supernova explosion in real time.

3:18 But, [music] they had no time to waste.

3:20 In addition to being impossible to predict,

3:23 the initial breakout period of a supernova explosion is incredibly short-lived.

3:28 Once they go off, their original geometry remains intact for just hours.

3:33 After that, the ejected material crashes into surrounding gas and dust,

3:38 warping and obscuring the blast's original shape.

3:41 The pristine fingerprint of the core collapse,

3:44 the very thing needed to understand how massive stars die,

3:48 would be gone forever.

3:50 So, to get a true reading, the researchers had to act fast.

3:54 While most of the world slept, Yang and his team worked through the night

3:58 drafting an emergency proposal to the European Southern Observatory.

4:02 They urgently needed time on one of the most powerful instruments on Earth,

4:07 the Very Large Telescope in Chile.

4:10 If they were going to measure the shape of the explosion,

4:13 they had to start gathering data right away.

4:17 Luckily, ESO immediately approved their request

4:20 and the VLT swung towards galaxy NGC 3621.

4:25 Yang and his team waited with their hearts

4:28 in their throats and their [music] eyes locked on their screens.

4:31 This was the moment of truth.

4:34 Had they taken too long?

4:36 Would they get a pure reading back or a scrambled

4:39 mess of inputs from an already warped explosion?

4:43 By the slimmest of margins, they'd pulled it off.

4:47 Precious data started streaming in, painting a picture of true,

4:52 undistorted supernova geometry.

4:55 Yang and his team were witnessing the birth cry of supernova 2024

5:00 GGI and learning at last how the universe makes its most violent fireworks.

5:09 Supernovae come in two main types, but we'll focus on type two here,

5:15 the kind Yang and his team observed.

5:17 Through a mix of physical theories,

5:20 astronomical observations, and computer simulations,

5:23 we've managed to build a pretty respectable model of how we think stars die.

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6:53 Stars spend their lives fusing lighter elements into heavier

6:58 ones by combining nuclei into more tightly bound configurations.

7:04 This fusion process releases excess energy,

7:07 creating an outward pressure that pushes back against the star's own gravity.

7:12 This keeps the star in a stable, spherical shape.

7:16 Over billions of years, the star's core works its way up the periodic table,

7:20 fusing hydrogen into helium, helium into carbon,

7:23 carbon into oxygen, neon, magnesium, silicon, and finally, iron.

7:30 But, this is where the chain stops.

7:33 If you want to fuse iron into something heavier,

7:35 that reaction won't release energy.

7:38 It will require an energy input instead.

7:41 Iron 56 has the lowest mass per

7:44 nucleon out of every possible nuclear configuration.

7:48 So, every chain of nuclear reactions will ultimately

7:51 turn every other type of nucleus into iron 56.

7:56 Eventually, the star's entire core becomes iron.

8:00 It's a dead end.

8:01 With no more energy releasing fusion reactions available,

8:04 the star has run out of fuel.

8:07 It stops burning.

8:09 The outward pressure that has been holding

8:11 it up against its own gravity disappears.

8:14 Without that support, the core collapses on itself at breakneck speeds.

8:19 The outer part of the core can reach velocities of 70,000 km per second.

8:24 That's 23% the speed of light.

8:29 The collapse is so violent that it crushes

8:32 protons and electrons in the inner core into neutrons.

8:36 It also releases a flood of neutrinos,

8:39 nearly massless particles that carry away enormous amounts of energy.

8:44 The collapsing material slams into the newly formed neutron core and rebounds,

8:49 creating a shockwave that propagates outward

8:51 at thousands of kilometers per second, a process known as core bounce.

8:58 But, the shockwave doesn't make it far.

9:00 Within milliseconds, it stalls nearly 100 to 200 km from the center

9:06 as it plows through the dense outer iron core.

9:10 The extreme temperatures and pressures tear apart the heavy

9:13 iron nuclei back into lighter elements like helium,

9:17 a process that absorbs enormous amounts of energy from the shock.

9:21 This is the moment where one of two things can happen.

9:25 If the shock stays stalled for more than about a second,

9:29 the star will keep accreting mass until it collapses into a black hole.

9:34 There's no bang, no brilliant burst.

9:37 Everything slips into the void of a failed supernova.

9:42 The more common and mysterious alternative is

9:44 that the shock doesn't stall, it gets revived.

9:48 Something, neutrinos, jets, magnetic fields,

9:51 or some combination we don't yet understand,

9:55 transfers enough energy to allow the shock wave

9:57 to continue and turn the implosion into an explosion.

10:02 Exactly how this happens remains one

10:04 of the biggest unsolved mysteries in astrophysics

10:07 and is why events like SN2024GGI are so tantalizing to the scientific community.

10:16 The shock wave travels up through thousands of kilometers of stellar material.

10:21 The journey takes several hours until finally it

10:24 breaks through the outermost layer of the dying star.

10:27 This is the breakout phase.

10:30 As it emerges, the shock wave creates

10:32 a brilliant flash of ultraviolet and optical light,

10:36 releasing huge amounts of energy and heating the star's

10:40 outer layers to tens of thousands of degrees.

10:44 It's only at this point that the supernova becomes visible for the first time,

10:48 reaching peak brightness over the next week or two.

10:51 And the beginning of this rising brightness [music] is exactly what ATLAS

10:55 saw in the early morning hours of the 11th of April 2024.

11:01 ATLAS catching SN2024GGI as early as it did was a triumph.

11:07 But detection was only the opening move.

11:10 The real work began after the alert went out.

11:13 A race against the clock to put the right

11:15 instruments on target before the explosion's original geometry vanished forever.

11:21 That urgency was exactly why Yang and his team had fought for time on the VLT.

11:28 They weren't just trying to see the supernova.

11:30 They were after a very specific signature hidden in its light,

11:35 something only one instrument could reveal.

11:40 Mounted on the VLT is an instrument called FORS2,

11:44 the only one of its kind in the Southern Hemisphere.

11:48 It measures spectropolarimetry,

11:50 how light is polarized across different wavelengths of the visible spectrum.

11:56 Normally, light waves vibrate in all

11:58 directions perpendicular to their direction of travel.

12:01 This is unpolarized light.

12:04 But, polarized light has waves that only vibrate in one direction.

12:08 For example, when sunlight reflects off a flat surface like water or glass,

12:13 it becomes partially polarized.

12:16 It's also why when you put on polarized sunglasses, this glare disappears.

12:21 They're designed to filter out that kind of polarized light.

12:25 In the context of SN2024GGI,

12:29 Yang was interested in polarized light because of what

12:32 it could reveal about the shape of the explosion.

12:36 A perfectly spherical explosion scatters light equally in all directions,

12:41 producing no polarization at all.

12:44 An asymmetric explosion, on the other hand,

12:46 one that is symmetrical along one axis,

12:50 like a football or a peanut-shaped explosion,

12:53 would polarize light in a specific pattern.

12:57 FORS2 can help scientists decode this pattern.

13:02 When light enters the instrument,

13:03 it first passes through a rotating crystal plate that can

13:07 shift the orientation of the polarized light by a precise amount.

13:11 By moving this plate to different angles,

13:14 the instrument effectively rotates any polarized light,

13:17 allowing it to measure polarization in all directions.

13:22 Next, the light hits a Wollaston prism,

13:25 a a optical component which physically separates the incoming

13:29 beam of light based on the polarization direction.

13:32 Light waves vibrating in one plane go one way,

13:36 waves vibrating perpendicular to that go the other way.

13:39 Each beam is then split into its component colors,

13:43 creating two side-by-side spectra on the detector,

13:47 one for each polarization orientation.

13:50 By comparing how bright [music] each wavelength is across the two spectra,

13:55 astronomers can determine how strongly the light

13:57 is polarized and in which direction.

14:01 Knowing how the light from SN2024GGI is

14:04 polarized tells us the shape of its explosion.

14:09 From there, we can predict the mechanism that caused it.

14:14 For over 50 years, scientists have tried to explain the physical

14:18 processes that revive a stalled shockwave and trigger a supernova explosion.

14:23 From the creator of Rebel, image two leading ideas.

14:28 The first says that neutrinos must have something to do with it.

14:32 We know the core collapse sends huge amounts of neutrinos shooting into space,

14:36 but in the incredibly dense region just above the neutron star,

14:41 even neutrinos have trouble getting through.

14:44 It's thought a tiny fraction of them deposit

14:46 their energy into the material above the core, heating it from below.

14:51 This creates a violent convection.

14:54 If enough energy accumulates,

14:56 the shock surges back to life and the star literally blows its top,

15:02 exploding furiously.

15:05 But here's the thing, the convection doesn't heat the neutrinos evenly.

15:09 State-of-the-art 3D simulations led by Bernard Müller

15:13 at Monash University show these explosions are highly asymmetric.

15:17 And we've observed real supernovae that fit this model well.

15:21 The Cassiopeia A supernova remnant, for example,

15:24 shows an asymmetric distribution of radioactive

15:27 chemicals like titanium 44 and nickel 56,

15:31 which trace back to asymmetries present from the onset of the explosion.

15:36 The second theory says jets are behind the boom instead.

15:39 The collapsing core forms a rapidly rotating neutron

15:43 star surrounded by a disk of infalling material.

15:47 Magnetic fields threading through this disk get

15:50 wound up and amplified by the rotation,

15:53 twisted tighter and tighter like a rubber band.

15:56 These amplified magnetic fields channel material into two powerful jets

16:00 shooting out along the star's rotation axis like cosmic blowtorches.

16:05 They punch through the star's envelope with tremendous force,

16:09 driving a fundamentally different explosion.

16:13 In this scenario, the blast would produce actual symmetry along the star's spin.

16:18 The explosion then expands along this axis,

16:21 leading to a football-shaped or olive-shaped geometry.

16:25 We've seen this kind of structure in other supernovae like SN 2023 IXF,

16:31 whose explosion was also organized and elongated.

16:35 So, what do you think Yang's VLT observation show for SN 2024 GGI?

16:41 Pause the video and comment below.

16:43 Explosion if you think it's highly irregular

16:46 and asymmetric like the neutrino-driven model predicts,

16:50 or an American football if you think it's

16:53 symmetrical along an axis like the jet model predicts.

16:57 So, moment of truth.

17:00 This is what SN 2024 GGI look like.

17:05 The spectropolarimetry data collected by the VLT's FORS2 revealed a clear,

17:12 well-defined axis of symmetry and an olive or football-shaped explosion,

17:18 technically a prolate ellipsoid.

17:22 At first, it seemed like a clear win for the jet-driven theory.

17:27 If the explosion were driven by neutrinos,

17:29 it would have been completely asymmetrical.

17:32 Finally, we had the conclusive observational data we'd always been missing.

17:38 Case closed.

17:40 Right?

17:42 If only it were that simple.

17:44 These models are based on simulations, theoretical physics,

17:47 and extrapolated information from supernovae we caught much later,

17:51 after the critical breakout phase window shut.

17:54 The neutrino-based model certainly has its shortcomings.

17:58 A 2015 article from the Royal

18:00 Astronomical Society showed that 3D neutrino-driven simulations

18:04 produce explosions an entire order of magnitude

18:07 less than what we've seen observationally,

18:10 even in the most favorable of conditions.

18:13 This suggests neutrino heating alone simply isn't enough to explain the blast,

18:18 but the jet-driven theories aren't flawless, either.

18:21 2D models seemingly work well,

18:24 but when astrophysicists simulate jet-driven explosions in 3D,

18:29 something strange happens.

18:30 A team at Caltech introduced a 1% wobble around

18:34 the axis of symmetry of a rapidly rotating magnetized stellar core.

18:39 That tiny change made the jets unstable.

18:42 They twisted and kinked,

18:44 winding around the rotation axis like water streaming out

18:48 from a garden hose left lying on the ground.

18:52 Instead of punching cleanly through the star and driving an explosion,

18:56 the jets produced two misshapen lobes of twisted,

19:01 highly magnetized material that slowly pushed outward.

19:05 The explosion never happened.

19:08 There simply wasn't enough energy to trigger an explosion.

19:12 Yet, SN2024GGI clearly exploded.

19:17 So, there must be something else going on.

19:20 Either our models are incomplete or we've missed something else entirely.

19:26 The solution might lie in flipping the old theory on its head.

19:31 What if the fatal wobbling and kinking

19:34 is actually an essential part of the explosion?

19:38 If jets rapidly jitter rather than maintain a stable axis,

19:42 they can't drill a clean channel through

19:44 the star like in the classic jet-driven model.

19:47 Instead, they're forced to deposit their energy close [music] to the core,

19:51 roughly 1,000 km from the center through shock waves.

19:55 These shock waves create hot, pressurized bubbles that merge and expand,

20:00 pushing the stellar material outward and driving the explosion.

20:04 The jets themselves are chaotic and unstable,

20:07 but the overall explosion geometry remains actually symmetric,

20:11 just like SN2024GGI.

20:15 So, where does this leave us?

20:18 Well, SN2024GGI definitely showed us

20:22 that explosions can have organized actual symmetry.

20:27 And how it arises, whether through

20:29 narrow jittering jets or a magnetorotational mechanism,

20:34 or something else, remains a mystery.

20:38 We're still unsure what role neutrinos play in explosions like this, if any.

20:44 It also raises questions about

20:46 alternative mechanisms that could cause asymmetric

20:48 supernovae like Cassiopeia A and others which don't follow actual symmetry.

20:54 Are they in fact caused by neutrinos or something else?

20:58 Which factors determine what kind of explosion occurs?

21:02 This might come as a shock, but we can't know without more data.

21:06 SN2024GGI is the only breakout phase we've observed

21:11 in such detail and measured in real time.

21:14 If we could somehow know when [music]

21:16 and where supernovae were going to explode,

21:19 we could make a point to track them all and look for the patterns.

21:23 And that's exactly what researchers plan to do.

21:26 Thanks to new full-sky surveys like ATLAS becoming increasingly popular.

21:31 2025 alone saw the inauguration of the Vera C.

21:34 Rubin Observatory Legacy Survey of Space and Time, NASA's SphereX,

21:40 and the latest data release of the Sloan Digital Sky Survey.

21:44 Astronomers are optimistic that with these new tools,

21:48 they'll be able to regularly catch

21:50 supernovae within 24 [music] hours of explosion.

21:54 If they notice an object is absent from previous night's images,

21:58 but then appears within a galaxy,

22:00 they'll follow up immediately with spectroscopy to determine

22:03 the object's characteristics as a fast as possible.

22:07 As more automated survey systems come online worldwide,

22:10 our ability to catch transient events

22:12 in their earliest moments will only continue to improve.

22:16 SN2024GGI showed us our models aren't quite there yet.

22:22 The next generation of observations should help

22:24 us find the missing piece and solidify our understanding of how stars all over

22:29 the universe draw their final [music] breath.

22:32 And I mean, who wouldn't want to see

22:34 more supernovae getting caught in their explosive moments?

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