
Across the Universe, a star exploded so violently that it annihilated itself - Santosh83
https://www.syfy.com/syfywire/across-the-universe-a-star-exploded-so-violently-that-it-completely-annihilated-itself
======
ajross
For those interested in the science and less in the fluff: this is notable not
because it's "bigger" but because it's the first really good observation that
fits with a "pair instability supernova", which is a well developed existing
theory but has never been observed.

The basic idea (I'm not a professional here) is that the gamma rays produced
in the stellar core provide the bulk of the pressure that holds up the outer
star against collapse. But past a certain temperature, the energy of those
gamma rays starts preferentially resulting in electron/positron pair
production. And the effective pressure per energy of an e-/p+ pair is
significantly lower than it is for photons. So the outer core layers start
falling downward, picking up energy as they do, and getting hotter. And the
still higher temperatures result in gamma production which is even more likely
to result in pairs, so the pressure drops still further and the core contracts
more, and you get a runaway reaction.

But the runaway is of temperature and not so much density, so what's left
after the outer layers of the star all get blown away is a comparatively low
density core of extremely hot plasma, which simply diffuses without
recollapsing into a neutron star of black hole.

~~~
zazagura
Why don't the e-/p+ annihilate back into photons? Where do the p+ go?

~~~
whatshisface
Some do, but if they are moving apart fast enough they'll never meet again.

~~~
zazagura
Yeah, but I'm not talking about the original pair.

Since there is way more matter than anti-matter, shouldn't the p+ eventually
hit something and annihilate?

~~~
ajross
It will. But due to the way things work (and here I'm mostly ignorant of the
actual physics) the likelihood of this happening (or really for it to scatter
against anything else in the plasma) for a high energy lepton is much lower
than for a photon of the same energy. So where the old, cooler photons were
hitting things locally and providing pressure that was keeping the core from
collapsing, the positrons are escaping farther out and allowing the core to
collapse.

~~~
raattgift
The range given by Plait in the article at the top for SN 2016 iet is 120-260
M_{sun}, and towards the lower end would be a _pulsational_ pair instability
supernova (PPISN) (the "pulsational", the first P of PPISN, part starts to
fall off above 130 M_{sun} and vanishes around 150 M_{sun}).

If you're feeling ambitious you can digest Woolsey 2017
[https://arxiv.org/abs/1608.08939v2](https://arxiv.org/abs/1608.08939v2) which
is about _pulsational_ pair instability supernovae (PPISNs) and which by
coincidence I had on hand because I was reading about LIGO's 50-135 M_{sun}
remnant mass gap[1].

The first couple paragraphs of Woolsey 2017 are a good basis for an answer to
the question, "what happens to the positrons?", and the answer is that they
and the electrons contribute to complicated nuclear fusion chains more
centrally within the star.

The central regions in which these gammas are being produced are extremely
dense, and maybe it is helpful to think of a piece of some oxygen or silicon
nucleus being squeezed in between the e+e- pair such that electron capture
"steals" the electron and its part of the gamma's momentum, and the daughter
products include neutrinos (which tend to carry momentum right out of the star
system, since practically everything in the area is transparent to neutrinos).

In effect, the momentum of a centrally-produced gamma ray radiation kicks
inner parts of the star outwards, but when the gamma ray's momentum
"condenses" into e+e- pairs, a good fraction of the momentum ends up trapped
within denser nuclei, or converted into neutrinos.

The electric charge is very strong so any "excess" positrons will quickly find
another electron to annihilate with -- and there are plenty in the star (say,
in less-central regions) to meet. The positron will be "pulled" part way up,
and prospective partners with the opposite charge will be "pulled" part way
down. They're likely to meet somewhere away from the central region,
especially if there is a significant positron excess centrally. An
annihilation gamma produced much closer to the surface can only lift the
surface matter with the gamma's momentum, doing nothing to lift much more
next-to-central regions away from the most central regions. Moreover, since
the e+e- annihilation gamma can go in any direction, it has a greater chance
of pushing less-central regions towards the centre than a nuclear fusion gamma
produced very centrally.

Finally, Plait's _Bad Astronomy_ article at the top also links to Plait's
earlier [https://www.syfy.com/syfywire/the-star-that-blew-up-a-
little...](https://www.syfy.com/syfywire/the-star-that-blew-up-a-littlethen-
blew-up-a-lot) which tries to describe PISNs for the readers following the
sentence, "What follows is still somewhat hypothetical, but astronomers are
working on this problem, and many think this can explain this very odd class
of exploding star …"

\- --

[1] There's a lack of observational evidence for black holes in that mass
range, and if PPISNs are commonplace that might be why they don't exist, as
opposed to other possibilities such as massgap BHs exist but their near-
regions don't radiate much compared to the background). In essence PPISNs and
PISNs reliably throw away enough mass that ~ 55-133 M_{sun} SN remnants are
prohibited, and we can only get compact objects in that massgap through
mergers or the like. More here
[https://arxiv.org/abs/1709.08584](https://arxiv.org/abs/1709.08584) if you're
very interested, and the two authors are worth an author-search as they are
prolific in this area.

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the8472
An interesting tidbit is that the region of stellar parameters where the
explosion is so massive that it leaves behind no compact remnant borders
directly on the region where stars can collapse into a black hole without a
supernova, they just wink out of existence.

[https://upload.wikimedia.org/wikipedia/commons/1/18/Remnants...](https://upload.wikimedia.org/wikipedia/commons/1/18/Remnants_of_single_massive_stars.svg)

~~~
FakeComments
Am I reading wrong, or is no remnant in the middle of a black hole formation
region?

It looks like “direct black hole” is on both sides of “no remnants”.

~~~
wallace_f
I guess on both sides they start out as star -> collapse, but one either side
the collapse continues into a hole. Within those parameters the star will stop
collapsing and go supernova and never collapse back. I guess that's what it
means.

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jackyinger
Total tangent.. but there’s no innate reason to be totally boggled by huge (or
incredibly tiny) numbers. I think getting all in a huff about them is an anti-
pattern in science writing. I mean, we deal with gigabytes of data on machines
with clock periods of nanoseconds every day.

What is cool is learning about new-to-you physical mechanisms.

~~~
aasasd
Counterpoint: Graham's number.

It's like the Emperor's “enough dakka,” but with digits.

~~~
perl4ever
It's faintly ridiculous to have all the over the top expressions of amazement
about something a couple hundred times the size of the Sun. If you're the
layperson this is aimed at, who doesn't have a clue about stellar physics,
then why not imagine a star 1,000 or 1,000,000 times the mass?

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peter303
Perhaps we are witnessing a war between Type II civilizations. A Type II
civilization on the Kardashev Scale directly harnesses the energy of a star.

[https://en.m.wikipedia.org/wiki/Kardashev_scale](https://en.m.wikipedia.org/wiki/Kardashev_scale)

Improbable, but an amusing hypothesis.

~~~
CapricornNoble
Could have been a weapon's test gone wrong, like that recent Russian nuclear-
powered cruise missile that blew up. A very amusing line of thought indeed.

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YeGoblynQueenne
>> It was pretty much at this point reading the journal paper that my brain
wanted to leap out of my skull and run around in panicked circles screaming.
I’ve run out of adjectives to describe an event like this.

Is it OK if I'm interested in science without acting as if I was watching a
football game? Or do I lose my nerd cred for not expressing exuberant
excitement with colourful language every time I hear how big the universe is,
or how vast the cosmic scales, etc?

~~~
gjm11
I don't remember Phil Plait being quite so _breathless_ when he was blogging
at, er, wherever he was before. (ScienceBlogs, RIP?) Is it a SyFy house style
thing, perhaps?

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blattimwind
How do you explode without annihilating yourself?

~~~
saagarjha
You leave behind a collapsed core.

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cronix
Stuff like this just boggles my mind. Imagine if we could capture/store even a
small fraction of that energy that was released in mere seconds. It could
probably power Earth for millions of years. Sorry if my maths are off a bit.

~~~
marcinzm
The energies of even less exotic stars are insane. If we could capture 1
second worth of our sun's output then we could power earth for almost a
million years.

~~~
sametmax
The earth already does that, we just call it our ecosystem. It's why we have
clean water, carrots, coal and, and the end the chain, the internet to talk
about it.

~~~
maxerickson
They are talking about capturing the full irradiance of the sun for 1 second.
You are talking about the little sliver of it that smacks Earth.

~~~
sametmax
True, but it's enough. We don't need more that what we already got. We don't
even use most of it.

~~~
human20190310
No, we need more. The main barrier to achieving the dreams of science fiction
is a lack of energy to accelerate human-scale amounts of material to high
speeds (then slow them down later).

~~~
lbatx
No, we need to use more of what we get. We don't use hardly any of what hits
the earth. I believe I read that the Sun supplies 50X the amount of energy we
consume (if of course we could capture it as energy).

~~~
osamagirl69
You are off by a few orders of magnitude, the amount of sunlight hitting just
new mexico (after taking into account the losses of converting it to
electricity) is about 50x the energy we consume.

~~~
lbatx
Worldwide?

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perl4ever
If the brightness didn't go away, and that's unusual, why are people
concluding it "annihilated itself"? I don't see why that would be the natural
assumption. Maybe there _is_ a black hole in the center and something about
the dynamics is producing more radiation than usual from the stuff falling
into it?

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rolltiide
Science Nonfiction channel?

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geertj
Photoid?

~~~
chongli
Wow, it's impossible to Google that term. Every single result is related to
"Photo ID."

~~~
ivalm
Supernovae resulting from an impact by a large object moving close to the
speed of light. Was used/popularized in a series called "Three Body Problem"
as a weapon (book premise is the "black forest theory", which is since every
sentient civ is a potential existential threat advanced civs would genocide
any aliens they detect, photoids being one of the exotic weapons to accomplish
this).

~~~
perl4ever
Also see "The Fourth Profession" by Larry Niven. Aliens didn't annihilate
everyone out of fear, but they would cause a nova when there was profit in
doing so.

