PBS Digital Studios
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The cosmos has so many catastrophes in store
for our fragile little planet.
Among the scariest is that one day,
we will almost certainly find ourselves
in the path of a gamma-ray bursts: death ray.
The end of the world may not be nigh,
but it will come.
In fact, there are quite a few ends of the world on their way.
Each one were interesting
and less avoidable than the last.
There’s a good chance we can prevent the most imminent,
like asteroid strikes,
or at least deal with their effects,
like the damage caused by gamma-ray bursts and supernovae.
But the later ones will be beyond any conceivable technology to prevent,
the gradual heating and eventual death of the sun.
Hopefully, our super advanced
and probably not quite human descendants
will be able to escape those by traveling to other star systems.
But what about the Milky Way’s inevitable collision with Andromeda,
or the final burning out of the last stars,
or the evaporation of the last black hole
and decay of the last proton?
The ends of the world and of the universe will come.
Better to be forewarned.
We’ll get to each of these inevitable cosmic catastrophes,
but let’s start with the one that could happen anytime–
a supernova or gamma-ray burst frying our atmosphere.
Every hundred million years or so,
a good deal of Earth’s life gets wiped out.
At five or six different point in time
over the past half billion years,
a large fraction of species simply vanished from the fossil record.
Some of these mass extinctions were due to giant asteroid impact,
including the most recent,
which wiped out the dinosaurs 65 million years ago.
But this is the most preventable end of world scenario.
In fact, we’ve already talked about asteroid impacts and how to deflect them.
However, at least one mass extinction may have been caused
by something we’ll never have the technology to stop.
The Ordovician-Silurian extinction 440 million years ago,
may have resulted from the earth being blasted
by the intense radiation jets
from a distant exploding star.
It may have been caused by a gamma-ray burst.
In this episode, we’re going to look at the evidence
that the O-S extinction was caused by a GRB.
And we’ll also figure out how long before the next gamma-ray burst hits.
But first let’s go over the proposed scenario.
As many of you know, a supernova is the explosion
that follows the catastrophic collapse of a massive star at the end of its life.
Our sun won’t die that way,
but any star more than around eight times the sun’s mass will.
The resulting explosion sprays high-energy light.
比如紫外线 x射线 伽马射线
So ultraviolet, x-rays, gamma rays
and near-light speed particles, so as cosmic rays,
into the surrounding interstellar space.
Any planet within a few tens of light years of a supernova is in trouble.
It’s even more dangerous if the star was rapidly rotating before it exploded.
In that case, the powerful magnetic fields
can channel the explosion into narrow jets
that massively focus and amplify the blast.
Roughly once per day, the jet from such an explosion in a distant galaxy
reaches the earth and is detected by the Swift or Fermi satellites.
The observed faint flash of gamma-rays from exploding stars
can last anywhere from a couple of seconds to a few minutes.
These are long-duration gamma-ray bursts.
Short-duration bursts that last less than two seconds
are caused by merging neutron stars.
So how close is too close for a GRB?
Well, the main danger of the burst within the Milky Way
is not the direct radiation itself.
本质上来说 所有的伽马射线 x射线
Essentially all of the gamma-rays and x-rays
are going to be blocked by atmosphere.
Some extra ultraviolet radiation will reach the ground,
but not at seriously dangerous levels.
Instead, the danger is in the long term effects on the atmosphere.
Gamma-rays break apart nitrogen and oxygen molecules in the atmosphere,
which then recombine into various oxides of nitrogen.
Those molecules are the real killers.
Nitric oxide catalyzes the destruction of ozone molecules,
depleting the ozone layer that protects us from solar UV.
And nitrogen dioxide absorbs visible light,
reducing the energy received from the sun.
These dangerous molecules can remain in the atmosphere for a few years.
And that’s potentially long enough to cause a UV increase
deadly to many species
and to initiate runaway global cooling.
Also they result in nitric acid rain.
It’s estimated that a typical gamma-ray burst within ten thousand light years
could deplete ozone enough
to cause up to a 30% increase in ultraviolet at sea level.
This is enough to devastate the most sensitive organisms
including phytoplankton, the basis of the marine food chain
and Earth’s main oxygen producer.
That alone is enough to cause a mass extinction event.
And this could be exacerbated by the global cooling
triggered by a few years of NO2 absorption of sunlight.
So why do some scientists think that
the Ordovician-Silurian extinction event resulted from a GRB?
Well, a couple of pieces of evidence fit nicely.
Looking at the fossil record, there seems to be a strong correlation
between the likelihood of a given species going extinct
and the exposure that species would have had to ultraviolet light.
Species in the late Ordovician that live near the ocean surface
or in shallow water were more likely to go extinct,
or went extinct earlier than those living in deeper water.
The same pattern isn’t clear for the other mass extinctions.
One explanation for this unusual extinction pattern is that
deeper dwelling organisms had more protection
against the increased UV following a gamma-ray burst.
Now I should mention that the O-S extinction is
definitely associated with the beginning of an ice age.
Scientists agree that many of the extinctions of that era
resulted from the changing climate.
The Ordovician was a very warm period,
and the relatively sudden onset of glaciation
is hard to explain without some triggering events.
That event may have been the increase in sunlight absorbing NO2 after a GRB.
Also, extinctions appear to have started before that ice age really got underway.
That fits the hypothesis of a GRB.
Extinction started due to the sudden UV exposure,
and continued due to climate change.
Whether or not this particular extinction event was due to a gamma-ray burst,
we’re pretty confident that the earth does get blasted periodically.
Based on the rates of GRBs we see in other galaxies
and the population of stars in the Milky Way,
it’s estimated that every billion years Earth finds itself in the path
of between 1 and 3 GRBs within 10,000 light years.
Unfortunately there’s no way to tell
whether a GRB will point it our way until it happens.
The nearest potential GRB in the brewing
is 8,000 light years away, so within the danger zone.
这是一颗沃尔夫-拉叶星 编号WR 104
This is a Wolf-Rayet star, WR 104,
the massive star in the last phase of its life,
currently blasting off its outer shells into a pinwheel-like nebula.
The exposed inner star shines several times hotter
and hundreds of thousands of times brighter than the sun.
This star is part of a binary system
and it’s this binary orbit that produces the spiraling nebula.
The fact that the spiral appears to be face-on
suggests that the axis of the entire system is pointed directly at the earth.
The rotational axis of the star will define the direction of the jet
in the event that this Wolf-Rayet star does produce a gamma-ray burst.
If it does, the orientation of the system suggests
we could be right in its firing line.
Well, no need to pack up and leave the solar system just yet.
Firstly, WR 104 could have up to half a million years of life in it,
although it’s hard to tell exactly how close a star like this is to exploding.
Also further observations with the Keck telescopes indicate
that the system’s orbital axis isn’t pointed directly at the earth.
That doesn’t necessarily mean we’re safe.
It’s a star’s rotational axis that defines the direction of the jet.
But the orbital axis of a binary system
and the rotational axis of its stars are often correlated.
So we may have dodged the bullet in this case.
Gamma-ray bursts are much less common than regular supernovae,
and in fact, regular supernovae can do just as much damage as a GRB.
However, for a supernova to produce the same effects,
it needs to be much closer, within 20 to 30 light years.
There are definitely no stars in that range that could explode any time soon.
However the sun isn’t stationary.
It orbits the Milky Way and its galactic neighbors come and go.
Maybe in a few 250-million-year orbits,
a stellar time bomb will wander into our vicinity.
However, it’s really the GRBs that are most likely
to hit us first and hit us more often.
We should certainly expect one in the next half-to-one-billion years,
even if it’s not WR 104.
And when that happens, well, we won’t see it coming
and anyway there’s nowhere in the solar system to hide.
But with any luck, we’ll have advanced to the stage
where geoengineering of the entire atmosphere is possible.
Perhaps we’ll be able to rebuild the ozone layer
and clean the bad molecules from the sky.
Maybe we can hold out a little longer
against the series of calamities flung at us
one after the other from outer space time.
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Last week we talked about our first detection of a visitor from outside our solar system.
It’s a giant chunk of rock that we’ve named Oumuamua.
supreme84x asks what would happen if this thing hit us.
Well, based on its brightness and variations in that brightness,
the body is estimated to be 180 meters long and maybe 30 meters wide.
Now assuming an albedo or reflectivity of 0.1,
that gives it a volume of around 130,000 cubic meters.
A typical asteroidal density is 2,000 kilograms per cubic meter,
so that gives it a mass of around a quarter of billion kilograms.
It was moving much faster than most solar system objects
at around 50 kilometers per second at its closest approach to Earth.
So its kinetic energy, 1/2 × m × v^2
大约为3 × 10^17焦耳
was around 3 by 10 to the power of 17 joules
or 70 megatons of TNT.
That’s greater than the yield of Tsar Bomba,
the greatest hydrogen bomb ever detonated.
Assuming Oumuamua is entirely rocky and hit the atmosphere pretty much head-on,
it might actually reach the ground before disintegrating to deliver that energy.
It would become what we call “a city killer”.
The global effects however would be limited.
PixelatedDonkey asks whether there’s a chance
that some stars like solar systems will be ejected from the galaxy,
when the Milky Way collides with Andromeda?
Well, in fact, yes.
Recent studies suggest there’s a 3% chance
that the sun will jump galaxies on Andromeda’s first fly-by.
And there’s some smaller chance that it’ll miss that jump, coming up short,
and end up in intergalactic space.
And some stars will be slingshotted out of the galaxy by the two supermassive black holes
of Andromeda and the Milky Way as they fall together.
And what will the residents of one of those exiled systems see?
Well, a very dark night sky, dark everywhere,
except for the gigantic elliptical galaxy–Milkdromeda,
the final result of the merger.
But don’t worry.
We have a few billion years
to come up with a better name for that galaxy.
网友Timothy Judge在Portegies Zwart及其合作者的一篇文章中
Timothy Judge points out a significant potential error
in the paper by Portegies Zwart and collaborators.
“The stone is lonely.”, not “Lonely rock”.
Let’s hope the referee caught that one before publication.