Surprisingly, for all its complexity and highly radioactive and dangerous nature, the universe
itself outside earth actually has only a short list of ways to cause human extinction, as
delicate as we are.
Typically we think of this question in terms of the most obvious way, an asteroid impact.
That’s the big danger, but there are a few other scenarios where some celestial event
could cause our demise.
One of these is the possibility of a nearby supernova.
Now what defines a near-earth supernova is the question of whether it could affect Earth’s
biosphere in some way.
Here it’s important to note that much of earth’s raw materials are present because
there was once a close supernova that populated the solar system with its heavier materials.
But as to how we would be affected today, the supernova would need to be at a distance
of less than a thousand light years away to have an effect, in most cases much less, to
do much other than certain very high energy particles linked to supernovas that hit earth’s
upper atmosphere on a regular basis.
Surprisingly, given the criteria, there actually have been a somewhat alarming number of close
supernovae just over the last 11 million years.
The number is a whopping 20 supernovae that close in that short of a time frame.
And due to the nature of the movement of stars in the milky way, there are going to be more
in the future as the sun makes its way through the galaxy encountering countless future stellar
encounters, and some will no doubt be dangerous.
Really close, highly dangerous supernovae within a distance of just over 30 light years
happen on average once every 240 million years.
As to the damage they can cause, the primary damage is going to be from gamma rays.
In earth’s case, the major damage here would be to the ozone layer.
A close enough one would basically strip the ozone layer off and the earth’s surface
would be very rapidly bathed in ultraviolet light from the sun.
This is dangerous to life on earth because while obviously skin cancer rates would go
through the roof and we’d have to take precautions when going into direct sunlight, the real
threat is to the phytoplankton in the ocean because these and related ocean life effectively
form the base of the oceanic food chain and could be heavily impacted.
Knock that food source down, and we have a severe reverberating food chain problem for
all macrofauna on earth.
But this equation also depends on the type of near earth supernova that might occur,
or have occurred in the past.
Supernovae are classed types of events, and the classes differ significantly from each
other.
Most of the speculation done on how bad things could get on earth after such an event are
based on the Type II supernova, of which there are subclasses.
Type II supernovae are the most famous type, where a change happens in a giant star's equilibrium
as it hits iron on the periodic table as it fuses its way along the periodic table at
the end of its life.
This happens to stars that are at least 8 times more massive than the sun, but interestingly
not more than about 40 or 50 times, above that there is no supernova at all, the whole
thing just collapses into a black hole.
The stars that meet this kind of end tend to be young, very massive stars that live
short lives, and these types of supernovas are hallmarked by the heavy presence of hydrogen
in their spectra.
Weirdly, astronomers see these types of supernovas in spiral galaxies like the milky way, but
not really in elliptical galaxies which are composed of mostly old, low mass stars.
What happens here is that when the supernova gets to iron and nickel, the fusion output
of the core basically stops, reactions of iron or nickel produce no net energy, so the
star’s core literally goes inert.
This means that there is no degeneracy pressure coming from the star’s core to keep the
star from collapsing under its own gravity.
As it collapses, electron degeneracy pressure is overcome, this is called the Chandrasekhar
limit, and the star continues to implode, but that causes the outer core of the star
to implode at speeds of one fifth the speed of light, causing the temperature of the core
to rise cataclysmically but there is a limit, neutron degeneracy stops the whole process
and the core rebounds.
The shock wave from that along with the high temperatures allows for a brief period of
fusion to occur that produces elements heavier than iron and basically blows them out into
the universe.
This type of supernova, if it were close enough, could strip earth of half of its ozone layer
if it was within 26 light years.
These are estimated to happen about 10 times per billion years, so earth has been affected
in its history by about 45 of these types of supernovas.
Life has survived them all during the period it existed, but there actually is a worse
type of supernova.
This is known as a Type 1a supernova, and this one is tricky because we may not know
about it in time to prepare.
A type 1a supernova is where you have a binary system consisting of a white dwarf and another
type of star or even another white dwarf.
Here the stars are exchanging material, and the white dwarf is accreting material robbed
from the second star until it reaches a certain point, and boom.
We don’t really have a good handle yet on nearby systems hosting white dwarfs in this
position, and it could well take us by surprise.
But we do have an example for this, in the star system IK Pegasi at a current 154 light
years away, but it’s estimated that by the time this one goes off, the proper motion
of the sun and this star system will have carried it to a safe distance.
But it’s of course possible that a system like this lurks much more closeby that we
don’t yet know about.
As far as Type II supernovae, we’re currently safe.
There are candidates, not the least of which is Betelgeuse, that will eventually go supernova
but are too far for damage, but will present an absolutely spectacular sight to anyone
that may still be here when they go off.
This list actually includes some of the most prominent stars in the night sky other than
Betelgeuse.
Also on the list are Antares, Rigel, and Spica which is the closest other than IK Pegasi
at about 250 light years.
So what would this be like to watch?
Well, let’s take Betelgeuse as an example.
Betelgeuse is a red supergiant at a distance of about 590 light years.
Here the star’s core is no longer fusing hydrogen, but that is happening in an outer
shell of the core.
This means helium, and often helium is fatal to stars.
Here the helium is accumulating at high density, and will eventually get hot enough to fuse
heavier elements.
We don’t know exactly if Betelgeuse has hit this stage yet, though recent evidence
suggests it hasn’t.
But when it does, it has only a few hundred more years to live.
When it finally explodes it will leave a neutron star that will probably become a pulsar after
a type II supernova explosion.
Despite the recent dimming event at this star, which it basically belched out a dark cloud,
it’s probably at least a hundred thousand years from exploding, perhaps significantly
longer.
Interestingly however, there will be an invisible in visual light flux of material that arrives
from the Betelguese explosion about 100,000 years after it happens.
We’re safe from this, the heliopause of the sun will block the flow and won’t be
a problem, it’s likely getting assaulted by supernova shock waves as we speak, but
will give future scientists if we’re still here a lot to study.
We’d also see a prolonged X-ray flux, to the tune of tens of thousands of years, but
these will be blocked by earth’s atmosphere so we won’t know the difference but it may
change the equation for space travel to a degree.
Shielding will have to be adjusted.
We will also know about this event before we actually see it.
Type II supernovae like this will produce a pulse of neutrinos warning us to turn the
instruments towards it.
It’s brightness will certainly exceed anything in the night sky except the moon, and if we
haven’t altered our visual capabilities a hundred thousand years from now, it’s
going to be hard to look at a point source that bright and might even cause damage to
your retina.
Never stare at a near earth supernova the few weeks it’s bright.
But what of past extinctions related to supernovae?
Well first is the aforementioned population of the solar system by heavier elements produced
by a nearby supernova.
The evidence for this comes from the presence of long lived daughter products from the decay
of short half life isotopes, and it has been suggested that this supernova event actually
precipitated the formation of the solar system.
It’s also an open question as to whether elements produced by this supernova were instrumental
in abiogenesis on earth, and that had it not happened, life wouldn’t be here opening
up a route for a solution to the Fermi Paradox simply being you need a nearby supernova to
happen that produces the right amount of heavier elements and just the right time for life
to be possible in a star system.
At the same time however, supernovae over long terms are not rare, so it may not be
a hard solution to the paradox.
But the effects of past supernovae outside of that on earth are not well explored.
But there are a few indicators.
The isotope Iron-60 has been found in pacific ocean sedimentary rock deposits.
This points to a nearby supernova having occurred about five million years ago, or less, because
the abundances are high and otherwise the supernova would have had to have happened
VERY close, enough for a mass extinction, but that does not seem to have happened.
That’s actually at odds with other evidence however.
For the iron-60 deposit to be there, the quantity would still suggest that the supernova was
less than 100 light years away, but for one that close the chances are rather small, only
about 5% for the last five million years.
It’s been suggested that the reason for this is that the odds are wrong due to the
solar system entering the orion arm of the galaxy.
More recent evidence shows interstellar dust found in the Antarctic surface snow that’s
very recent that again shows Iron-60, bolstering the case for a near earth supernova in the
not too distant past.
But then we get back to the gamma rays, and there we may have some evidence of supernova
mass extinctions in earth’s past.
Specifically the End Orovician extinction.
This was the first of the really bad ones as far as earth’s extinction events go.
It came in two pulses about a million years apart starting about 443 million years ago.
Earth largely only had marine life at this stage and only evidence of very primitive
plants are associated with land.
This mass extinction was horrific, killing 85 percent of all marine life and altering
the course of marine evolution permanently.
One possibility is that the initial extinction pulse was caused by a gamma ray burst.
But not just any burst of gamma rays from a supernova, but this one from a hypernova.
A hypernova is a type of supernova where a very specific, and very extreme core collapse
happens resulting in a rotating black hole, but not only that, but also an accretion disk
and relativistic jets coming out of the poles ready made.
These things are much more energetic explosions than normal supernovas and are at least ten
times greater in luminosity.
The result of the conditions of a hypernova is a sustained, very long gamma ray burst,
which can be on order of two minutes in duration.
That’s a solidly damaging dose of gamma rays for earth, and worse, a hypernova could
do damage at much greater distance.
In this case, it could have been as far as 6000 light years away.
That’s scary.
If that were the case, even just a ten second gamma ray burst would have almost immediately
stripped the ozone layer, like near instantly in this context, and started the extinction
in short order due to exposure from ultraviolet light from the sun.
Shallow living plankton would have been immediately hit, more so than anything on the seabed,
and this would then have set off a period of severe glaciation due to the production
of nitrogen dioxide which would have caused global cooling, worsening conditions for extant
life on earth.
This is far from a proven hypothesis, but it’s a scary one, especially in that if
you were alive in those days you’d probably never even know anything was wrong, unless
someone had a gamma ray detector.
You’d just be having lunch unaware the ozone layer was being stripped above you by the
gamma ray burst and that a mass extinction was beginning in short order, all caused by
something that happened to a star 6000 years previous.
But there is also the strange case of the supernova remnant RX J0852.0-4622.
This is a weird one in that it was detected in front of a larger supernova remnant, like
it’s superimposed on it from our line of sight.
Gamma rays from the decay of Titanium-44 suggest that this supernova happened recently and
fairly close.
In fact it’s thought that this star exploded at the beginning of the 13th century here
on earth from our perspective.
Yet there were plenty of people recording what was going on in the sky at that time
across the globe, yet not a single reference in the historical record indicates anyone
saw this, even though they very well should have since the distance could have been as
close 650 light years away.
This is statistically weird because a supernova that close should happen less than once per
100,000 years, yet there it is.
This remnant needs more study, but it stands to show that we may not know everything about
the possibilities of near earth supernovae and perhaps even less about hypernovae.
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently eyeing the universe
suspiciously again.
It sure seems to throw a lot of stuff at us, ultra high energy cosmic rays, interstellar
meteorites, dinosaur killing asteroids and gamma ray bursts.
It may be beautiful, but it definitely has a vendetta against us.
But it also physically allows candy bars to exist, so it can’t be all bad and be sure
to check out my books at your favorite online book retailer and subscribe to my channels
for regular, in-depth explorations into the interesting, weird and unknown aspects of
this amazing universe in which we live.