At a distance of about 150 Light-years lies a ticking time bomb known as the star system
IK Pegasi, a system that ranks among the the few star systems that can directly affect
earth if and when it goes supernova.
This is the nearest known supernova candidate star, and is a member of a binary system.
It's an A type main sequence star with a white dwarf companion.
In the future, it’s somewhat uncertain just when, the main star will exhaust the hydrogen
fuel in its core and swell into a red giant phase, an eventuality that will see the star
gain up to a hundred times its current radius.
Once that happens, a very strange and potentially dangerous chain of events will begin.
The first step will be an exchange of material between the two members of the system, in
that the white dwarf will begin to accrete material from the bloated red giant star.
This is a recipe for disaster.
As that white dwarf gathers that material, two things will happen.
First the stars orbit will shrink, and then that hydrogen and helium from the red giant
will begin to compress and become increasingly hot.
Eventually, the white dwarf which is normally seen as a dead cooling remnant of a star,
comes back to life as a kind of zombie or vampire star feeding off its main host.
The pressure and heat eventually become so great that fusion occurs once more at the
white dwarf and a recurrent nova cycle results.
This is known as a cataclysmic variable star, where wild and very pronounced changes occur
in its luminosity as the novas periodically spark off.
There are known examples of this in the galaxy, thankfully none as close as IK Pegasi.
But there’s a possibility that it won’t simply end at recurrent novas, but that overall
the white dwarf will continue to gain mass despite what gets blown off during the nova
periods.
An alternate model suggests that a sustained fusion burn could occur instead, but regardless
if the white dwarf’s mass continues to grow, it will approach something known as the Chandrasekhar
limit, after a revision by Dr. Subrahmanyan Chandrasekhar in 1930 of a previous set of
calculations.
This limit, which is currently thought to be about 1.4 solar masses, is the largest
mass a stable white dwarf can achieve without a cataclysmic collapse happening.
This is because of electron degeneracy pressure and gravity.
In a dense white dwarf, electrons are forced into higher energy states, they start moving
faster, and that creates electron degeneracy pressure which in turn supports the white
dwarf from undergoing further gravitational collapse.
But when you hit the Chandrasekhar limit, the electrons are moving near the speed of
light, and that pressure is overcome.
And this is where things get uncertain depending on what the white dwarf’s core is composed
of.
During the collapse, if it’s mostly oxygen, Magnesium and neon, it should form a neutron
star with relatively little of the star's mass ejected.
But if it’s carbon and oxygen, runaway carbon fusion will occur before it hits the Chandrasekhar
limit and boom.
This is known as a Type 1a supernova, and given the proximity of the IK Pegasi system,
this could pose a threat to life on earth.
When will this mess of uncertainty unfold?
It’s not likely within a very long time, though no one is certain when it will happen.
What is certain is that the star system is moving away from us, so the risk lessens with
time, but by the time it does eventually go supernova, if it indeed goes that route, it
may still be close enough to have an effect on earth, but not to cause a major mass extinction,
it’s simply not close enough under current thinking to be able to do it.
But there are also averages, and on average a supernova will occur near earth within about
33 light-years that can cause mass extinction about every 240 million years.
How they do it is gamma rays and their effect on the earth’s atmosphere.
The effect is that the gamma rays cause molecular nitrogen and oxygen to react and become nitrogen
oxides, which depletes the ozone layer which in turn allows the sun’s ultraviolet light
and other dangerous radiation to reach the surface and start the chain that leads to
mass extinction, and indeed a supernova is a main candidate for the end Ordovician mass
extinction in earth’s past.
Deplete the phytoplankton and reefs, and that takes out the base of the marine food chain.
Type 1a supernovae however aren’t the only threat.
So are type II, which a number of prominent night sky stars are predicted to eventually
produce such as betelgeuse, but none are close enough to pose a threat, though in the future
as the sun makes its way through the galaxy it could run into giant stars that do.
This has happened, it’s thought that as many as 20 supernovae have occurred near earth
in the last 11 million years.
For example, a type II supernova happening within about 26 light-years would burn off
half of earth’s ozone layer very rapidly.
This has been observed, supernova 1987A in the large Magellanic Cloud kicked off radiation
of this magnitude.
But the Type Ia supernovae are the worst because they involve dim white dwarfs, which are something
we are not fully aware of as far as populations nearby earth go.
There may be worse ones out there than IK Pegasi.
But within all of this there is a mystery.
With so many supernovae going off in the past, why didn’t they cause very obvious periods
of extinction?
Sediment studies to look for isotope signatures of supernovae in earth’s oceans show that
they happen, particularly with Iron-60.
The scientists studying that found comparatively a lot of it.
That means that either the supernova that caused that was recent, within the last 5
million years, or it was extremely close.
If it were close, then why wasn’t there a mass extinction?
And the mystery deepens with the supernova remnant RX K0852.0-4622, which had to be a
recent supernova due to its chemical signatures.
As in about 800 years ago, and should have been prominently visible in the night sky
for a time.
But no humans keeping records on such things in those days, and there were quite a few,
seem to have seen it.
Even weirder, it’s a statistical outlier in that you should only see one that close
every 100,000 years, yet it appears to have happened without anyone seeing it.
And it’s not just the ocean, further work in Antarctica has shown once again iron isotopes
associated with supernovae.
It’s been argued that the probability of supernovae happening nearby earth is actually
rising due to Earth entering the Orion arm of the milky way.
There are many mysteries involving supernovae in our galaxy.
But it doesn’t end there.
There are two other odd supernova related phenomena that may very directly affect the
chance for life in the universe.
The first is the formation of the solar system itself.
It’s unclear how the dust and gas that made up the material for the solar system started
collapsing and lumping together to form what would eventually be the planets.
It has been suggested that the concussion from a nearby supernova at the dawn of the
solar system could have been this trigger mechanism.
No nearby supernova, no earth.
On the other hand, that probably isn’t that rare of an occurrence, we see planetary systems
everywhere and supernovae happen regularly around the galaxy on geologic scales, and
indeed were once more common.
We also see evidence for near earth supernovas in meteorites.
They exhibit isotopes that show that between 1 to five million years before they fully
formed, there was a nearby supernova.
In addition to this, a great deal of the material that comprises the solar system originated
in supernovae over a period of billions of years.
There’s also the question of how our solar system was populated with heavy metals during
its formation.
This may have been due to a succession of supernovae based on studies of star forming
regions.
This particularly has to do with the presence of the isotope Aluminum-26.
This isotope is formed in stars, and is expelled during supernovae but also to some degree
in the winds of wolf-rayet stars, but there appears to have been more of this element
around during the formation of the solar system than a single supernova could account for,
and wolf-rayet stars are less likely.
But within this, there is another mystery.
For multiple supernovas to have delivered aluminum-26, to account for the observations
in meteorites all of that aluminum from different sources and different times would have had
to be heated by some unknown event early in the solar system to reset things so that all
of the aluminum appears to have formed at the same time as far as radiogenic dating
goes.
So what happened there?
No one knows, but whatever this heating event was, it would have had to vaporize all the
material in the solar system for a time.
That may invoke yet another nearby supernova, but that would require the supernova being
close enough to heat everything up, but far enough not to destroy everything.
That’s asking a lot.
But it’s also been suggested that the very young sun might have gone through some sort
of outburst, which has been observed in protostars elsewhere in the galaxy, but they don’t
appear to be powerful enough for a full scale reheat of the solar system’s materials.
It’s also possible that there’s something about aluminum-26 generation that we simply
don’t know, so the whole thing is up for revision as more discoveries are made.
And there’s a huge problem here.
Some meteorites also exhibit interstellar grains, material that was picked up that predates
the solar system and originated in other star systems.
If there was a giant heating event from the protosun having an outburst, or a nearby supernova,
these would have been melted.
There’s also a question surrounding the abundance of heavy water in the solar system,
this should not be if there were a heating event.
And finally, there is the question of life.
Firstly, most of the elements used by life were forged in supernovae.
No supernovae, no heavier elements essential for life.
But it may also be the case that supernova activity in the universe directly contributes
to habitability.
This is through the production of cosmic rays that interact with the earth’s atmosphere.
It’s thought this might create a chain of events in the atmosphere that ultimately leads
to a cooler climate more conducive to life, and also plays into nutrient generation and
distribution.
So all of this creates a question.
If supernovae are essential for life, but too many of them are bad for life, and too
few are also bad for life, then is it possible that the solution to the Fermi Paradox is
that the right balance of supernovae is required for a galaxy’s habitability, from planet
formation to abiogenesis itself, to moderating the atmospheres of exoplanets.
That’s probably not that rare of a situation, but it would make suitable galaxies a bit
scarce.
The question of would we even be here if there hadn’t been enough supernovae aside, I leave
you with this.
What we do know is that this galaxy is habitable.
So this one does not solve the Fermi paradox, but highlights it.
If we’re here, then there should be many others in the habitable zone of the Milky
Way.
Where are they?
Thanks for listening!
I and futurist and science fiction author John Michael Godier currently wondering if
a common first contact message in the universe is instructions for supernova mitigation as
a public service, or worse, selling Supernova-B-Gone anti supernova and general lubricant spray.
And then you find out it doesn’t work as advertised.
Very questionable on the part of the aliens 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.