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about 1,200 years ago earth may have
experienced one of the rarest and most
powerful cosmic events a planet can be
exposed to a gamma-ray burst we're not
sure yet if it actually happened but if
it did well let's just say that we as
living things on earth are lucky it
wasn't worse gamma rays are the most
energetic form of light and when
explosions in space released them in
sudden bursts lasting anywhere from a
few milliseconds to a minute or more
they release radiation that's a million
trillion times as intense as the Sun
astronomers think these enormous blasts
of energy come from a couple of
different sources some birds last on
average for about 30 seconds and occur
when massive stars die in a violent
supernova creating a black hole in the
process but as gamma bursts go 30
seconds can be a long time short bursts
like the one astronomers think might
have hit the earth last for just a
second or two and almost always occur
when two neutron stars or in some cases
a neutron star and a black hole collide
neutron stars are almost inconceivably
dense picture the entire mass of the Sun
crammed into just a couple of kilometers
if a couple of these extremely dense
stars collide the result is so powerful
that it releases some of its energy as
gamma rays which have the smallest
wavelengths and the most energy of any
wave in the electromagnetic spectrum
these gamma-ray bursts or GRBs aren't
exactly rare on a universe wide scale
because they're so bright they're easy
to spot with telescopes even at the
outer reaches of the universe but within
our galaxy astronomers believe they only
happen about once every million years
and gr bees only release a very narrow
beam of powerful radiation which means a
planet has to be in just the right place
at the right time to be struck by one so
what makes scientists think a GRB hit
earth the answer lies in ancient trees
in ancient ice where in 2012 scientists
discovered unusually high levels of two
particular isotopes carbon-14 and
beryllium ten these isotopes formed when
radiation from space collides with
nitrogen atoms in the atmosphere causing
them to decay because cosmic rays are
constantly bombarding our atmosphere we
expect to find certain amounts of these
two isotopes lying around but they
turned up at levels ten times higher
than normal in two distinct places in
the tree rings of old Japanese Cedars
and in ancient layers of Antarctic ice
both of these sources have been dated
- about the same time around the Year
775 based on what they're seeing
scientists estimate that the energy that
caused these anomalies was roughly
equivalent to 13 Hiroshima bombs spread
out over half the planet's surface
unlike the famous supernova of 1054
which was recorded by cultures all over
the world there doesn't appear to be any
historical record of people noticing
anything unusual in the sky around 775
but that doesn't actually tell us much
because the bursts probably only lasted
two seconds and may not have emitted any
visible light but of course there are a
couple of other explanations a
historically large solar flare could
have bombarded earth with enough
radiation to create the clues we're
seeing but it also would have caused a
truly spectacularly large display of
Northern Lights which again doesn't seem
to have been noted by anyone and nearby
supernovae is another possibility but
for such an event to send out comparable
levels of radiation the explosion would
have been easily visible from Earth and
again there's no record of such a thing
happening so we're left with a GRB
scenario scientists believe the
explosion that caused it must have
occurred between three thousand and
twelve thousand light-years from Earth
because if it were in closer the
radiation would have caused the sudden
extinction of at least some life though
highly unlikely what would happen if a
comparable GRB struck earth today well
it depends how far its source was from
us if it was more than a few thousand
light-years away Earth's atmosphere
would absorb most of the dangerous
radiation so humanity would be okay but
our satellites and power grids would all
be fried likely causing some post
technological scenario worthy of a cable
TV drama that you wouldn't be able to
watch if the burst occurred closer than
3,000 light years away the radiation
could destroy a fair amount of our
atmosphere particularly the ozone layer
which is made up of triple oxygen
molecules and protects our planet from
the ultraviolet radiation from the Sun
with our shields basically down Earth's
surface could be exposed to potentially
lethal levels of radiation which while
not killing everything would certainly
add to the list of mass extinction
events that our planet has endured so
far so on the bright side if a gamma-ray
burst did happen to graze earth 1,200
years ago at least we don't have to
worry about it for another nine hundred
and ninety nine thousand years or so
thanks for joining me for this episode
of scishow space if you'd like to learn
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