[image: unnamed.jpg]
*Excalibur: * Sword in the Stone
It is perhaps the most famous example of metal-meets-rock in the entire
mythological multiverse.     Excalibur, the  sword in the stone.      The
first recorded account of this mythical sword shows it appearing on
Christmas Eve, when it was embedded within an anvil poised on a rock.   In
a more famous version, dating to the 15th century, next to the sword, anvil
and stone was found the inscription,  "Whoso pulleth out this sword of this
stone and anvil, is rightwise king born"       Merlin, himself, was
responsible for both the inscription and the embedded sword.  As every
noble aspired to the throne, each one of them attempted to extract the
sword, but to no avail.   Their lack of success was hardly surprising.
 Merlin made sure that the greater the exertion applied to its attempted
withdrawal, the stranger its resistance to removal became.   Even the
mightiest noble couldn't so much as lift the sword  a micrometer out of
place.    Finally, a humble squire in the service of Sir Kay, who knew
nothing of the challenge but saw a sword within reach, effortlessly removed
it from the anvil to everyone's amazement.      When challenged to repeat
the deed, he yet again did so with only the slightest effort.    That
simple squire, Arthur, then realized that he was not the son of Sir Ector,
but of Uther Pendragon,himself and, consequently, the rightful heir to the
throne.

THE SOUTHWORTH PLANETARIUM
207-780-4249   www.usm.maine.edu/planet
<http://www.google.com/url?q=http%3A%2F%2Fwww.usm.maine.edu%2Fplanet&sa=D&sntz=1&usg=AFQjCNHulkHuLP13bOG2PkNrPazsGWFs2A>
70 Falmouth Street   Portland, Maine 04103
43.6667° N                   70.2667° W
Altitude:  10 feet below sea level
Founded January 1970
Julian Date:  2459242.18
2020-2021:  LXXVIII


THE DAILY ASTRONOMER
Wednesday, January 27, 2021
Exploratorium XIII:   A Long Time Ago, a Hundred Light Years Away


*Location*
         100 light years (or so) from Earth

*Time*
          3 million years ago

In today's exploratorium, we venture into the distant past to watch a
supernova explode.      The time is 3 million years before modern day.
 The location is about 100 light years from Earth.           Astronomers
believe that a supernova exploded around this time and place.       Today's
topic pertains to how astronomers could possibly know this occurrence.
And, how that knowledge relates to, of all things, ocean crust.

[image: 7iXUPoququbMmAPMTLsoKK-1200-80.jpg]
Supernovae happen at the end of a highly massive star's life cycle.  A star
generates energy and sustains its structure by core thermonuclear fusion
reactions.  These reactions transform light elements into heavier ones
(such as hydrogen into helium or carbon into oxygen, to cite two common
examples.)    The fusion transmutes some of the initial material into
energy, the pressure from which balances the relentless gravitational
compression.  If a star is sufficiently massive, its core pressures are
high enough to permit fusion of heavier elements, such as silicon and even
iron.  However, no star can produce the temperatures necessary for iron
fusion.   The exhaustion of the core iron reserves disrupts the balance
between the contracting gravity and expanding energy. The outer layers
collapse onto the core, precipitating an explosion.

Not only does the supernova destroy the star, it also imparts enough energy
to facilitate the high-powered fusion reactions needed to create all the
elements heavier than iron.   The resultant heavy-element nebula expands
through space, conveying the heavy elements to other star systems.    So,
if a supernova is close enough to a star/planetary system, its nebula will
sprinkle particles onto the bodies within it.

Now, one might think that if we can identify these particles, we could
conclude that a supernova happened nearby, right?  Well, unfortunately, the
issue is not quite that simple.  You see, our solar system (including us)
formed from a nebula that, itself, was rife with supernova elements.  We're
all composed of exploded star bits.    How could we distinguish between
recent supernova fragments and those that have been around awhile?

It's time to examine that ocean crust.

Scientists extracted pieces of what's known as "deep ocean
ferromanganese crust"
from the Pacific bottom.  These iron rich-metallic crusts serve as handy
geological repositories because they grow at the painfully slow rate of 2.5
millimeters per million years.   By studying the different layers,
scientists can identify the sediments that settled onto the crust at
various epochs.   Well, at the layer corresponding to the era about three
million years ago, they discovered a sharp increase of iron-60: a
radioactive iron isotope with a half life of 2.6 million years.*  (Follow
that asterisk if you'd like more information related to that last sentence.)

[image: What is the difference between oceanic crust and continental
crust-geology in.gif]
The presence of this radioactive iron compels us to ask: "how could it be
there?"      We know that nothing in the solar system could have smothered
the planet with Iron-60.     The only mechanism with which we're familiar
is a supernova explosion:  the event that creates heavy elements, including
the radioactive Iron-60, and sends them in all directions around it.
(Note: the radioactive Iron-60 within the supernova cloud that formed the
solar system would have vanished long before.)

Based on the detected Iron-60 amounts, scientists estimate that the
supernova exploded about 100 light years from Earth.    (Had it been
closer, the Iron-60 abundance would have been greater.)     So, examination
of the ferromanganese crust led to the Iron-60 discovery, that, itself, led
to the supernova's "discovery."   This "discovery" is not a direct sight
discovery, but instead, a theory based on chemical analysis of the crust.
   The crust is acting like a telescope: enabling us to learn about a star
that might have exploded within our vicinity.

Some worry that such proximate supernovae could destroy Earth life.  Not
only did this event not diminish life, but, as mentioned previously, it
could have led to Earth's rapid human migration from its African origin.
The idea is that the supernova energized cosmic rays.  These enhanced rays
might have precipitated more cloud particle condensation, resulting in
increased cloud cover.   Such cover would have reduced the solar heating
and cooled the planet.  Such cooling might have made the African region
drier, prompting the inhabitants to migrate away to more favorable climes.

Now, that last paragraph is stuffed with assumptions!
The notion that this supernova indirectly led to human migration is a
fascinating idea, but we cannot categorically state that this correlation
existed.  Again, it is a theory.      However, it is intriguing to think
that not only could a supernova have created the solar system, but that
another one might have driven humans to populate the world.


*One can think of an "isotope" as being a type of element.  Within a
nucleus one finds protons and neutrons.  The proton number defines the
element; for instance, an Iron nucleus contains 26 protons.  The Iron-60
nucleus also contains 34 neutrons: the sum of protons and neutrons defining
the number 60.   Iron-60 is radioactive, meaning that it spontaneously
emits radiation as it transmutes to Cobalt-60.    Half-life pertains to the
transmutation rate.   If we had an Iron-60 sample in front of us, half of
it would change into Cobalt-60 in 2.5 million years.   Another 2.5 million
years would elapse before half of that sample transmuted.


To subscribe or unsubscribe from the Daily Astronomer:
https://lists.maine.edu/cgi-bin/wa?SUBED1=DAILY-ASTRONOMER&A=
<https://lists.maine.edu/cgi-bin/wa?SUBED1=DAILY-ASTRONOMER&A=1>