[image: acrisius_219.jpg]
*King Acrisius*:  The Bad Dad
Remember the mythological adage:  resisting a prophecy merely ensures its
fulfillment.  Such was certainly the case with Acrisius, the ill-fated,
ill-tempered, ill-humored but otherwise robustly healthy king of Argos.
 He had one child, Danae, who was not only a dutiful daughter, but an
adoring one.  She offered her father unconditional love and kindness.   He
repaid her devotion with cool disdain for he, being a king in need of an
eventual successor, only wanted a son.  In desperation he traveled to
Delphi and through the Sibyl asked Apollo if he would ever be given one.
Apollo told him sternly that he would never have a boy. However, his
detested daughter Danae would give birth to a son.  Moreover, her son would
mature into a great warrior and would, according to the oracle, kill King
Acrisius.    The terrified king rushed home and imprisoned Danae in an
underground chamber that, curiously, was cast in bronze and sumptuously
furnished.    Perhaps Acrisius experienced some regret at the necessity of
his daughter's confinement and tried to make her comfortable.    She
wasn't.   She languished in isolation for many months. Night after night
she would lie in her bed looking up through the ventilation grate at the
stars. Night after night she cried herself to sleep.   One day, to Danae's
astonishment, a shower of shining golden droplets fell through the grate
onto her lap.    The droplets, cast down by Zeus, dissolved into her body
and by them she became pregnant.   We should point out that in the
mythological universe the sexual act was considered the mortal means of
conception.    Other interactions, such as the intermixing of  seas, could
also produce offspring.   For this reason some exalted figures could claim
direct descent from the Nile, a mountain or some other equally imposing
geological construct.    Months after this golden rain fell onto Danae, the
servant who had been charged with feeding her noticed that she had given
birth and promptly ran to inform the king. (No, he didn't notice her
pregnancy, apparently.)   Acrisius removed her and her child immediately
from the bronze prison and shut them in a chest which he then pushed out
onto the Aegean Sea.    Why, one might wonder, would he have not just
killed them both, thereby confounding the prophecy? Simple.   The
consequences of slaying his own daughter and grandson would have likely
have been far worse than death.    We recall from some of our previous
mythological encounters that shedding kindred blood was deemed the worst of
all transgressions.    Had Acrisius killed them, he might have been
besieged by furies whose unrelenting persecution would have driven him
mad.  Or, he might have been condemned to suffer ineluctable torments in
Tartarus for all eternity.   By casting Danae and her child adrift,
Acrisius was increasing the probability of their deaths while not actually
causing them:  a subtle difference that might not sway a jury in our modern
age, but at least protected Acrisius from divine punishment.
Naturally, when Zeus saw this chest bobbing on the Aegean waves, he
directed it to the island of Seriphus,    Dictyus, the estranged brother
of  Polydectes, king of Seriphus, found the chest against the shore and
opened it.    He was shocked to have discovered a beautiful young woman and
her infant had been locked inside it.  Although Dictys lived the life of an
impoverished fisherman, he took the woman and child to his house and agreed
to raise the child as if he were his own.    That child, as it turns out,
was none other than Perseus, the greatest of the early age heroes.    It
was this Perseus who would slay Medusa and then would rescue Andromeda from
the sea monster Cetus (a matter for another day.)   Later on in life, when
his exploits had earned him wide acclaim, Perseus traveled to the
Thessalian kingdom of Larissa to participate in the funeral games held in
honor of their king who had just died.  The then famous Perseus was invited
to attend as a guest of honor and, if he wished to, to also take part in
some of the games.      Perseus opted only to compete in the discus
throwing competition.    Determined to exhibit his strength, which remained
formidable despite his advancing age, Perseus threw the discus with so much
force that it sailed into the audience and struck one of the attendees on
the forehead, causing him to fall backward onto the ground.   Perseus
rushed forward to the attendees's aid and found that he had accidentally
struck his own grandfather, King Acrisius, who had also been attending the
games to honor the dead Larissian king.     To his shame and horror,
Perseus quickly realized that Acrisius lay dead before him. The prophecy
was fulfilled, as we all knew it would be.

THE SOUTHWORTH PLANETARIUM
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Julian Date: 2458984.16
2019-2020:  CXLVII

THE DAILY ASTRONOMER
Thursday, May 14, 2020
Remote Planetarium 34:  The Solar Exterior

Let's start by admiring the Sun: the enormous plasma sphere generating
prodigious amounts of energy that constantly radiate through space in all
directions.    We on Earth receive about one billionth of this radiant
energy, a trifling amount by solar standards, but enough to sustain our
world.

[image: 462977main_sun_layers_full.jpg]

Yesterday we plumbed the Sun's unfathomable depths and explored its inner
regions, from its nuclear furnace core to the comparatively cool gaseous
layer rendered perpetually turbulent by convective cells.     We also
learned that the constant photon stream issuing from the core requires
approximately 300,000 years to migrate to the outer layers.  Each and every
day we are literally bathed in ancient starlight.

Today, we begin our tour of the sun's outer regions. Whereas yesterday we
proceeded from the deepest depths outward, today, we'll examine the various
regions alphabetically.


*CHROMOSPHERE:*
[image: HI6563_fulldisk.jpg]
The Sun as viewed through a hydrogen-alpha filter.

The Sun's atmosphere consists of three main layers: the *photosphere*,
*chromosphere* and *corona*.  Extending between 3000 - 5000 kilometers
deep, the chromosphere is just above the photosphere.    Because the
chromosphere's density is 1/10,000 that of the lower photosphere, it is
generally invisible except during solar eclipses when it appears reddish.
  The chromosphere is a highly complex atmospheric layer responsible for
such phenomena as *spicules, *plasma jets about 300 - 350 kilometers in
diameter and *prominences,* large loops of heated gases that can extend
thousands of kilometers above the "surface." The lengths of some of the
most powerful prominences have exceeded 500,000 kilometers.

[image: 800px-Halpha_+700_limb_spicules_08Aug2007_SST.png]
*Spicules*

[image: faq1.jpg]
* Prominences *with an image of Earth juxtaposed for size comparison
purposes

Perhaps the most curious feature of the chromosphere is the temperature
gradient. The chromosphere's temperature actually increases with increasing
distance from the solar interior, the exact opposite of the situation in
the photosphere which is hottest at its lowest level.     While the
mechanism responsible for this temperature inversion remains unknown, it is
possibly the result of magnetic reconnection, the phenomena by which
magnetic fields connect and then disconnect causing the release of copious
energy.     Magnetic reconnection might also explain the dynamism we
observe within this layer, such as prominences and spicules.

*CORONA:*
[image: corona1.en.jpg]
One can often observe the corona during a total solar eclipse

Deriving its name from the Greek word for "wreath," the corona is the Sun's
outermost layer.   This rarefied gaseous region extends for millions of
miles beyond the photosphere.    Its light, which is faint owing to the low
coronal density, originates from three sources: sunlight reflected off dust
particles, sunlight scattering off free electrons and emissions from ions
located within the surrounding plasma.  With a temperature between 1 - 3
million Kelvin, the corona is also the hottest region along the Sun's outer
layers.  Spanish astronomer Jose Joaquin de Ferrer (1763-1818) coined the
term corona after having observed it during a total solar eclipse, the only
time it becomes visible unless one observes the Sun through a coronagraph.
Even though it is so hot, the corona's density is one billionth that of the
photosphere and so emits faint light.

[image: 800px-Coronal_Hole_Magnetic_Field_Lines.svg.png]

Within the corona one will find "coronal holes," cool, less dense regions
within the corona.  These holes tend to migrate toward and then away from
the poles during the 11-year solar cycle.  As the cycle progresses toward
the solar maximum, the holes move toward the poles and then as the cycle
approaches a minimum, they migrate away from them.    We can see in the
graphic above that when the magnetic field lines do not loop back to
ensnare the energy, solar wind escapes into space.

A corona based event that can affect Earth is a *coronal mass ejection*,
the expulsion of great quantities of charged gases from the corona into
outer space.  These CME's have been responsible for significant auroral
activity on Earth and other planets surrounded by magnetic fields.

*PHOTOSPHERE*
[image: download.jpg]
*Photosphere:  *the Sun's radiative layer, often mistakenly called the
Sun's "surface."

We often mistakenly refer to this layer as the Sun's "surface," because
this is the region from which light and heat are radiated into space.
 The photosphere's effective temperature is approximately 5,500 degrees C,
even though the temperature can range between 4,230 - 5,730 degrees C.
 The photosphere is about 100 kilometers thick and consists of short-lived
"cells" called *granules.   *

[image: 3-newinsightsi.jpg]
*Granules: *looking very much like droplets of molten gold

With a diameter rarely exceeding 1000 miles, granules are miniscule
relative to the Sun.   Convection currents produce them, causing them to
form and re-form quickly.  Granules generally last about 10 - 20 minutes
before dissipating, only to be replaced by others.

The best known phenomena along the photosphere are *sunspots*: cooler, and
therefore darker regions where magnetic fields have drawn forth the gases
and in the process cooling them.         A sunspot consists of a dark inner
area called an *umbra*, and the lighter surrounding area known as the*
penumbra, *not to be confused with the umbra and penumbra associated with
solar and lunar eclipses.    Sunspots tend to persist for weeks or
sometimes months, depending on their size.     Astronomers recognize an
11-year sunspot cycle when the number of sunspots increases and decreases
in response to changes in the Sun's magnetic field.         Currently, this
cycle has been disrupted as we seem to be moving into a protracted solar
minimum.   The year 2020 has so far had 102 spotless days.  There were 281
spotless days in 2019 and 221 in 2018.

[image: ctZ4jzL3eaTdiXNrC7XtHM-320-80.jpg]
*Sunspot:* If seen in isolation, the sunspot would appear reddish and
brighter than the full moon.  Only when we see the sunspot relative to the
hotter surroundings does it appear dark to us.

This brief tour of the Sun's outer layers illustrates the complexity and
dynamism of that brilliant orb that illuminates the day time sky and
sustains life on Earth.      Even though the Sun is the closest star,
astronomers still have much to learn about it. Hence, the deployment of the
Parker Solar Probe that was launched in 2018 and has already four
perihelion passages around the Sun.  The next one occurs next month.


Next week we finally begin our exploration of the other stars.
Quiz tomorrow.

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