[image:
Rhéa_présentant_une_pierre_emmaillotée_à_Cronos_dessin_du_bas-relief_d'un_autel_romain.jpg]
*Rhea:  *Mother of the Gods
Ouranos was the primordial deity personifying the sky.   Gaia was the
female personification of Earth.   Both arose from Chaos or the "timeless
nothing." Gaia emerged first so as to serve as the abode of the future
deities who would preside over the Universe.   From Chaos Gaia drew forth
Ouranos, the sky, so it could encompass her along all sides.  Earth and sky
thus coupled to sire the three Cyclopes (Brontes, Steropes, and Arges), the
Hecatoncheires (the hundred-handed ones) and the Titans:   Oceanus (the
sea), Coeus (the axis around which the cosmos rotates), Crius (the spring
Ram), Helios Hyperion (the first Sun), Iapetus (the piercing point), Theia
(the divine), Mnemosyne (memory goddess), Tethys (river god mother), Themis
(the good counselor), Phoebe (the moon), Cronus (time), and Rhea (the
ground).      Fierce of spirit all, these children evoked pride in Gaia as
much as they inspired fear in Ouranos.  He was most terrified of the
Cyclopes and the Hecatoncheires who, even in infancy, exhibited immense
physical strength. Ouranos lured the Cyclopes and Hecatoncheires back into
Gaia before collapsing onto the Earth, trapping them in her womb.     This
action induced excruciating pain in Gaia and she pleaded for Ouranos to
withdraw.  Despite her agonizing wails, he refused to do so.    Gaia then
conspired with Cronus, the youngest Titan, to liberate his imprisoned
siblings.    Gaia presented Cronus with a sickle of adamantine with which
he castrated his father.  Ouranos rapidly recoiled off Gaia to such a
height that he remained attached to her only along a thin seam encircling
Earth.   Cronus discarded Ouranos' manhood into Thalassa, the sea.  (More
on that important matter at another time.)   With Ouranoa vanquished,
Cronos became the dominant deity despite his youth. He soon thereafter
married Rhea.    Cronus and Rhea sired the six gods/goddesses:   Demeter,
Hestia, Hera, Hades, Poseidon and Zeus.      As Cronus overthrew his
father, so, too, did he fear the usurpation by his own children.  He
commanded Rhea to give him each child immediately after birth so he could
swallow him/her.    While Rhea dutifully presented the first five infants
to Cronus, she was determined to save the youngest, Zeus.     Rhea secretly
gave birth to the infant within the Diktean Cave on Crete.   She then
concealed him within the cave and enshrouded a rock in swaddling clothes.
 Rhea presented this disguised rock to Cronus which he promptly swallowed.
  Meanwhile, Zeus grew rapidly to maturity under the protection of the
Dactyls and the care of Amalthea, the goat maiden.     As Gaia had allied
herself with Cronus to depose Ouranos, so, too, did Rhea unite with Zeus to
fight against Cronus.   Zeus tricked Cronus into swallowing an emetic
-provided by Rhea- that caused him to disgorge his five siblings and the
rock used as a substitute for Zeus, called there after the "Omphalos
Stone."   Zeus and the other Olympian gods then fought a decade long war
against the Titans, called the "Titanomachy." The Olympian gods prevailed
and established their dominance over the Universe.  While Cronus and some
other Titans were condemned to Tartarus, Rhea was spared by her children.
 Rhea rarely made an appearance after the Titanomachy.   She did arrange
the compromise by which Persephone, who had been abducted by Hades, was
permitted to return to Earth for part of each year.   Although Persephone
was generally believed to have been Demeter's daughter by Zeus, one version
claims that Rhea was Persephone's mother by him, instead.     Soon after he
finished his war against the Titans, Zeus was already exhibiting the
licentious behaviours with which he would come to be associated.
 Suspecting that her youngest son would prove to be a particularly poor
husband -which he was- she forbade him to marry anyone.   Infuriated, Zeus
threatened to violate his mother if she didn't relent and permit him to
marry.  Rhea, understandably disgusted by her son, transformed herself into
a snake to prevent Zeus from committing such an abominable act.
 Undeterred, Zeus changed himself into a snake and followed through with
his threat.     Rhea then gave birth to Persephone and permitted Zeus to
marry his sister, Hera.  While this story does explain Rhea's intervention
on Persephone's behalf, it doesn't explain Demeter's distress at
Persephone's abduction.    We remember that Persephone was only delivered
back to her mother because Zeus feared that Demeter's despondency would
chill the entire world permanently and the populace would starve.    Oh,
well.   Rhea's tales comprise just a few twisted strands in the fist-thick
mythological cable.    Were we to slice that cable open, as Alexander did
with the Gordian Knot, we'd realize that not even the most jumbled pile of
tangled wires tucked away in a neglected corner of an underground
planetarium could compare with the overlaps, triple twists and interweaves
of the mythological story lines.  For this reason, each day (Mon-Thurs) we
loiter up here in the sub empyreal realm, we can expect to encounter
ghastly beasts, intrepid heroes, aged sorcerers and fiery chariots.
However, we should never expect to find coherency.

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: 2458996.16
2019-2020:  CLVI


THE DAILY ASTRONOMER
Thursday, May 28, 2020
Remote Planetarium 43:  H-R Diagram Part I:  The Main Sequence

__________________________________
*CORRECTION!*
In Monday's article/lesson I wrote the following sentence:
If Orion is due south at 8 p.m. on February 2nd, it will be due south at 6
p.m. on March 2nd, 4 p.m. on April 2nd, noon time on May 2nd, and so
forth .  Well, that was a mistake!  That should read    6 p.m. on March
2nd, 4 p.m. on April 2nd and* 2:00 p.m*. on May 2nd.
My thanks to subscriber TT for bringing this error to my attention.
My apologies for the mistake.
____________________________________

Today we introduce the "Astronomer's Stone," technically known as the H-R
Diagram.    Named for its two inventors,  American astronomer Henry Norris
Russell (1877-1957) and Danish astronomer Enjar Hertzsprung (1873-1967),
this diagram plots stars according to their spectral types and
luminosities.       Starting today we will learn how astronomers have used
this diagram to ascertain so many stellar properties.        We offer the
first glimpse of this powerful tool below:

[image: Image31_med.gif]
The lower horizontal axis lists the spectral types we discussed yesterday.
  OBAFGKM.   On this scale, O is the hottest and M is the coolest.    We
can see the associated temperature range at the upper horizontal axis.
Along the left vertical axis is the absolute magnitude.  Absolute magnitude
equals the star's magnitude at a distance of 10 parsecs.  Absolute
magnitude is the measure of a star's intrinsic brightness.     At the right
hand axis we see the luminosity in terms of the Sun.   A star with a
luminosity value of 1 is as luminous as the Sun.  A star less luminous
would appear lower on the graph and one more luminous would appear higher.

The luminosity equation equates a star's energy output with its size and
temperature.       The larger and hotter the star, the more luminous it
will be. When we plot the stars according to luminosity and spectral type
(temperature), we should expect to see a direct correlation.   Indeed, we
do observe this correlation for the majority of stars, those that appear
along a band called the *main sequence.*

[image: HR2.jpg]
Even though each star within the main sequence is considered a "dwarf"
star, their sizes and masses vary considerably.  As we will discover next
week, a star's luminosity is related to its mass.   (In fact, mass
determines a star's entire life cycle.)      Main sequence stars are those
within a band that begins at the lower right and continues to the upper
left.   At the lower right one finds red dwarf stars, such as Ross 248.
 These are the low mass, low temperature, low luminosity stars. They are
also the most common, representing approximately 76 percent of all main
sequence stars.   Red dwarfs are those stars that just became hot enough to
ignite and sustain the core thermonuclear fusion reactions that power
stars.     As one proceeds toward the left of the main sequence, it ascends
up the diagram as we should expect.   When we reach the G-section we find
the Sun, classified as a G2 V star.  (The V is a luminosity class
distinction meaning dwarf.) Continuing on to the left, we climb higher
until reaching the pinnacle of the main sequence at the ultra hot,
blue-white stars that are about 100 times more massive and a million times
more luminous than the Sun.
The image below shows us the relative sizes and predominant colors of the
various main sequence stars.

[image: Morgan-Keenan_spectral_classification.svg.png]
At the left the M type red dwarfs; at the right the 0 type blue "dwarfs."


*Main Sequence Stars:*

   - Every star begins its life cycle on the main sequence.  As a star
   evolves, it will move away from the main sequence to other regions within
   the H-R diagram.
   - Every main sequence star is fusing hydrogen into helium
   - The more massive and hotter the main sequence star the more luminous
   it will be.
   - The Sun serves as a demarcation within the main sequence.  The stars
   that are more luminous are part of the *upper main sequence*; those less
   luminous comprise the *lower main sequence*
   - The more massive and therefore luminous main sequence stars have a
   shorter life cycle than those that are less massive and luminous.    The
   life spans of the high mass/high luminosity stars range between 8 - 12
   million years while the red dwarf lifetimes can exceed one trillion years.
   - Every main sequence star is a "dwarf."
   - The majority of main sequence stars are M type dwarfs (76%).  As we
   move left along the spectral classes, the proportions decrease.   K stars
   (12.1%),  G (7.6%), F (3%), A (0.6%), B (0.12%), O (0.00003%)    '


The H R Diagram will enable us to follow any star's evolutionary cycle.
 As we continue studying the HR Diagram next week, we will learn how we can
use it to ascertain a star's mass, radius and life span.            The HR
Diagram truly is the stellar astronomer's most powerful tool.    The main
sequence is just the beginning.

Week 9 Quiz tomorrow.


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