[image: Hundred-Handed-Ones-380x240.jpg]
*The Hecatoncheires:   Hundred-handed*
Of all the monstrous entities that ever arose out of the mythological
Universe, the three brothers known as the Hecatoncheires were arguably the
most fearsome in nature and ghastly in appearance.    Each brother sported
fifty heads and one hundred arms, hence the name "Hecatoncheires." ("The
Hundred Handed Ones").   They were named Aegaeon ("The Sea Goat"), Cottus
("The Furious" or "The Striker") and Gyges ("The Big Limbed.")     Ouranos
(sky) and Gaia (Earth) sired them along with the Titans and Cyclops.
Fearful of being overthrown by his powerful children, Ouranos trapped the
Titans within Gaia and then imprisoned the Cyclops and Hecatoncheires in
Tartarus, the deepest region of the Underworld.     Enraged at her
childrens' imprisonment, Gaia conspired with her son, the Titan Cronos, to
overthrow his father and liberate his siblings.  Although Cronos was
successful in this endeavor and did release the Cyclops and Hecatoncheires,
he, too, feared them and soon locked them back up in Tartarus.  He
recruited the loathsome Campe, a creature covered in snakes and scales, to
guard them.    Cronos then swallowed all the children he sired with his
sister Rhea, except the infant Zeus.   As soon as Rhea delivered Zeus, she
wrapped a rock in swaddling clothes and presented it to Cronos, which he
quickly consumed.   Zeus and Rhea conspired against Cronos just as Cronos
and Gaia fought against Ouranos.  Gaia, also, joined Zeus and Rhea in their
campaign against Cronos.   Gaia's affection for her son waned after he
relegated the Hecatoncheires and Cyclops back to their abyssal confines.
Gaia promised Zeus that both the Cyclops and Hecatoncheires would assist
him in his war against the Titans if he was able to free them from their
prison.  Realizing that he and the other gods were about to fight the most
formidable of foes, Zeus promptly descended to Tarturs, killed the Campe
and set the Cyclops and Hecatoncheires

THE SOUTHWORTH PLANETARIUM
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Founded January 1970
Julian Date: 2459045.16
2019-2020:  CLXXVII

THE DAILY ASTRONOMER
Tuesday, July 14, 2020
Remote Planetarium 64:  Exo-Planets I - The "Wobble" Method

As of today (14 July 2020) astronomers have discovered 4282 planets (3164
planetary systems).   Within the next decade, that number will most
assuredly exceed 10,000.    Based on these detections, we can assume that
our Milky Way Galaxy alone might contain as many as one trillion planets,
millions of which could possibly be life-bearing.    Presently, those
numbers are merely conjecture: optimistic extrapolations derived from these
early findings.     All the same, however, these predictions bode well for
those cheerful sort who believe that our Universe teems with life.

Today we begin a sequence pertaining to the detection of these
exo-planets.   How have astronomers managed to discover such small objects
in orbit around stars so remote they appear merely as pinpoints of light?
  The answer to this question is multi-faceted.    Astronomers have
developed multiple exo-planet detection techniques, some of which are only
theoretical as they haven't yet yielded any exo-planet discoveries.   Our
focus will be on the principal methods astronomers have employed in a
pursuit that, until relatively recently, many scientists dismissed as
futile.

[image: 3-onceuponatim.jpg]
*Astronomers Michel Mayor and Didier Queloz.*

The first technique we'll discuss was also the one first used to detect an
exo-planet around an active star.*  Astronomers call it the "wobble
method."  Astronomers Michel Mayor and Didier Queloz were the first to find
an exo-planet around a star.  In 1995 they announced the discovery of a
massive planet around the star 51 Pegasi, a faint star to the east of the
Great Square of Pegasus. (They shared the 2019 Nobel Prize in Physics for
this discovery.)

[image: unnamed.jpg]
51 Pegasi b, the first exo planet discovered around an active star orbits
the star 51 Pegasi, a star within the constellation Pegasus.    A pink
circle indicates the star in the image above.   51 Pegasi b is about half
as massive as Jupiter and completes an orbit around its parent star every
4.2 days.      This planet is officially named 'Dimidium,' the Latin term
for half because it is half the mass of Jupiter. The initial recommendation
to name it "Bellerophon," after the reckless youth who tamed Pegasus, was
rejected by the IAU (International Astronomical Union) who
characteristically opted for the more prosaic alternative.

As its name implies, the "wobble" method involves detecting wobbles in a
star induced by any attendant planets.    According to Newton's Universal
Law of Gravitation, every massive objects exerts a gravitational pull on
every other massive object.  The magnitude of the force a massive object
exerts on another massive body depends on its mass and separation
distance.      The more massive the object, the stronger the pull.  The
greater the separation distance between two massive objects, the weaker the
pulls becomes.      Even though a star is far more massive than any planet
in orbit around it, the planet will still "tug" on the star.  Consequently,
the barycenter, or center of gravity," will shift away from the star's
core.    The extent of this displacement depends on the mass of the planet
and its distance, as we would expect from our knowledge of Newton's
gravitational laws.    The star will therefore also revolve around this
barycenter, making it appear to "wobble" periodically.  The size of the
wobble relates to the separation between the star's nucleus and the
system's barycenter.

Below we can see a graphic pinpointing our solar system's barycenter
between 1944 and 2020.

[image: main-qimg-c345edfee69ca8fa25eccb1be0d94c01.gif]


We can see that at times the barycenter is located within the solar
interior and at other times is well outside of it.   [The variation relates
to the changing planetary configurations.  For instance, when Jupiter and
the other as gas giant planets are located along one side of the Sun, the
barycenter will be "pulled" well outside the Sun.  When Jupiter is on the
opposite side of the Sun relative to the other gas giant planets, the tugs
will almost cancel out and the barycenter will be nearly at the Sun's core,
such as in 1990.]

The barycenter is currently outside the Sun and has been since 2017.  Any
alien astronomer observing the Sun might actually observe our parent star's
wobble due to the barycentric displacement.   However, they would not "see"
the wobble.    The wobble method relies on observations of a star's
spectrum.

[image: f1big.jpg]
One can think of a spectrum as a rainbow.    If light emitted by a star
were to be separated into its component colors, a sequence such as the one
above would appear.  The spectrum above is from the Sun.     Notice the
series of dark lines.  Those are absorption lines.   They form because
atoms within the Sun's outer layers absorb light at different wavelengths.
  Recall that we saw spectra in early June when we discussed the H-R
Diagram.

The image above shows a spectrum as it appears at rest.    However, if the
Sun were to move away from us or toward us, the lines within the spectrum
would shift.    If the Sun is moving away from us, the light waves would be
elongated,  which would increase the wavelength and decrease the
frequency.      If the Sun is moving toward us, the light waves would be
compressed.  The wavelengths would shorten and the frequency would
increase.

[image: doppler_shift_light.png]
We refer to this compression and elongation of waves as the "Doppler
shift."  You likely notice an audial version of this shift whenever you
hear a siren.  If the police car is approaching you, the sound waves are
compressed and the pitch increases.   When the car is next to you, you hear
the same normal pitch you'd hear were the car at rest.  When the car is
moving away from you, the waves are elongated and the pitch decreases.
 The Doppler effect occurs when the speed of the wave emitting source is
fast enough to affect the speed of the waves, themselves.

[image: radial-velocity-method-e1570809992113.jpg]

In our simplified version, when a planet is between us and distant star,
the star will be pulled toward us.  Consequently, the star's light waves
will be compressed and the star's spectrum will shift toward the blue end
of the spectrum.  Because blue light is more energetic than red light, its
wavelength is smaller and its frequency higher.   For this reason, light
compressed by the approach of an object is referred to as *blue-shifted.*
When a planet is moving along the far side of the star relative to us, the
star is tugged away from us and its light is elongated.    The wavelength
increases and the star's light is said to be *red-shifted* because the
wavelength of red light is longer than that of blue light.

Astronomers have found hundreds of planets using this technique, which is
also called the* radial-velocity method. *    Radial velocity refers to the
component of a celestial object's velocity along our sight line.      The
limitation of this method pertains to the planets' masses.   Only large
planets will induce measurable wobbles in their parent stars.      For
instance, Jupiter's gravitational influence on the Sun exceeds the combined
influence of the other planets.    As massive as Earth might feel to us,
its effect on the Sun's barycenter is negligible.

Fortunately, astronomers have developed other planet detection techniques,
such as the transit method that we will discuss tomorrow.




*In 1992 astronomers Dale Frail and Aleksander Wolszczan announced the
discovery of two planets in orbit around Pulsar  PSR 1257+12** , located
about 2,300 light years from Earth within the constellation Virgo.    These
were the first exo-planets discovered.    These findings are often
forgotten because the planets are in orbit around a stellar remnant, a
rapidly spinning neutron star.  One might wonder: how can a pulsar possibly
have planets?    The leading theory posits that some of the debris cast off
by the supernova explosion that destroyed the highly massive star quickly
coalesced into planets that soon established stable orbits around the
pulsar.    It is now known that this system contains at least three
planets.   Alien-seekers needn't bother scanning this system.  The chances
of life existing on a pulsar planet is about equal to the chances of
finding a lush botanical garden inside a nuclear reactor.


**Just for morbid fun:    This pulsar has been assigned a proper name:
Lich. Aficionados of fantasy literature and Dungeons and Dragons might
recognize the term "Lich," in reference to "undead" creatures such as
desiccated corpses or skeletal creatures that a necromancer reanimated.
 The name is certainly appropriate as a pulsar is merely the dead remnant
of a star as opposed to being  real star, itself.

_______________________________________________
SKYWATCHING TIP:  *Jupiter at opposition*
This is the best time to seek out Jupiter!   Jupiter is at opposition
today, meaning that Earth is passing between the Sun and Jupiter.    The
fifth world will rise at sunset and be  visible all night.     Planets are
opposition are also at their brightest.   At magnitude -2.7, Jupiter
outshines all the night sky stars and planets, save Venus.      One will
find Jupiter rising in the southeastern sky within the constellation
Sagittarius,
________________________________________________

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