[image: 220px-Lotto_Capoferri_Magnum_Chaos.jpg]
*CHAOS: Void*
What can one say about nothing at all?     Today's journey through the
upper realm has brought us to chaos: the formless, unbounded, indefinable
nothing from which the mythological Universe emerged. While for us "chaos"
denotes a maelstrom of frenetic activity, discordance of sounds, and/or a
complete loss of order, the mythological chaos was dark and silent.
 Hesiod and the other ancient poet-mythographers were at a loss to
understand what event transpired to bring the cosmos into being out of this
chaos.   As this eruption of physical reality predated the creation of the
Titans and gods, no deity was present to midwife the Universe's  birth.
   It just happened for reasons beyond all mortal and even divine
fathoming.   From it arose Tartarus, the abyss; Gaia, the Earth mother, and
Ouranos, the sky father; Nyx (night); and Erebus (darkness).      Actuated
by a bestial lust, Tartarus lay upon Gaia, but its weight was such that it
sank to the lowest depths and there forever remained.  It now serves as the
remotest abyss to which only the wickedest souls are consigned.    Gaia and
Ouranos then coupled to sire the Cyclopes,the Hecatoncheires and the
Titans, including Cronos and Rhea, parents of the gods.  Fearing that his
children would overthrow him, Ouranos trapped them inside Gaia until Cronos
castrated him with a sickle, causing him to recoil high above Gaia while
retaining a connection only around her perimeter.   When the gods defeated
the Titans in the 10-year Titanomachy, the three principal gods Zeus,
Poseidon and Hades drew lots to select their respective domains.   Hades
became ruler of the Underworld, a region including Tartarus.  Poseidon
governed the seas. As Zeus' domain was the Sky and Earth, he became the
greatest of the gods.     Long before this differentiation occurred, there
was only chaos:  the dark, unbounded deeps from which the mythological
Universe took form.

THE SOUTHWORTH PLANETARIUM
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Founded January 1970
Julian Date: 2459040.16
2019-2020:  CLXXIV


THE DAILY ASTRONOMER
Thursday, July 9, 2020
Remote Planetarium 62:  Interstellar Extinction/Reddening

So often in astronomy the theory is simple, but the practice proves
difficult.   During our discussion about stars so far we have often
mentioned *apparent magnitude, *the star's apparent brightness.    This
concept appears at first blush to be beautifully straight forward:  look,
measure and then write down the number.    This method assumes that the
star's light is passing through a perfect vacuum.      Unfortunately, it
isn't.    The galaxy contains substantial amounts of gas and dust, most of
which is naturally concentrated in the galactic plane.         While the
average density of interstellar material is only about one atom per cubic
centimeter, the cumulative effect of all the dust grains over the vast
stretches of space is sufficient to cause both *interstellar extinction *(the
dimming of incoming light) and *interstellar reddening* (the apparent
reddening of incoming light cause by the scattering of blue light).

Below we see an image of an interstellar dust grain. The white line at the
lower right equals one micron (one millionth of a meter).   The core of
such grains generally consists of silicates, iron or carbon while ices of
carbon dioxide, water, methane and ammonia comprise the outer "mantle." The
surface will often be coated with simple organic compounds.

[image: grain_b.gif]

As the diameters of these dust grains tend to be comparable to the
wavelength of blue light, they will tend to either absorb the blue light
entirely to scatter it away from the light's pathway.

_____________________________________________________
*Reminder:  the visual section of the electromagnetic spectrum.*
The visible band represents a minuscule segment of the EM spectrum, which
extends from the long wavelength radio waves to the very short wavelength
gamma rays.    As we can see  below, the visible band range extends from
the 400 nm (nanometer = one billionth of a meter) to 700 nm.   Blue light
waves are of higher energy than red waves and so their wavelength is
shorter.
[image: wavelength-nm.jpg]
____________________________________________________________

As a consequence of this scattering and absorption, the incoming light from
a celestial object will be reduced or reddened.     The extent of this
extinction is distance dependent.  Within the galactic plane, the
brightness reduction equals about 1.8 magnitudes per kiloparsec.  (A
kiloparsec equals 1000 parsecs, or about 3,260 light years.)

[image: dustefct.gif]


The longer wavelength light, toward the reddish end of the spectrum,
doesn't interact with these dust grains as directly and so will tend not to
be affected by them unless they encounter a local region of unusually high
density.      Astronomers who observe astronomical objects in the optical
region must take these effects into account when measuring the brightness
of stars and other celestial objects.

The other complication is that dust concentrations vary along the various
directions within the galaxy as well as above and below the galactic plane,
where the extinction is at maximum.    Fortunately, through infrared
observations, astronomers have developed a "dust map" of the Milky Way
Galaxy.   Infrared observations also enable astronomers to see regions
beyond dust clouds.   Below, for instance, we see six different images of
the dark nebula Barnard 68. The top three images were captured in visible
light, the lower three in infrared.  We can notice immediately that the
infrared images permit us to see objects behind this dark nebula.

[image: extinction2-400x270.jpg]

As we'll discover, dust is just one of many complications we'll encounter
as we proceed.   Tomorrow, the next quiz.


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