THE SOUTHWORTH PLANETARIUM
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70 Falmouth Street  Portland, Maine 04103
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Founded January 1970
             "We love the Universe. It's great!"


THE DAILY ASTRONOMER
Thursday, July 28, 2016
Dark Galaxy  (Parts I and II)

Hello!
So, here's the rub.   The e-mail system I'm using at the planetarium has
gone a bit off the rails.
Presently, we're attempting to fix the problem, which will likely prove a
bit tricky until we can find someone with enough stamina to constantly turn
the crank connected to my medieval computer.   However, all is not lost, as
I am equipped with a more modern machine at home.    So, I am sending the
DA late.    However, it is a repeat.
When the new DA school year begins anew on Sept 1, all will be in proper
working order.
No repeats.
No delays
No excuses
No worries
No frets, frowns or furrowed brows.

And, now that the DA has fallen into your inbox like to much manna from
Heaven, your lives -which had been trapped in suspended animation- can
resume.

Yours hopelessly,
Edward



PART I

  Today we revisit the adage "If it is not observed, it is inferred."
Therein lies astronomy's true miracle: the astronomer's ability to
ascertain an invisible object's existence merely by noting how it interacts
with the visible material in its proxmity.    Astronomers observe the
light* that visible objects either produce (stars) or reflect (moon and
planets.)   This light provides them with copious information about the
body, be it its chemical composition, discernible through  analysis of dark
lines within its spectrum,**  or its motion relative to nearby bodies.  It
is through a celestial body's motions, of course, that an astronomer can
deduce the existence of objects that neither emit nor reflect light.   The
most important example of this inference through indirection observation
involves dark matter, the mysterious and opaque material believed to
comprise nearly twenty five percent of the physical Universe.

We start with the simple notion that our solar system and billions of other
star systems revolve around the galactic nucleus: the galaxy's center.  By
our lethargic terrestrial standards, these stellar motions are quite rapid,
often exceeding 200 miles a second.     Unlike solar system body movements,
which are comparatively simple to model as they result principally from the
Sun's gravitational influence, galactic stellar motions are more complex.
The central galactic region likely harbors a supermassive black hole and is
certainly home to myriad stars, clusters and prodigious amounts of dust.
 This abundant material exerts a gravitational influence on the galaxy's
orbiting stars, though astronomers are still trying to determine the
precise nature of the motions and how interactions with all the galaxy's
matter affects stellar orbital velocities.

What is known, however, is that the visible matter is not solely
responsible for the observed motions.    The galaxy, itself, and the
innumerable galaxies within its vicinity and those farther afield, must all
contain vast quantities of unseen matter.  This conclusion, first reached
in the 1930's by Swiss astronomer Fritz Zwicky, is based on observed
motions of galaxies and of stars within galaxies.    All of which move far
more quickly than they should be moving if the only matter  within the
stars and galaxies are visible.   There is a direct relation between the a
system's mass -and, by extension, the gravitational potential energy- and
the kinetic energy of the system's particles.  It is called "the Virial
theorem," a particularly complex little relation that, at its base, enables
physicists to relate the speed of a system's components with the system's
mass.  By knowing one, one can calculate the other.       The greater the
gravitational potential energy (or matter) the greater the kinetic energy
(velocity.)

Astronomers have measured the speeds of stars within the galaxy***.  These
speeds are quite high and, by the estimations through the Virial theorem,
astronomers have realized that much more matter must be present within the
galaxy to produce such high stellar speeds.   They have concluded that
ninety percent of the galaxy's material must consist of 'dark matter.'
The physics is quite clear on this point.  The problem involves the missing
matter's nature and location.  Where is it is and of what is it composed?

These  matters pertaining to dark matter have not yet been resolved. For
the last few decades - as long as the notion of dark matter became widely
accepted- astronomers have tried to develop the means by which to detect
the unseen material.    For scientists accustomed to observing visible
matter,  such a task as finding dark matter is no mean feat.   These
efforts have yielded few results.  However, this lamentable, but hardly
surprising lack of success, might have finally come to an end, as some
astronomers might have finally found evidence of dark matter.

PART II
 In our last riveting episode, we rapidly reviewed how astronomers
determined that dark matter exists.  "Dark matter," for the benefit of
those subscribers who didn't find the episode quite that riveting, is the
blanket term applied to all material that doesn't emit or reflect light.
Detecting such material proved a considerable challenge for astronomers who
have been accustomed to studying celestial objects through their
electromagnetic emissions (e.g. visible light, radio waves, UV rays).
Scientists realized that such material must exist in abundance through its
gravitational interactions with visible matter.   For instance, the stars
in our galaxy move far more quickly than they would if the galaxy consisted
solely of the stars and other objects we either directly observe, or know
to exist, such as the super massive black hole with the galactic nucleus.
 The stellar motions directly correspond to the galactic material through a
complex, yet handy relation, called the "virial theorem." It tells us that
the kinetic motion of any system's particles increase with increasing
matter.     Astronomers can measure the velocity of stars within the galaxy
and thereby have estimated that approximately ninety percent of the Milky
Way's material must consist of dark matter.    Moreover, cosmologists
estimate that dark matter comprises twenty-five percent of the Universe
entire.

As dark matter represents a significant proportion of the Universe,
locating the unseen material and then identifying it has become a
profoundly important issue in modern astrophysics.  Unfortunately,
discerning the nature of matter that one cannot scrutinize through
traditional means (i.e. studying its light) presents considerable
difficulties.    However, astronomers have surmounted such challenges
before.  (We recall the early 19 century view that humans would never know
a star's chemical composition.)  Now, hundreds of researchers are embroiled
in a determined effort to ascertain precisely what makes up a quarter of
everything in the Universe.

While they have made no definitive discoveries as of yet, a recent
experiment conducted on the International Space Station might yield
promising results.   The device used for this experiment is the Alpha
Magnetic Spectrometer, a highly sophisticated (billions of dollars worth of
sophisticated) series of instruments designed to detect and identify deep
space cosmic rays: those emanating from far intergalactic and
extra-galactic sources.     The collection of these rays could allow
astronomers to indirectly detect dark matter, itself.

To understand the relation between cosmic rays and dark matter, we must
introduce a new concept, that of the WIMP (Weakly interacting massive
particles.)   As their name suggests, they are highly elusive because they
rarely interact with material.   For instance, neutrinos produced by the
Sun's core nuclear reactions, distant supernovae, and even from the
Universe's infancy, are passing through you at this very moment.   Billions
of them every second.  As they don't interact with you, you don't notice
them at all.  And, they move extremely fast.    Try this: Blink.   The
neutrinos that entered your body when you closed your eyes already exited
the opposite point of Earth by the time you opened them.

WIMPS could very represent a large proportion of matter in the galaxy and
Universe.    Though they don't "interact" for the most part, they could
still exert a strong gravitational influence owing to their staggering
abundance.   Think analogously of those harmless little water droplets that
can form a shore-assaulting tidal wave if you get enough of them in one
place.    The problem, which isn't trivial, is their detection.     That is
where the cosmic rays become necessary.

While WIMPS don't interact with much of anything, generally, they can
interact with each other.   Such interactions or collisions, should produce
two types of particles:  electrons, and their positively charged
counterparts, positrons.    These highly energetic 'rays' are far easier to
detect than WIMPS.




 *We use "light" in reference to all electromagnetic radiation, which
includes, but is not limited to, the visible light spectrum.

**We’ll be brief.   Imagine you divide light into its component colors
(Roy, Orange, Yellow,  Green, Blue Indigo, Violet) through a prism or
diffraction grating.      A star emits light at all wavelengths, but
chemicals within the star's outer atmosphere absorbs light at some of these
wavelengths, thereby producing a sequence of dark lines.  Each chemical has
its own 'signature' sequence within the spectrum.  By observing this dark
line pattern, an astronomer can identify chemicals within a star.

***Now, we won't be brief:
How do you measure a star's 'space velocity?'  We first have to explain
that such a velocity consists of two components: radial velocity and
transverse velocity.   Radial velocity refers to a star's velocity along
our line of sight: a radial velocity is positive if the object approaches;
and negative if it recedes.  Transverse velocity refers to motion along
one's line of sight.  Somebody running side to side in front of you
exhibits a high transverse velocity, but a cannon ball fired at your chest
has a zero transverse velocity.
Astronomers can measure a star's radial velocity by measuring how its light
is elongated or compressed as it either moves away from us or approaches.
This elongation/compression is called the "Doppler Effect," a phenomenon we
notice in sirens that have a high pitch when they approach (compression)
and a lower pitch when they move away (elongation.)

Astronomers can also observe a star's 'proper motion' defined as a star's
angular displacement relative to the other stars.   If astronomers know a
star's distance, and also the proper motion, they can determine a star's
transverse velocity.       By combining the radial and transverse
velocities, the astronomers measure the star's velocity through space.