THE SOUTHWORTH PLANETARIUM
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Altitude:  10 feet below sea level
Founded January 1970
Julian Date: 2458796.16
2019-2020:  LII
            "I truly have no idea about reality, to be honest. How
fantastic!"


THE DAILY ASTRONOMER
Thursday, November 14, 2019
A Local Group

[image: Local_Group_and_nearest_galaxies.jpg]

How remarkable is it that merely a century ago, astronomers were fiercely
debating the notion that parts of the Universe exist outside our own Milky
Way Galaxy.*    Today, we know that not only is our Milky Way not alone, it
is merely one of billions, if not trillions, of galaxies in the cosmos.
    Astronomers know that these galaxies are not scattered indiscriminately
throughout the Universe, but are instead arranged in clusters and then in
staggeringly huge collections called "Superclusters."     We are not
inclined to ascend to that level today.  Instead, we want to remain
comparatively close to home in a vast galactic region called "The Local
Group."

The Local Group, a term coined by Edwin Hubble in his 1936 publication "The
Realm of the Nebulae" is approximately 10 million light years across,
contains material with a mass exceeding two trillion solar masses, and is
populated by about 54 known galaxies.    Most of these members are dwarf
satellites of the Local Group's two largest members, the Andromeda and
Milky Way Galaxies.    One can think of the Local Group as being
dumbbell-shaped, with the Milky Way and Andromeda at opposite ends.  Dwarf
galaxies cluster around both.       One cannot think of this group as being
stagnant, however, for even at this level one finds the same restless
dynamism one observes throughout the Universe.     Despite the cosmological
expansion that drives most distant galaxies apart, gravity remains the
predominant force impelling most of the constituent galaxies toward each
other.   The Milky Way and Andromeda Galaxies, for instance, are drawing
together at 300,000 miles per hour and will collide within 4 - 6 billion
years.

Much remains unknown about the Local Group.  Principal among the mysteries
is the actual galactic population.  Astronomers know of 54 galaxies, but
suspect that other remain undiscovered, hidden by the Milky Way Galaxy,
itself.   As our planet is tucked away inside one of the galaxy's
spiral arms, a region directly beyond the galactic disk are largely
unobservable due to the material obscuration.    Astronomers refer to this
region as the "Zone of Avoidance."

We do know that the Local Group, though unfathomably immense by our
reckoning, is reduced almost to negligibility at the level of Superclusters
which number around 10 million in the observable Universe.    We also know
that the night sky above reveals a wafer thin layer of this Local Group.
  And, of course, when we look at the cluster of galaxies within the Local
Group that extend far beyond our fathoming, we know that we're looking at
our home.


*In April 1920, we'll be celebrating the 100th anniversary of the "Great
Debate" between Harlow Shapley and Heber Curtis about the existence of
extra galactic objects.    Shapley argued that the Milky Way comprised the
entire Universe, whereas Curtis insisted that the cosmos was populated by
myriad galaxies.     We know now who was correct.



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