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Date:  2459359.18
2020-2021: CXXXV


THE DAILY ASTRONOMER
Monday, May 24, 2021
The Andromeda Nebula

was the name of the Andromeda Galaxy when its true nature was unknown to
astronomers.     It came by the name "nebula" honestly, for even the galaxy's
discoverer, the 10th century Persian astronomer Abd al-Rahman al-Sufi,
referred to it as a "nebulous splotch."     Throughout the intervening
centuries, it was assumed to be a vaporous cloud lingering within nearby
star streams.      Charles Messier included the object in his catalog and
assigned it the designation  M31.     Even though the Andromeda Galaxy is
readily visible to the unaided eye on a moonless night and even though the
Persian astronomer al-Sufi was the first to record his observations of it
in AD 964, Messier extended credit for its discovery to Simon Marius.
 Simon Marius had mentioned the Andromeda Nebula in a 17th century
publication, one that Messier cited when including it in his catalog.


*The Andromeda Galaxy.       The closest major spiral galaxy to the Milky
Way.  This large galaxy is more than 150,000 light years in diameter and
might harbor more than 500 billion stars.    Once thought to have been a
cloud of vapor among the stars,  it is now known to exist more than 2.2
million light years from the Milky Way.      Image by Adam Evans.*

The Andromeda Nebula's true nature was still unknown even into the early
20th century.  It was Heber Curtis who had observed that a series of novae
in his cloud were considerably fainter than other novae.    From this
observation he concluded that the nebula was not a cloud of gas, but a body
well beyond the bounds of the Milky Way Galaxy.    His claim was disputed
by the acclaimed astronomer Harlow Shapely, who believed that the nebula
was just that: a cloud within our galaxy.   This dispute led to the "Great
Debate" on April 26, 1920 at the Baird Auditorium at the Smithsonian Museum
of Natural History.    Shapley contended that the spiral nebulae seen
around the sky were not, as Curtis described them, "island universes."*
 He based part of  his argument on the work of astronomer Adrian van Maanen
who claimed -erroneously as it turned out- that he could observe the
"Pinwheel Galaxy" rotating. Direct observation of such a rotation would not
be possible if the object were extra galactic.   Soon after the Great
Debate-which was like the current Presidential debates, only toxic and
vicious- van Maanen's observations were found to have been in error.    In
1925 American astronomer Edwin Hubble established that the Andromeda Nebula
was outside our Milky Way by observing Cepheid variables within it.   This
discovery destroyed the notion that the Milky Way Galaxy encompassed the
entirety of the Universe.    We now know that it is just one of billions of
galaxies scattered throughout the cosmos.

Although most galaxies are moving away from our own as a consequence of the
Universal expansion, the Andromeda Galaxy and the Milky Way are moving
toward each other at about 300,000 miles per hour.   At this rate, they
will collide in about 3.8 billion years to form a giant mega galaxy with
perhaps as many as one trillion stars.     There is obviously much more in
that nebulous splotch than humans first realized.


**The term "island universe" was first coined by philosopher Immanuel Kant *
*____________________________________________________________*

*PAST ASTRONOMER:  Simon Marius*


*Simon Marius (1573-1625)   A German astronomer who claimed to have
discovered Jupiter's four largest moons just before Galileo Galilei first
observed them on January 7, 1610.   Marius even insisted that he had
discovered them just a day earlier, a claim that Galileo vigorously
disputed.  In his 1623 book "The Assayer," Galileo portrayed Marius as a
plagiarist, which destroyed Marius' reputation for centuries.    Artist of
this portrait is unknown.  *

Galileo is credited with having discovered Jupiter's four largest moons,
Io, Europa, Ganymede and Callisto.     He was said to have first observed
these four Jovian satellites on January 7, 1610.     Even though he named
them for his patron Cosimo De' Medici, they are now called the Galilean
moons after their discoverer.        However, the history is not quite that
simple.  (When is it ever?)

German astronomer Simon Marius claimed to have discovered these moons just
before Galileo found them in early 1610.   Marius even insisted that his
discovery preceded Galileo by only 1 - 2 days.   Unfortunately for Marius,
Galileo highlighted this discovery  in his famous book  Siderius Nuncius
("The Starry Messenger.") , published on March 13, 1610.   Although Marius
announced his own discoveries in some local publications soon after first
finding these moons, he waited to publish his own book "Mundus Iovialis"
until 1614.     Galileo disputed Marius' claim to be the true discoverer of
these moons in his 1623 book entitled "The Assayer."  In this publication,
Galileo described Marius as a "plagiarist" and succeeded in destroying
Marius' reputation as a result.

In the early 20th century, a Dutch court adjudicated the dispute between
Galileo and Marius and found in favor of the latter.   The court concluded
that Marius did independently discover these moons, but, having waited to
publish his findings, was denied due credit for his discovery.        It
should be noted that even though these four moons are still called the
"Galilean moons," Simon Marius did give them their individual names that we
still use today:  Io, Europa, Ganymede and Callisto.
________________________________________________________________________



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