70 Falmouth Street Portland, Maine 04103 43.6667° N 70.2667° W Altitude: 10 feet below sea level Founded January 1970 Julian 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.
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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
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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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