Gaia:  Earth mother

Impermanence is the one trait that truly distinguishes gods and mortals.  Even though the immortals are just as apt to occasionally behave as irrationally and miserably as the rest of us, they have all of eternity in which to exhibit these disagreeable behaviours.    Even Apollo, himself, once wondered why the gods even bothered to become involved with mortals, whose lives were so comparatively brief as to be akin to leaves falling along mountain slopes.    The gods are as eternal as the aspects of the mythological Universe they personify:   the broad, boundless sky; the fierce and powerful sea; and -with apologies to the geologists- the immutable and everlasting Earth.    Gaia is the Earth mother, one of the primordial deities, those whose emergence predated the advent of the Titans.        Before the Universe took form, only Chaos existed: a formless, unbounded void.     From this Chaos arose the first creatures:    Nyx (the night), Erebus (darkness), Tartarus (the underworld), Eos (procreational love) and Gaia (Earth). From Gaia came forth Pontus (the sea), Oros (mountains) and Ouranos (the sky).     Ouranos mated with his mother to sire Oceanus and Tethys (the great seas coiling around the world) , the three original Cyclopes, the Hecatoncheires, as well as the Titans and Titanesses, including Cronos and Rhea, the parents of the gods.      Ouranos imprisoned the Cyclopes and Hecatoncheires inside Gaia, for he feared they would usurp his position as the dominant deity.  He pushed them into Gaia's womb and they lay on her to keep them trapped.   Gaia conspired with her son Cronos to liberate her imprisoned children.   Cronos castrated Ouranos with a sickle.  He rapidly ascended high above Gaia to form the sky.   Eventually, Cronos and Rhea sired the gods who rebelled against them after Cronos swallowed all of them, save Zeus, for he, too, feared overthrow.    A decade-long war, the Titanomachy, ensued.  The gods were victorious and Zeus consigned most of the Titans to the underworld forever.    Gaia, distressed at the fate of her children, coupled with Tartarus to sire Typhon, the fiercest creature ever let loose onto the world.  It bore a hundred arms and a hundred heads.  Each head was monstrously disfigured and issued the most horrific sounds imaginable: from shrieks of despair to thunderous shouts and deafening screams of rage.  Gaia hoped that Typhon would avenge the Titan's defeat and release them from their prison.  Instead, Typhon was bestial and set about destroying the Universe indiscriminately.    Suddenly, Gaia, herself, was in the gravest danger.    After a protracted battle, Zeus vanquished the fearsome Typhon with a barrage of thunderbolts.    Typhon joined the Titans in Tartarus and the cosmos was spared.  Gaia was reconciled with her grandson Zeus and made no other attempt to save the Titans.      She figured little in subsequent mythological tales apart from having borne many other children.    Gaia remains permanently affixed to the mythological Universe:  the inexhaustible source from which all life emerges.   While some Olympians govern the seas, fields and forests along her surface, none will ever presume to claim dominion over her.    

THE SOUTHWORTH PLANETARIUM

207-780-4249   www.usm.maine.edu/planet

70 Falmouth Street   Portland, Maine 04103

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Founded January 1970

Julian Date: 2459115.16

2020-2021:  XVI

THE DAILY ASTRONOMER

Wednesday, September 23, 2020

Remote Planetarium 94:   "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, 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.   To understand how Cepheid variables were useful in this capacity, we begin with a famous relation in astronomy:  "The Distance Modulus," which relates a star's apparent magnitude (apparent brightness), absolute magnitude (actual brightness), and distance.  (We covered this concept earlier this year.)If one knows two of these values, one can determine the other.    This modulus is quite intuitive.  We measure a star's apparent magnitude directly just by observing it on Earth.    This brightness value does not yield information about its actual brightness, however.  For all we know, a given bright star could be comparatively faint, but close; conversely, a faint star could be quite intrinsically brilliant, but far away.    If we can figure out the star's distance, we would know its true brightness.  Or, if we can somehow ascertain a star's absolute magnitude, we can compare it to its apparent magnitude and measure its distance.

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"The Distance Modulus"

m - M = 5log(d) - 5

The equation shown above is one of astronomy's most famous mathematical formulae.  It relates three variables: M, a star's absolute magnitude; m, a star's apparent magnitude, and d, a star's distance in parsecs.    (Yes, of course we'll explain what we just wrote.)

"Magnitude" measures a celestial object's brightness. The small "m" refers to the object's apparent magnitude, or how bright it appears.   The large "M" refers to the object's absolute magnitude, or its intrinsic brightness.  Technically,  the absolute magnitude equals the celestial object's apparent magnitude from a distance of ten parsecs.   (A "parsec" is equal to about 3.26 light years.)

The magnitude system is a logarithmic scale that assigns lower numbers to brighter objects.  For instance, a star of magnitude 1 is approximately 2.5 times brighter than a star of magnitude 2, but 2.5 times dimmer than a star of magnitude 0.  

If we know two of these variables, we can determine the third.      As is the case with the Andromeda Galaxy, an astronomer will know the Cepheid variable's apparent magnitude (m) through direct observation.    Observation of its variability period yields its luminosity or absolute magnitude (M). Then, the astronomer can utilize her prodigious mathematical skills to calculate the distance.    

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The trick is knowing a star's intrinsic brightness.   And, with a certain type of variable star, Cepheid variables, we can directly know its brightness by observing its variability period.   Cepheid variables are giant stars that pulsate, growing larger and then smaller and then larger again over a period that can last many weeks.  Conveniently, the variability period, between successive minima (least brightness) or maxima (greatest brightness), depends on its intrinsic brightness.  The brighter the Cepheid variable, the longer the variability period.   As brightness relates directly to a star's luminosity, or energy output, this correlation between the variability period and brightness is called "The Period-Luminosity Relation."

This relationship was first established by a survey of Cepheid variable in the Small Magellanic Cloud,  a satellite galaxy to the Milky Way approximately 160,000 light years away.  It was noticed that the brighter Cepheid variables in the SMC had longer periods than the dimmer ones.     As astronomers assumed that the stars within the SMC were at equal distances from Earth, they determined that the Cepheidvariable's luminosity affected the star's variability period.**

Edwin Hubble measured the distances to Cepheid variables within the Andromeda Galaxy and, by extension, to the Andromeda Galaxy itself   He knew the absolute magnitude by the amount of time they needed to cycle through one variability period.  He then compared it with the star's apparent magnitude, discernible through his powerful telescope. He determined that the Andromeda Galaxy was about 900,000 light years from Earth.  Though this measurement was enough to place the Andromeda Nebula, as it was then called, well outside the Milky Way Galaxy's boundaries, it was less than half the currently accepted value.   Astronomers later learned that there are two classes of Cepheid variables, each of which has its own period-luminosity relation.   Hubble initially used 'cluster variables,' which yielded an incorrect distance to the Andromeda Galaxy.

Cepheid Variable Curve.   A simplified light curve of a Cepheid variable star. As its outer layers contract and expand, the star dims and then brightens.   Astronomers know that the variability period is directly related to the star's luminosity, or  intrinsic brightness. (A variability period is the time separating successive minima)  The longer the period, the brighter the Cepheid.   Therefore, an astronomer can determine a Cepheid variable's true brightness by observing its variability period.  

When the two classes were defined, and the measurement to the Andromeda Galaxy was refined accordingly, astronomers realized that that splotch of light in the sky we had called the Andromeda Nebula was 2.2 million light years away!   The most distant object one can observe with the unaided eye.

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 4-6 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 Andromeda Galaxy, located in the constellation Andromeda, is well positioned for viewing this time of year.    Find it high in the  evening sky, just north of the Andromeda constellation; or to the northeast of the Great Square of Pegasus.   Though approximately four degrees across, the Andromeda Galaxy is somewhat diffuse and therefore observable only in darker regions.   Note that the gibbous moon will often obscure it, even when it is in a different part of the sky.   So, one might want to wait until the weekend to start trying to find the Andromeda Galaxy.   

**We know the stars in the Small Magellanic Cloud are not equidistant from Earth.  However, we can assume they are for the purposes of this observation.   People in Los Angeles are not all precisely the same distance from us, but Los Angeles is far enough away that we can make a safe assumption that they are.