Hades: God of the Underworld
The Titanomachy, the decade-long struggle pitting the Titans against the gods, came this close to destroying the Universe. The antagonists were not only equally powerful, but both of them were fiercely determined to prevail over the other. One can well understand this determination for the victors won dominion over the cosmos while the vanquished were condemned to the lowest reaches of Tartarus. The gods ultimately did defeat the Titans and won the right to rule over all. The three Olympian brothers Zeus, Poseidon and Hades then drew lots to determine each one's domain. Zeus became god of the sky and consequently over the other gods, as well. Poseidon's domain was the sea. Hades was left to rule over the Underworld, the gloomy region to which the shades of the dead are consigned. And there Hades remained, residing in an elaborate palace that appeared so dark as to be all but invisible against the stygian landscape. He had very little to do himself apart from presiding over the region. Cerberus, the three headed hound, guarded the Underworld's entrance. Charon ferried the dead across the River Styx. Most of the dead shades merely loitered about. The precious few who in life proved unusually brave or angelic wandered through the lush Elysian fields, while those who were preternaturally wicked suffered the torments of Tartarus. As he preferred to remain largely in his own palace, Hades rarely appeared in any of the mythological tales. Of course, he did venture onto Earth when he kidnapped Persephone and made her his queen. We might remember having encountered Persephone on a much earlier date. Her abduction so distressed her mother, Demeter, goddess of the harvest, that the world grew cold and barren. Fearing that all the mortals might die of starvation, Zeus commanded Hades to relinquish Persephone back to her mother. Before Persephone left, however, Hades offered her four pomegranate seeds. Persephone had been so miserable during her captivity that she refused to eat. As she was ravenously hungry prior to her departure, Persephone accepted the seeds and quickly devoured them. Unfortunately, by consuming those seeds, Persephone was obligated to return to the Underworld each year for four months, one month for each seed. During the four months when Persephone resided with Hades, Demeter grew so morose that the world grew cold again. She was enlivened whenever Persephone returned and the world became warm and lush again. Thus, the mythological explanation for the transition from summer to winter and then back to summer again. Persephone was with Hades when Orpheus ventured into the Underworld to rescue his love Eurydice. Orpheus sang to Hades and pleaded with him to allow Eurydice to return to Earth to live out the remainder of her life. It was said that Orpheus' music was so enchanting that Hades wept tears of iron. He permitted Orpheus to take Eurydice back with him on the condition that she follow him. He was not to look back to see her until they both reached the threshold of his home. Alas, just before arriving home, he turned to be sure she was there for he had heard no footfalls during their journey. He saw her for a fleeting moment before she returned to the Underworld forever. When another mortal, Heracles, descended into the Underworld to capture Cerberus, and thus complete his final labor, Hades gave him permission to take the hound temporarily so he could present him to King Eurystheus. At the same time, Hades allowed Heracles to pull Theseus up from the chair of forgetfulness. Theseus and Pirithous had ventured down into the Underworld to kidnap Persephone. Hades, knowing full well their intentions, invited them into his home and seated them in chairs that caused them to immediately forget who they were. They had remained there in an insensible state ever since. While Hades permitted Heracles to save Theseus, he refused to release Pirithous, as he had been the one who had truly wanted to kidnap Persephone. Theseus agreed to assist him because he had earlier helped Theseus kidnap Helen, then a young girl. Hades has been unjustly described as a malevolent deity who revelled in the misery of those languishing in his domain. Yet, he wasn't as much an evil overlord as he was simply a taciturn one. He was as gloomy in spirit as his surroundings. It would have been exceedingly difficult for anyone to develop and retain a happy-go-lucky disposition among throngs of the dead. Yet, he was a just ruler who gave to each soul his/her proper station. Those entitled to an eternity of bliss received it. Those deserving of ineluctable torments suffered them. And those who merely had to remain in the Underworld were kept there, despite their ardent desires to return to Earth. He didn't delight in denying these souls the opportunity reawaken back to life. He was merely enforcing the rules of the Underworld, the domain that he didn't want, but was chosen for him by lots.
THE SOUTHWORTH PLANETARIUM
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2019-2020: CLXXXIX
THE DAILY ASTRONOMER
Tuesday, August 4, 2020
Remote Planetarium 76: Nebulae Part II of II
PLANETARY NEBULAE
Cat's Eye Nebula
Distance: 3,262 light years
Constellation: Draco the Dragon
The term "planetary nebula" is one of astronomy's greatest misnomers. The astronomer William Herschel (1738-1822) coined the term when he described these nebulae as appearing similar to planets when viewed telescopically. Today we know that these nebulae are not associated with planets, at all. They are the gaseous material cast off by red giants at the conclusion of their life cycles. Earlier in this course we encountered these nebulae when we were discussing stellar life cycles. We know that when a star that is less than nine times as massive as the Sun it turns into a white dwarf remnant surrounded by an envelope of expanding gases. Radiation excites the gases, causing them to glow through the same fluorescent process that illuminates emission nebulae. These nebulae eventually dissipate over thousands of years and through this attenuation gradually fade.
Ring Nebula
Distance: 2,283 light years
Distance: 2,283 light years
Constellation: Lyra the Harp
DARK NEBULAE:
Horsehead Nebula
Distance: 1,500 light years
Constellation: Orion
A dark nebula is, just as its name implies, is a denser region of gas and dust that obscures the light of objects behind it. The above image shows the "Horsehead Nebula," located close to Alnitak, the easternmost star in Orion's Belt. Looking very much like a horse head protruding out of the darkness of space, this nebula is seven light years across. Lights within the base of this nebula originate in stars that have recently formed within the region.
Coalsack Nebula
Distance: 586 light years
Distance: 586 light years
Constellation: Crux (a.k.a. "The Southern Cross.")
Unlike the Horsehead Nebula, which is only visible in a time exposure photograph, the Coalsack Nebula, pictured above, is visible to the unaided eye, provided one travels south of the 30th parallel. The Coal sack appears as dark splotches against the light of the Milky Way. At mid northern latitudes, we can also see such dark regions along the Milky Way band, places where foreground gas and dust obscure light from more distant sources.
SUPERNOVA REMNANT:
Crab Nebula
Distance: 6,523 light years
Distance: 6,523 light years
Constellation: Taurus the Bull
The final nebula type is a supernova remnant, the remains of a star that exploded from the inside out. The image above shows the Crab Nebula, produced by a supernova that was observed in 1054 AD. (As the nebula is 6,523 light years away, the explosion actually occurred 6,523 years earlier.) While much of the emission within a supernova remnant is similar to that of emission nebulae that absorb radiation and then re-radiate it at a lower wavelength, some of it is caused by synchrotron radiation, radiation produced by electrons curving in a magnetic field. The Crab pulsar, the rapidly spinning neutron star within the nebula, generates a magnetic field sufficiently powerful to produce that type of radiation.