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Boötes: * A simple, celestial sheepherder's tale*
When entering the mythological Universe, one expects to encounter all
manner of grotesque monsters, prodigiously powerful witches, and a vast
assortment of noble warriors. However, today's excursion into the upper
reaches brings us into the company of a simple sheepherder named Boötes
(pronounced -we think- "Boa-tease.") Yes, even this fantastical realm
teems with non-magical folk who are happily -and sometimes not so happily-
engaged in all manner of quotidian activities. (After all, if you backpack
through Middle Earth, you will encounter many maids and men milking cows,
tending fields and repairing thatched roofs.) The sheepherder Boötes spent
his days, naturally enough, tending his flock of sheep. Like all others
who shared his vocation, Bootes often experienced sheer exhaustion for he
was in constant motion. Unlike many others, however, Boötes also had to
contend with a vicious bear who often attacked the flock at night and
devoured a few sheep. Not only was this bear monstrously large and
ferocious, it was also craftier than bears tended to be. It always knew
when Bootes was not attending to his flock, such as when he ate or slept.
The bear always laid siege to the helpless sheep at those precise times
when the usually attentive Boötes relaxed his vigilance. By the time
Boötes resumed his duties following one of these assaults, he found himself
a few sheep short. In frustration, Boötes constructed a wooden stall
in which to house the sheep. The morning after its construction, he
awoke to find the wall in fragments. He also found that he was down three
sheep. Angry, but undeterred, Boötes and his assistants constructed a
circular rock structure into which he then directed the sheep. That night
he slept soundly, confident in the belief that he finally thwarted that
accursed bear. He felt less confident when he awoke to find a pulverized
wall and four fewer sheep than he had the night before. One can tell by
now that, though well intended, Boötes was no Daedalus. He built no
other walls. Instead, he dug a deep hole. Once completed, he covered it
with a carpet of grass. Boötes then took the cadaver of an old sheep who
had just died and placed it at the hole's edge. The sheepherder embedded
its feet into the turf to make it look as though it was grazing in the same
spot. Then, he took his flock away and waited. All day he saw no sign
of the bear. Before evening, though, he heard what sounded like a scream
issuing from the region where he had left the trap. Boötes excitedly ran
back and found, to his delight, the hole exposed and the decoy sheep out of
sight. He rushed over and looked into the pit to find one of his
bewildered assistants looking back up at him. "I fell into a hole trying
to catch this sheep, but, well, I think it's dead." The furious Bootes
rushed back to his flock and discovered the hated bear chasing after them.
Actuated by a rage he had never before experienced, Boötes pursued the
bear and tried desperately to seize it. The bear moved so swiftly,
however, that he managed only to grab its tail. Boötes pulled the tail
with all his strength and such was the power of his anger, he managed to
lift the animal from the ground. Boötes then swung it around his head
repeatedly and released it. The bear, suddenly helpless itself, careened
into the sky and became the constellation Ursa Major. This story
explains why the bear is circumpolar: it is still describing wide circles
around the heavens just as it did around Boötes head. Also, this tale
explains why Ursa Major, unlike any terrestrial bear, has just a long
tail.
Today, one can see Bootes and Ursa Major together in the sky. While Ursa
Major always remains above the horizon, Bootes does rise and set. Bootes
stands behind the bear, as though it is still swinging it around its head
by a tail: a practice which no wildlife biologist would recommend.
THE SOUTHWORTH PLANETARIUM
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2020-2021: XLII
THE DAILY ASTRONOMER
Tuesday, November 10, 2020
Remote Planetarium 121: Big Bang II
Quite a lot happened in the Universe's first second. After its birth, the
cosmos quickly inflated, the four fundamental forces fractured, quarks,
electrons and neutrinos formed, and it cooled down to a frosty temperature
of one trillion degrees.
Today, we continue the timeline starting one second after the Big Bang. On
Thursday, we'll learn how humans realized that the Universe began in this
single event.
The first second of the Universe ended with the Hadron Epoch, which
literally lasted one millionth of a second. The following epoch, the
Lepton, is by contrast as long as the life-age of Earth: 1 second to three
minutes.
*Lepton Epoch*
1 second - 3 minutes after the Big Bang
The predominant activity during this epoch involves the annihilation of
leptons, such as electrons and anti-leptons such as the anti-particle
equivalent of electrons called positrons. The collisions of electrons and
positrons creates photons that, in turn, recombine to form electrons and
positrons.
*Nucleosynthesis*
3 - 20 minutes after the Big Bang
The Universe's temperature had decreased to a billion degrees. At this
time, protons and neutrons, positive and neutral subatomic particles,
respectively, can combine through the process of fusion to create primarily
hydrogen, helium and lithium, the first three elements. At the end of
the period (20 minutes after the Big Bang), the temperature is too low to
allow for further nucleosynthesis to occur.
*Photon Epoch*
3 minutes - 240,000 years after the Big Bang
The Universe continues to expand, albeit at a slower rate than it did
during the inflationary epoch. Plasma pervades the cosmos. This
plasma consists of the atomic nuclei that arose out of the nucleosynthesis
epoch and electrons. Also, the photons generated by the electron-positron
collisions are dominant. These photons often interact with the nuclei and
electrons. Important to note that the cosmos is still opaque throughout
this epoch.
*Recombination/Decoupling*
240,000 - 300,000 years after the Big Bang
Finally, the Universe cools to a temperature of about 3,000 degrees. At
this point, the atomic nuclei can capture free electrons. These nuclei
become electrically neutral and, with the electrons now bound to atoms, the
photons are free and the Universe becomes transparent. Photons that have
also been bound up with the electrons and protons are released, a process
known as "decoupling."
*Dark Epoch*
300,000 to 150 million years after the Big Bang
As its name suggests, this epoch was particularly dark, despite the
existence of photons. No stars had yet formed. This protracted period
was defined by low energy and dominated by dark matter. The Universe
continued to expand and cool throughout.
*Star and Galaxy Formation Epoch*
300 million years after the Big Bang
During the 150 million year gap between the end of the Dark Epoch and the
beginning of the star formation period, quasars (to be discussed in a
later article) form and their immense power re-ionizes much of the
Universe. Meanwhile, the first stars and galaxies form out of the vast
swaths of hydrogen and helium within the still-expanding Universe. These
bodies formed around small regions of higher density. This first
generation of "metal-poor" stars called Population III formed in clusters
and many of these coalesced into galaxies. Thermonuclear fusion
reactions within these stars produced the heavier elements. Supernova
explosions created by their deaths dispersed this material through the
galaxies which became incorporated into the next generation of stars.
The star formation and the Universal expansion continue to this day and
will continue to do so for billions and billions of years in the future.
On Thursday, how did astronomers realize that the Universe had a specific
beginning?
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