THE USM SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
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70 Falmouth Street Portland, Maine 04103
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43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
Julian date: 2458687.5
"In some cultures, it's considered bad luck to build
your house on quicksand."
THE DAILY ASTRONOMER
Monday, July 22, 2019
Sunshine Science
isn't simple. Then again, what is? The astronomical sages have shown us
that even the daintiest thing loitering about the cosmos formed after a
series of highly complex stages. Even if you snip a leaf or a siphon
off a drop of remote pond, you will encounter far more than meets the eye.
So, in this vein, the astronomer languishes on his vine-entwined gazebo and
gazes into the recession of cloud forms molding and dissolving against
boundless blue. The world awakens to a sultry summer and with it a
landscape awash in sunshine: the most pervasive of all astronomical
phenomena. So close and intimate does the Sun seem that it is difficult to
remember it is more than ninety three million miles away. And, so
beloved and adored is Sol among all people that many refuse to accept that
it is an astronomical entity. It's far more porch light than star fire.
Today, we shed some light on sunshine. (And, yes, we're the very first
people to come up with that witty word play.)
The sunshine that smarts the shoulder and dazzles the irises was once among
the greatest mysteries. Its origins, and, by extension, its longevity,
were unknown until the 20th century. Humanity was reliant upon a fiery
sphere of indeterminate composition and duration. We know now that it
took form about five billion years ago and will continue to issue
prodigious amounts of energy for billions of years to come. We're so
confident about the Sun's life span because astronomers determined that it
uses its own material as a fuel source: converting hydrogen into helium
deep within its core. We also know that the Sun contains enough matter
-and therefore sufficient fuel stores- to continue energy production for
aeons to come.
The sunshine that you see left Sol's photosphere about 8.3 minutes ago.
Light travels at about 186,290 miles a second and requires just over eight
minutes to move from the Sun to Earth. Though a 93 million mile excursion
through the unsounded deep might seem exhausting, it's the easiest part of
the trip, at least for the photons comprising sunlight. The actual
odyssey began in the solar core: the nuclear reactor mentioned in the
previous paragraph.
Comprising 1.5% of the Sun's volume, the core is a furious energy machine:
every second, 647 million tons of hydrogen is converted into millions of
tons of helium. A minute percentage is transmuted into energy.* The
Sun's interior is so hot (27 million degrees F; 15 million degrees C) that
the protons-positively charged subatomic particles- move quickly. As they
have the same charge, the protons would repel each other in cooler
environs. In the sun's core, they have so much energy they can fuse
together despite the electrostatic repulsion that would otherwise keep them
separated. The hydrogen turns into helium and energy!
This energy doesn't then just snap out of the Sun and onto Hawaiian
beaches. As soon as the energy emerges, it is almost immediately
absorbed by intervening material. Then, it is eventually re-emitted, only
to be re-absorbed; then re-emitted; re-absorbed, and so on and so forth and
back again. Meanwhile, it is struggling to liberate itself from the
roiling solar inferno. We use an example in the Star Dome Astronomy
class. Imagine that you have to walk to Los Angeles. You have to walk 16
hours a day and we provide accommodations and nourishment along the way.
How long would the trip be? Perhaps 4 - 6 months? Now, imagine, that
you walk back to Portland from Los Angeles, but this time you stop at every
third house and sit in the living room floor until the owner kicks you
out. The problem is that you have to move in whatever direction the
furious owner tosses you. Though your destination is to the northeast, if
you are kicked toward the southwest, you have to walk toward the SW until
you reach the third house. Your direction will change with every
explusion. With this new complication added, how long would the journey
require? Well, forever. You'll never be in Portland again.
The core energy photons face the same exasperatingly protracted journey
involving innumerable absorptions and re-emissions until they finally go
through the radiative zone and then the outer convective zone before
reaching the uppermost layers and then freedom! This process requires
more than 100,000 years! The sunshine that strikes you everyday is truly
ancient starlight.
Also, the solar photons that hit Earth comprise less than a billionth of
those the Sun produces. The planets receive precious little of the Sun's
energy. Most of those photons scatter through interstellar space. Few
reach extragalactic distances and if an alien astronomer in Andromeda
captures an image of the Milky Way Galaxy, perhaps a negligible part of the
glow will consist of the Sun's contribution.
Sunshine is possible because a few of the Sun's photons strike Earth soon
after their 100,000 year ordeal of emissions and absorptions. One can look
outside on a clear day and almost hear the impacting photons murmuring,
"Oh, not again."
Deep in the Sun's core right now, our star is generating the energy that
will shine down on Earth 100,000 years or so from now. An optimist's
dream: an assurance that we have myriad sunny days not yet experienced.
*Here, we offer a bit more detail for those who crave scientific
satiation. When the astronomer tells you that the Sun generates energy
by converting hydrogen into helium, what does that actually mean? What is
happening: Well, in the Sun, we see a series of reactions called the
"proton-proton chain," which, itself, has three variants. We'll limit
ourselves to the most common sequence, as a means to elucidate the concept
without driving the reader to hopeless despair.
We start with two hydrogen atoms that combine to produce deuterium, a
hydrogen isotope with an extra neutron. This deuterium connects with
another hydrogen atom to make helium-3 (it has two protons, hence the
element changes to helium. Deuterium contains only one proton. Remember
that the proton number determines the element.) Two helium-3 nuclei fuse
into a helium-nucleus (two protons; two neutrons) and in so doing eject two
hydrogen nuclei. Energy is released during this sequence: the energy
that eventually becomes sunshine. The gamma photon is released in the
second stage: the collision of deuterium and the hydrogen nuclei.
Other proton-proton chains have been identified, but hopefully discussing
this one serves to explain that the fusion reactions are not just one-hit
wonders: they happen in stages.