THE USM SOUTHWORTH PLANETARIUM
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.