April 20, 2018

A Peek at the Structure of the Sun.

(Blogger’s Note: This paper is a Bloggified version of an essay I submitted last year in a quest for my science teaching degree…it has been re-edited to fit this format.)

The Sun is often touted to introductory astronomy students as our nearest star. Of course, its proximity gives us an unprecedented opportunity to study the behavior of a star up close. But how do we really know what we know? Our friendly neighborhood Sun powers life here on Earth. A stable G2V type star, it fuses hydrogen into helium, while releasing copious amounts of energy in the process. Hopefully, last weeks’ review of Death from the Skies! drove home the fact that our Sun has an angry side, as well. As we gear up for another climb towards a solar maximum in a few years, it’s useful to appreciate the grandeur that is ol’ Sol. Let’s delve into the structure of the Sun, and how we know this.

SOHO: A Montage.

Until the discovery of the nuclear process, what powered the Sun and its interior was a complete mystery. Sunspots were monitored and recorded; a rough rotational rate of 27 days was deduced, as well as the fact that the Sun couldn’t be solid, because different latitudes rotated at different rates. But at the time, chemical processes were the most powerful known; for example, it had been calculated that a Sun made of coal would burn itself out in a thousand years! Modern day knowledge would have to wait for the discovery of nuclear physics and theories of stellar evolution.

How do we know the workings of the Sun? Modern day knowledge stems from patient examination of the Sun throughout the spectrum, coupled with studies of other stars in various stages of evolution, and computer modeling of the interior. Waves analogous to acoustic waves (these are modeled and analyzed visually) scour the visible photosphere and enable us to map the structure the interior, much like seismic waves on Earth. This emerging science is known as Helioseismology. And yes, Virginia, we can actually model the backside of the Sun with this process! Current models hold up well to predicted observations of solar behavior, although there is still much to be known about the Sun. One major mystery is why we do not see any polar sunspots; they never appear above or below certain latitudes. Clearly, a riddle of the Sun’s behavior is locked up in this fact. Also why is the solar cycle 11 years? What triggers it?

The Sun is divided into six main layers; at the heart is the core. This is where nuclear fusion begins. At a diameter of about 216,000 miles and a temperature of 28 million degrees Fahrenheit, the core has a structural density of 160 g/cm3, or over eight times that of gold.The function of the core is to create energy through the proton-proton chain process. Neutrino detectors buried deep within mines to shield them from cosmic rays are able to probe the core of the Sun. An amazing fact about the energy production of the Sun: any given photon generated at is core may spend its first 100,000 years or so shooting around like a pin ball until reaching the surface! It then has an 8 minute free flight to the Earth!

The next layer outward is the radiation zone. The width of the radiation zone constitutes about half the radii of the Sun; here, the mean density drops from 20 g/cm3 to about 0.2 g/cm3 and allows energy to begin to radiate through. Note that there are no distinctive boundaries between interior layers; transition can only be termed in the behavior characterized by each. The temperature of the radiation zone drops from about 12,000,000 to 4,000,000 degrees Fahrenheit, while its function is to allow light and energy to begin to travel outward as radiation.

Between the radiation zone and convection zone comes the tachocline layer. This layer produces torsion through fluid motion, and the shear here is thought to be responsible for the Sun’s immense magnetic field.

Next above the radiation zone comes the churning cells of the convective zone. This area is over 120,000 miles wide. It has a beginning bottom temperature of 3,000,000 degrees Fahrenheit, which drops to about 10,000 degrees Fahrenheit at the top. Its density is 0.0000002 g/cm3 in the upper portions. In this region, convection is the primary method that allows energy to be carried upward.

Next is the photosphere. This is the dazzling “surface” of the Sun. Note that the Sun is basically a ball of gas and does not have a solid surface in a literal sense. Its width, represented by the photosphere, is approximately 864,000 miles, and this layer is about 300 miles thick with a pressure of less than 1/100th of Earth’s atmosphere. Here, we see phenomena such as sunspots, granules, and faculae transiting the face of the Sun in visible light. Rarer are bright white light flares. Sunspots are actually cooler regions of intense magnetic activity, which only appear darker because of the intense regions surrounding them. Temperatures in the photosphere hold at about 10,000 degrees Fahrenheit.

Above the photosphere is the chromosphere. This is the active “inner atmosphere of the Sun”. Relatively cooler temperatures of about 7,300 degrees Fahrenheit occur in the bottom of this region, and then climb to 90,000 degrees Fahrenheit at the top. It is visible along hydrogen alpha transmission bands, at a transmission wavelength of about 656.281 nanometers. Here, looping prominences can be seen during periods of high activity. The chromosphere is about 1,200 miles in thickness.

Finally, the “outer atmosphere” of the Sun is the corona. At about half the brightness of the full Moon, it is much fainter than the photosphere. It is only visible from Earth during the brief minutes of a total solar eclipse, when the body of the Moon blocks the brilliant photosphere. Solar winds originate in this area. Occasionally, huge Coronal Mass Ejections (CMEs) can be seen in this region; such disturbances can eject billion ton clouds of ionized plasma and can wreak general havoc on the local solar weather if they’re aimed at Earth. Its size is highly variable with the solar cycle, but can extend millions of miles from the edge of the photosphere. It is composed of super heated ionized particles, at a temperature climbing to 5,400,000 degrees Fahrenheit.

How does the Sun produce all that energy? The method of energy production was a mystery until the discovery of nuclear fusion in 1932. The Sun is so massive that internal pressures cause nuclear fusion of hydrogen, known as the proton-proton chain reaction. This outward pressure counteracts gravitational collapse, keeping the Sun stable. In this reaction, four hydrogen protons are fused to form one helium atom, converting about 0.029 of their atomic mass to energy in the process. Incidentally, the Sun has shrunk slightly over time, brightening as it does so. A Population I type star, the Sun is currently about mid-way through its projected 9 to 10 billion year life span; once its hydrogen fuel is exhausted, it will expand into a brief Red Giant phase, leaving the Main Sequence and fusing “helium ash” into heavier elements. This bloated stage will be ultimately unstable; the collapsing Sun will then throw off huge shrouds of material as the core contracts. After a brief and dramatic “helium flash” the Sun will find its days as a normal star numbered. The final stage of the Sun will be that of a degenerate white dwarf about the diameter of the Earth, nested in a planetary nebula. Perhaps we’ll find ourselves as a brief lingering stop on an alien “Messier Marathon…”

Why bother to study the Sun? The Sun goes through a predicable cycle, reaching a peak in solar activity every 11 years. As our technology evolves, we become ever more vulnerable to the throes of our nearest star. Solar flares and coronal mass ejections can damage satellites, cause blackouts, and disrupt communications. As we venture out on longer duration manned space missions, what the Sun is doing will become essential information. Missions such as the European Space Agency’s Solar Heliospheric Observatory (SOHO) and the Global Oscillation Network Group (GONG) keep a perpetual eye on the Sun for any potentially disrupting shenanigans. We know that the Sun is capable of some pretty substantial CMEs, and that we are long overdue for one to smack the Earth.

In closing, the Sun provides us with a dynamic laboratory to study a star up close. An intimate knowledge of its inner workings is vital to life on Earth, and understanding its behavior is directly related to our own destiny as a species. And our “Nearest Star” has be strangely quiet these past few years… one wonders what wackiness might be in store this coming maximum!<–>


  1. magical250 says:

    i luv earth

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