Saturday, February 26, 2011

Our Sun - A Closer Look

In this section, we will discuss some of the more prominent features of the Sun. And we will also discuss the Solar Eclipse (found on its own page).

Here is a nice summary of some of the important features. Just click on the image to see a large version. The layers in this image is not to scale.

Corona This outer layer is very dim - a million times dimmer than the photosphere and oddly enough, it is the hottest. At 106 K it would seem the heat would be unbearable for us, but remember in Physics heat is a measure of molecular energy - the movement of molecules within a space. Because the Corona extends several million kilometers into space, there is a lot of room for molecules to move. It is this movement that is the source of the solar winds. The high temperature of the Corona can force ions to move as fast as a million kilometers per hour.
Chromosphere Chromosphere means "sphere of color," but this layer is 10-4 as dense as the photosphere so it is not that bright. In fact, the best way to view the chromosphere it to use a special narrow band filter called a Hydrogen-Alpha (Hα) filter. The wavelength is 656.3 nm which is in the red (lower energy) part of the spectrum. This wavelength is given by the single electron in the Hydrogen atom dropping to the second orbit (more in the physics section). The temperature of the inner portion of this layer is lower (at 4400 K) than the photosphere, but jumps suddenly to 25,000 K. From this point to the transition into the Corona, the temperature jumps sharply to 400,000 degrees. The reason for this is not clearly understood and remains an active subject to Astronomers studying the Sun, but suspect the magnetic flux as a result of (as well as resulting in) sunspot formation might provide some clues.
Photosphere The Photosphere, Chromosphere and Corona are the three layers that make up the "atmosphere" of the Sun. The Photosphere is the inner-most layer and is the layer we easily see every day. It burns at 5800 K. Oddly enough, the photosphere is opaque to light, only allowing transferred energy from the convection layer below. It is the opaque feature of the photosphere that shields us from directly viewing the thermonuclear core and provide the shape of the Sun. The transferred energy from the convection zone below occurs in the form of granules (see the photo above). As the hotter gas rises up, the cooler gas descends only to be re-heated by the convection layer and the process repeats itself. Sometimes disturbances in the magnetic field will produce sunspots, which occur within this layer, but more on that later.
Convective Zone We cannot visualize past the photosphere, but we can create models and examine particles emanating from the Sun to get an idea to the Sun's interior structure. Three internal layers dominate the anatomy with the outer layer (just under the photosphere) called the convective zone. Radiation is not an effective mean of generating the heat energy produced by the core, so the convective layer acts as the buffer to stabilize this energy. Once a photon enters the convective zone, it can take 170,000 years for it to reach the photosphere. The action of the photon is something called the "random walk" where the photon collides with other photons mainly because the opacity of this zone it a bit high. While there is some pretty unforgiving mathematics involved in determining exactly how much opacity there it, it is important to know the convective zone helps maintain the hydrostatic equilibrium within the Sun by acting as a buffer.
Radiative Zone This layer of the Sun is responsible for delivering the photons from the Core to the Convective layer. The radiative layer "radiates" the energy by the emission and reabsorbtion of photons.
Core The heart of the Sun is the core. Not much to say here accept the gravity was strong enough to bring Hydrogen together to initiate fusion. The magic temperature for fusion is 10,000,000 K. The energy released is balanced by the radiative and convective layers of the Sun to create the hydrostatic equilibrium necessary to prevent the Sun from flying apart or to burn its fuel to fast.

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