Photosphere


    The photosphere is where the light from the Sun that we normally see is radiate, which give rise to the name Photosphere or Light Sphere. This is essentially what we see from Earth with our naked eyes when we look at the Sun.

    It is also the nearest part of the Sun's atmosphere that is nearer to the interior of the Sun. It is opaque and realtively denser than the other parts of the Sun's atmoshphere (Chromosphere and Corona) which will be covered later. The gas content is relatively thin as compared to the interior of the Sun; about 104kgm-3.

    This part of the atmosphere is also opaque because of the extra electron that is attach loosely to the Hydrogen atom. This eletphoton. Thus it absorbs light readily and therefore appear opaque.


Limb-Darkening Effect

    If we look closely at photographs of the Sun, we will notice that the edge of the Sun ("limb") looks darker as compared to the center of the Sun which is bright. This is because the temperature increases as we go deeper into the photosphere. Gases in the photosphere are relatively cooler at high altitudes than gases at lower altitudes. When we look at the center of the Sun, we are actually staring deeply into the Photosphere. When we look at the limb of the Sun, we are actually looking at some sort of a section of the Photosphere and not looking deeply into it. Thus, the limb appears to be darker than the center of the Sun (Limb-Darkening Effect).

    When we look at the sun, our line of perception is as shown. This explain the limb-darkening effect. At the center of the photosphere, radiation emerges from C. At the limb, radiation emerges from A, several hundred kilometres above the base of the photosphere, near the temperature minimum of the sphere, therefore it appears to be darker than the at C. At B, radiation emerges some distance from the base of the photosphere but still less distance from the base of the photosphere as compared to at point A, so it'll appear to be slightly darker than at C but brighter than at A.


Granulation

    From observation using even a very simple telescope, one will notice that there are 'rice grain patterns' all over the Sun. The individual features that make up this patterns is called granule. High resolution pictures reveals more about these granules. The granules are light-colored and surrounded by darkish area. The difference in brightness is due to the temperature difference between this 2 regions to be about 300K.  
Granules (src: Universe (Fifth Edition))


Why are there granulations?

    Granulation is essentially caused by convection current. Gases from deeper down the photosphere are heated up and rises up. They then spill over the "surface". This is what is observed as the light-colored granulations. The hot gases spills over the granules ,cools and then plunges back into the Sun (this corresponds to the darkish region, which is of lower temperature because the gases that descends back into the Sun are lower in temperature).  
Convection current causing granulation (Picture modified from src: Universe (Fifth Edition))


Resource book: "Universe (5th Edition)"   
Pictures "Universe (5th Edition)"   

Page of GEM1506K, Group 2 (http://sg.geocities.com/apollonus02)   

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