The sun's atmosphere is made up of several layers, mainly the photosphere, the chromosphere and the corona. The photosphere is the visible surface of the Sun. All the light received from the Sun, in fact, comes from the photosphere. The photosphere of the Sun is like the crust of the Earth in some ways. Both the photosphere and the crust are many miles thick. The top of the crust is the surface of the Earth. If we could stand on the moon and look at the Earth, we would see earth's crust. If we look at the Sun, we see the photosphere. (DO NOT LOOK DIRECTLY AT THE SUN!) We walk around on the surface of the Earth. Not so for the Sun. The photosphere is not solid. We will fall. the Sun is very hot (about 5,000 degrees Centigrade).Most stars have photospheres.
Q.1 Why do not we receive the radiation in the same form as generated in the interior of the Sun ?Ans. At the center of the Sun, the energy is generated in the form of high energy photons called γ-rays. As these photons travel outwards, they collide with particles of matter and lose energy continuously. By the time these photons reach the surface − the photosphere − they are reduced to photons of visible region of electromagnetic spectrum. So, visible radiation is emitted from the photosphere.
- The density of photosphere is 3400 times less than the density of the air we breathe.
- The thickness of the photosphere is about 500 km.
- The temperature at its base is ~ 6500 K.
- The temperature decreases upward and reaches a minimum value of ~ 4400 K at the top.
- The Photosphere shows a granular structure(see the Figure).The surface of the Sun, like the surface of a boiling pot, is constantly changing. When we talk about solar granulation, we mean the cell structure, or bubbly look, of the photosphere. The "boiling" nature of the Sun allows heat from the core to be brought up through the convection zone and into the photosphere, just like heat from the bottom of a pan ends up heating the entire contents (and the air above it!).
- you can see that the photosphere consists of bright and irregularly shaped granules; each granule surrounded by dark edges.
- These granules are very hot and their typical size is ~ 1500 km.
- The hot gas in the granules rises up with a speed of the order of 500 ms−1 and bursts apart by releasing energy. The cool material subsequently sinks downward along the dark edges or lanes between granules. The rising hot granules are seen only for a very short time (~ 10 minutes) before they dissolve.
The granulation can be visualized (See the figures) as the top layer of a region where, due to convection, hot gas from below the photosphere moves upward. Thus, the center of the granule is hotter and it emits more radiation and looks brighter in comparison to the edges which are relatively cooler and emit less radiation. Convection based explanation seems valid because the spectra of granules indicate that their centers are much hotter than the edges. Further, the solar granulation provides observable evidence supporting the idea that there exists a convection zone below the photosphere.
Q.2 What is the chemical composition of the photosphere?
Hydrogen - 79% and the remaining 21% consists of nearly 60 other chemical elements. Interestingly, all the elements of the photosphere are known elements and their proportion in the earth is more or less the same as that in the photosphere.
- The photosphere is marked by bright, bubbling granules of plasma and darker, cooler sunspots, which emerge when the sun's magnetic field breaks through the surface. Sunspots appear to move across the sun's disk. Observing this motion led astronomers to realize that the sun rotates on its axis. Since the sun is a ball of gas with no solid form, different regions rotate at different rates. The sun's equatorial regions rotate in about 24 days, while the polar regions take more than 30 days to make a complete rotation.
- The photosphere is also the source of solar flares: tongues of fire that extend hundreds of thousands of miles above the sun's surface. Solar flares produce bursts of X-rays, ultraviolet radiation, electromagnetic radiation and radio waves.