(Chapter 16) < < prev   next > > (SOHO)
Sun's atmosphere is made up of three layers: photosphere, chromosphere, and corona.
The layer that we see in visible light is the photosphere. Its average surface temperature is 5800 K but ranges from about 7500 K at its base to about 4500 K at its top. Its about 400 km thick. The layers above the photosphere are transparent for visible light, so we have to look at those layers with telescopes that "see" UV and x-ray light
The absorption lines seen in a spectrum of Sun are called Fraunhauffer lines and are shown below. They originate in the photosphere. The interior of Sun produces a continuous blackbody spectrum. The gas in the photosphere absorbs particular wavelengths, depending on the elements present in the gas and the temperature of the gas.
N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
The absorption spectrum tells us about the composition and the temperature of the photosphere. This is true in general for any star.
The photosphere is also where we see sunspots. For more information on sunspots, see NASA's Sunspot Cycle web site.
At this time, sunspot activity on Sun is very low. (Last year, in 2006, on February 1, there were no sunspots.) Consider the following picture of Sun taken by SOHO on February 6, 2007. There are only two sunspots!
(from SOHO real-time photos)/p>
Now, consider the following picture by SOHO on February 1, 2001.
Sunspots come and go with an average period of 11 years. Here's a graph of number of sunspots each month as a function of time.
This somewhat outdated graph shows sunspots for the past cycle and its predicted minimum in 2006. In fact, sunspots did seem to hit a minimum in 2006, though we won't know until after 2007 if them monthly number of sunspots is increasing or not.
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