If the temperature of the solar surface is so high that it is white, why are sunspots black? To be black they would have to be cold, and how can there be something cold in the Sun?
The question, as formulated, seems like a real catch. In fact, in the early nineteenth century the great astronomer William Herschel He concluded that sunspots had to be cold because they were black. The only way to explain it was to assume that the Sun was not hot in its entirety. According to Herschel, it had an incandescent atmosphere, but underneath it was a cold solid body, which is what we saw through a series of cracks in the solar atmosphere. These cracks were sunspots. Herschel even thought that the inner cold of the Sun could be inhabited by living beings.
But this is false. Today we are completely sure that the Sun is hot in its entirety. Moreover, the surface we see is the coldest part of the Sun, and yet it is already too hot, without a doubt, for living beings. Radiation and temperature are closely related. In 1894, the German physicist Wilhelm wien He studied the different types of radiated light at different temperatures and concluded that, under ideal conditions, any object, regardless of its chemical composition, radiated a certain range of light for each temperature.
As the temperature rises, the maximum radiation wavelength becomes shorter and shorter, in the same way for all bodies. At about 600 ° C enough radiation slides in the visible portion to give the object a dull red appearance.
At even higher temperatures, the object becomes bright red, orange, white and bluish white. (At sufficiently high temperatures, the radiation would be mostly in the ultraviolet, and beyond.) By carefully measuring the wavelength of the maximum solar radiation (found in the region of the yellow color) it is possible to calculate the temperature of the solar surface: it turns out to be about 6,000º C.
Sunspots are not at this temperature. They are much cooler and their temperature in the center must be placed at 4,000º C only. It seems that sunspots represent gigantic gas expansions, and such expansions, whether in the Sun or in a refrigerator, lead to a significant temperature drop.
There is no doubt that in order to keep a gigantic sunspot cold for days and weeks against the heat that flows from the surrounding, hotter areas, a huge thermal pump is needed, and the truth is that astronomers have not yet found a mechanism completely satisfactory for the formation of those spots.
Even at 4,000º C, sunspots should be very bright: much more than a voltaic arc, and a voltaic arc is already too bright to look directly at. What happens is that sunspots are, indeed, brighter than a voltaic arc, and measuring instruments can attest to this.
The crux is that the human eye does not see the light in an absolute way, but judges the brightness by comparison with the environment. The hottest areas of the solar surface, which we could call normal, are four to five times brighter than the colder regions in the center of a sunspot, and comparing these with those, they seem black. That black is a kind of optical illusion.
That this is so can sometimes be demonstrated during eclipses. The eclipsing Moon, with its dark face turned towards Earth, is really black against the bright globe of the Sun. When the edge of the Moon passes over a large sunspot, so that the "black" of the spot contrasts with the Moon, then you can see that the stain is not really black.
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