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The Sun, like other stars, is a burning ball of mostly gaseous hydrogen, large enough to hold a million Earths. The temperature of its surface is about 6000°C-- as hot as a welder's flame--and the radiation that it emits peaks in a band of wavelengths called the "visible spectrum." It is all that our eyes can see, yet less than half of the solar radiation that reaches the Earth lies within this thin, central slice of a much wider spectrum of wavelengths. Our skin can feel as heat a part of the rest--the infrared--and is also acutely sensitive to another part: the unfelt but more energetic and potentially damaging ultraviolet. Solar X-rays, gamma rays, and radio waves make up the remainder, which constitutes less than 0.1 percent of the solar energy received at the Earth.
Observations from space reveal that the total radiation from the Sun is continually changing--with variations of up to 0.2 percent from one month to the next. The timing and nature of these shorter-term fluctuations are consistent with the Sun's 27-day period of rotation, and occur because persisting darker--or brighter--areas on the solar surface alter the amount of sunlight received at the Earth.
As the sunspot number rises or falls, the distribution of energy within the spectrum of sunlight also changes. High levels of solar activity enhance radiation in UV and X-ray wavelengths, and in radio wavelengths, far more than in the visible portion of the spectrum. At peaks of the eleven-year cycle, radiation at longer UV wavelengths, for example, increases by a few percent, compared with an increase of but 0.1 percent in the total radiation. Still larger changes--of factors of two or more--are found in extremely short UV and X-ray wavelengths.
Does the Sun affect climate on time scales of 11 years or less?