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The Sun is yellow in color, which correlates with a surface temperature of about 6,000°C, so it is classified as a yellow dwarf of spectral type G2. The visible white light of the solar spectrum can be split into many wavelengths (colors) by passing it through a raindrop or a prism. The red end of the spectrum is at a slightly longer wavelength (lower frequency) than the violet end.²

However, most of the Sun's output of electromagnetic radiation is invisible to our eyes. The first to demonstrate the existence of this invisible light was William Herschel. 19 years after he discovered the planet Uranus (see Chapter 10), Herschel made another notable breakthrough when he used a prism to split sunlight into its colors and then placed a thermometer just beyond the red end of the spectrum. Noting a higher temperature at this location, he concluded that it must be caused by the existence of “calorific rays” – now known as infrared (literally “below red”) radiation.

Figure 2.4 On July 14, 2000, high‐energy particles from a solar flare left their mark on the detectors of the SOHO spacecraft for several hours. SOHO is located about 1.5 million km from Earth, so that it can provide advance warning of solar storms heading for the planet.

(ESA/NASA)

Subsequent studies have shown that the Sun also emits invisible electromagnetic radiation at much longer and shorter wavelengths. In 1801, Johann Ritter proved the existence of ultraviolet radiation by using a prism to create a solar spectrum and then noting how paper soaked in silver chloride became much darker beyond the violet end of the visible spectrum.

In 1942, James Hey was assigned to find a solution to what seemed to be severe jamming of anti‐aircraft radars by the Germans. Instead, he found that the direction of maximum interference seemed to follow the Sun. On checking with the Royal Observatory, he learned that a very active sunspot was traversing the solar disc. Hey had discovered the first radio emissions from the Sun.

Seven years later, solar X‐rays were discovered using instruments on board U.S. suborbital rockets. The earliest detectors were simply pieces of photographic film shielded from visible and ultraviolet light by foil made of beryllium or aluminum. Later flights carried more sophisticated Geiger counters. By the 1950s it was possible to confirm that the solar corona consisted of million‐degree plasma and that active regions on the Sun's surface were also a source of X‐rays.¹