SP-402 A New Sun: The Solar Results From Skylab

SPECTRUM OF SOLAR RADIATION.

SPECTRUM OF SOLAR RADIATION. Visible sunlight is but one part of the total radiation Earth receives from the Sun; shown here is the full span of electromagnetic radiation from our nearest star. Electromagnetic radiation such as sunlight travels in waves, the wavelengths of which serve as descriptions, or identifiers, of the different forms of radiation. Our eyes see only a narrow band of wavelengths-the "visible spectrum" of rainbow colors from about 4000 to 7000 Å, violet to red. We see it on the chart as a rainbow of colors. To the left of the visible spectrum is the infrared, covering a wider band of wavelengths, reaching from the red of the visible to wavelengths of about 1 mm. The Sun emits light, or radiation, throughout this region. Although we cannot see it, we can feel infrared waves as heat on our skin. To the left of the infrared stretches the vast spectrum of radio wavelengths, where the Sun also emits energy that [41] is detectable by solar radio telescopes that "hear" it on radio receivers as a form of cosmic static. To the right of the visible spectrum stretch the shorter and more energetic wavelengths of ultraviolet radiation, X-rays, gamma rays and cosmic rays. All are invisible to our eye. These shorter, invisible wavelengths arise in the upper, more active layers of the Sun, and are thus especially valuable for the study of the active Sun. Special telescopes and sensors are required to measure the radiation at these wavelengths.

The atmosphere of Earth is transparent to visible sunlight; almost all the sunlight in the visible spectrum passes through the air to reach the surface of the ground. Gases in the terrestrial atmosphere, such as oxygen, ozone, or water vapor, absorb most of the infrared, ultraviolet, X-ray, and shorter wavelengths of solar radiation before it reaches us. On the chart Earth's atmosphere is shown in vertical crosssection, with a scale of height above sea-level at left. The depth to which each region of the solar spectrum penetrates is shown as a dotted line. In the radio region, like the visible, penetration is almost complete, and these regions are called "windows." X-ray radiation is totally absorbed far above Earth, at an altitude of about 100 km. Skylab, and other spacecraft and rockets, were at altitudes high enough to feel and observe the full range of electromagnetic radiation from the Sun-a feat impossible for solar astronomers on the ground.

Skylab carried special telescopes to observe the Sun in the region from about 2 to 7000 Å wavelength, in X-ray, ultraviolet, and visible regions of the spectrum. Its region of observation is shown in the expanded spectrum at the top, with spectral lines of special interest as dark, vertical lines.


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