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