"spectral signatures" of each element enables
astronomers to identify the elements present in
distant stars by analyzing their spectra.
There are three kinds of spectra: continuous,
absorption, and emission. The continuous
spectrum appears as a continuous band of
color ranging from red to violet when observed
through a spectroscope. An absorption
spectrum occurs when the light from a star
passes through a cloud of gas, hydrogen for
example, before reaching the spectroscope.
As a result, some wavelengths of light are
absorbed by the hydrogen atoms. This
selective absorption produces a spectrum that
is a broad band of color interrupted by dark
lines representing certain wavelengths of light
that were absorbed by the hydrogen cloud.
Such a situation occurs when a star is located
inside or behind a gas cloud or nebula. An
emission spectrum is observed when energy is
absorbed by the gas atoms in a nebula and is
reradiated by those atoms at specific
wavelengths. This spectrum consists of bright
lines against a black background. The light
from fluorescent tubes and neon lights produce
emission spectra. Stellar spectra allow astronomers to determine star temperature, chemical composition, and motion along the line of sight. This enables astronomers to classify stars into spectral categories and estimate their age, reconstruct their histories, and postulate their future evolution. When available, astronomers prefer stellar spectra collected by orbiting spacecraft over spectra collected by Earth-based telescopes since they are not affected by atmospheric filtering and are therefore more accurate. Included in the spectra collected by spacecraft are infrared, ultraviolet, x-ray, and gamma ray bands that simply do not reach ground-based spectroscopes. |
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