IDENTIFYING THE ELEMENTS FROM EMISSION OR ABSORPTION SPECTRA
An astronomical light source, such as a star or a gaseous nebula, contains a mixture of chemical species, each either emitting or absorbing its own set of wavelengths of electromagnetic radiation. With the aid of laboratory spectral analysis of the different chemical elements, astronomers can identify individual elements in the light source from the measured wavelengths of its spectral lines, regardless of whether they are emission 'or absorption lines.
Identification is done in the following way: Light from a celestial body is collected by a telescope and then passed through a spectrograph in order to disperse the white light from the light source and form its spectrum. The photographic plate on which the spectrum is recorded is called a spectrogram. As a standard against which unknown wavelengths in the astronomical spectrum can be measured, an emission spectrum of a known gas, such as neon or vaporized iron or titanium, is placed above and below the astronomical spectrum. (The mechanism for placing the laboratory spectrum on the astronomical spectrogram is a part of the telescope and spectrograph.) With these comparison lines of known wavelength the astronomer can determine the unknown wavelengths of the astronomical object's spectral lines. The absorption spectrum is gray with black absorption lines and the comparison spectrum of neon shows white emission lines on a black background.
Kirchhoff's laws of spectrum analysis tell us about the general physical conditions of the light source. And if the spectrum of the light source contains absorption or emission lines, we can measure their wavelengths and identify the chemical elements that are present.
Can more detailed information about the light source be found? Suppose we want to know the temperature of the light source. Can this be done? Yes it can, for special types of light sources known as ideal radiators, or blackbodies.
All objects radiate and absorb some form of electromagnetic radiation; the wavelength region and the amount of energy depend generally on the body's temperature and physical state. From laboratory experiments and from theory physicists in the nineteenth century analyzed how various bodies emit and absorb radiation as a function of temperature and wavelength. From this work they developed the concept of an idealized radiator called a blackbody.
An astronomical light source, such as a star or a gaseous nebula, contains a mixture of chemical species, each either emitting or absorbing its own set of wavelengths of electromagnetic radiation. With the aid of laboratory spectral analysis of the different chemical elements, astronomers can identify individual elements in the light source from the measured wavelengths of its spectral lines, regardless of whether they are emission 'or absorption lines.
Identification is done in the following way: Light from a celestial body is collected by a telescope and then passed through a spectrograph in order to disperse the white light from the light source and form its spectrum. The photographic plate on which the spectrum is recorded is called a spectrogram. As a standard against which unknown wavelengths in the astronomical spectrum can be measured, an emission spectrum of a known gas, such as neon or vaporized iron or titanium, is placed above and below the astronomical spectrum. (The mechanism for placing the laboratory spectrum on the astronomical spectrogram is a part of the telescope and spectrograph.) With these comparison lines of known wavelength the astronomer can determine the unknown wavelengths of the astronomical object's spectral lines. The absorption spectrum is gray with black absorption lines and the comparison spectrum of neon shows white emission lines on a black background.
Kirchhoff's laws of spectrum analysis tell us about the general physical conditions of the light source. And if the spectrum of the light source contains absorption or emission lines, we can measure their wavelengths and identify the chemical elements that are present.
Can more detailed information about the light source be found? Suppose we want to know the temperature of the light source. Can this be done? Yes it can, for special types of light sources known as ideal radiators, or blackbodies.
All objects radiate and absorb some form of electromagnetic radiation; the wavelength region and the amount of energy depend generally on the body's temperature and physical state. From laboratory experiments and from theory physicists in the nineteenth century analyzed how various bodies emit and absorb radiation as a function of temperature and wavelength. From this work they developed the concept of an idealized radiator called a blackbody.
