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helenwolfy

Spectrum.
When electromagnetic waves are ordered in accordance with their frequency or wavelength, this ordered array is called the electromagnetic spectrum. A source of radiation such as the stars never produces just one frequency of electromagnetic wave, but rather emits a mixture of waves of many different frequencies. These spectra may be resolved, or separated, by instruments such as prisms or grating spectrometers.

In principle the electromagnetic spectrum extends from zero, the short wavelength limit of the gamma-ray end of the spectrum, to infinity, the long wavelength limit of the radio end of the spectrum. Visible light, the portion of the spectrum to which the eye is sensitive, occupies a narrow band. The radiation dispersed by a prism actually extends farther in both directions but light outside of this range cannot be detected by the human eye.


The spectrum of stars.
When light shines on matter, the radiation energy is partly absorbed by the molecules of that matter. This is caused by the transition of electrons from a lower energy level to a higher energy level, when a photon is absorbed by the atom.

From experiments we know that every type of molecule or chemical compound absorbs only the radiation of a specific frequency, that is characteristic for that type of molecule. For example red colored matter absorbs mainly blue light from incoming sunlight, the red color frequencies are reflected back. So by measuring the absorbed and reflected frequencies of a mixture of all frequencies that shines on an object we can determine exactly which elements the object is made of.

Stars, like our sun, emit a full range of  frequencies, but the atoms in the cooler outer layers of the atmosphere of the star absorb certain frequencies. When the spectrum of starlight is measured, it shows various dark lines, the absorption lines. These lines reveal the presence of particular elements in the atmosphere of the star.


Luminosity.
The observed brightness of a star not only depends on its emission intensity, but also on its distance, far off stars seem less bright than nearer. For observational purposes a brightness indication (or magnitude) is used, which is the relative brightness as seen from Earth.
The luminosity, or intrinsic intensity, is an indication of the energy radiated by a star. It varies with the temperature of the surface of the star as well as with its radius. The light from stars with a higher surface temperature (like blue giants) is of a higher luminosity than that of colder ones (like red giants). Also the luminosity of  larger stars is higher than that of smaller ones (like blue giants compared to white dwarfs).


http://library.thinkquest.org/18188/english/universe/theory/spectrum.htm

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