The key parameter in this equation is the temperature. Each temperature corresponds to a unique spectrum of emission.
To a high degree of approximation, stars are blackbody radiatiors and hence we can use this ideal to describe the pattern of radiation given off. This pattern of radiation is called the Planck curve
Here are two examples:
Notice in the above sample, the curve with a temperature of 7500 degrees emits the most radiation (e.g. peaks) in the blue portion of the spectrum. Therefore, that object would appear to be blue.
In the example below, the curve with a temprature of 3000 degrees emits very little light in the blue and therefore would appear to be very red.
Examination of these curves shows a fundamental experimental result. As you go to cooler temperatures, the wavelength at which the maximum amount of energy is emitted shifts to longer wavelengths.
In more quantitative terms, we have this relation:
is a very small part of the total spectrum of radiation given off by objects in the Universe:
Furthermore, our atmosphere blocks up much of this spectrum and therefore to observe the total EM spectrum from celestial sources requires Satellites and/or telescopes in space.
In practice, astronomers use various filters and form filter flux ratios to determine the color. This is coded as the B-V color index, which is what we will use as an indicator of temperature.
In the applet below you should check the box labelled draw limits of integration. This will superpose the filter system on the black body curve. As you change the temperature, watch how the B-V index changes in the readout below the graph.
Wavelength in Angstroms | |
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