COLOR AND STELLAR TEMPERATURE

COLOR AND STELLAR TEMPERATURE

Today we will learn that stellar temperature can be measured directly from stellar color. In turn, stellar color is measured by using different filters.

The pattern of radiation given off by a perfect radiator is called Blackbody radiation (because the radiation pattern is independent of what the material is made of). This pattern is characterized by a particular function, called the Planck function, which mathematically is expressed as:

Don't be scared by this formula - it is not necessary to know it. The key parameter in this equation is the temperature. Each temperature corresponds to a unique spectrum of emission. In the interactive activity we will be doing later, you will see how temperature effects the nature of the radiation pattern given off by a blackbody.

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:





But remember, the optical part of the spectrum

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.

PRS questions about blackbody radiation using this applet

Wavelength in Angstroms

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In addition to the wavelength dependence on temperature, there is also a strong dependence of the total energy emitted. Let's see if we can ferret this out using this applet: