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Stellar Distance Page 11

Colour

Another method of determining distance to stars is by first measuring its temperature from its colour profile. Stars are very nearly “black body radiators”.

A star’s blackbody curve reveals its temperature (see graph). Notice that the peak of each temperature curve, moves to the left (shorter wavelength) when the temperature goes up.

Graph: Wikipedia

Think about a red hot poker you pull out of a fire. The colour you see (which is the peak colour) reveals its temperature.

The hotter it gets the shorter wavelength light it emits. Initially you may not see any light coming off the poker, then it radiates mainly in infrared.

When it gets hotter it will become red and later even white (yellowish) hot. If you use a material that does not melt at that point, you will see it become blue hot, etc.

Jewell Box

Even with the naked eye we can distinguish different colours of stars. Young stars are generally blue (hot), while older, cooler stars are red.

 

Image: http://outreach.atnf.csiro.au

One of the nicest examples of different colour stars in one group,
called the Jewel Box.

 

 

 

The Sun

In the graph to the right is the ideal black body radiation pattern for 6000 K (below) and the measured pattern for the Sun (top). This confirms that the photosphere temperature of the Sun is about 6000 K, because the 6000K graph best fits the observations, especially when looking where the peak of the graph is.

Method
Star light is passed through different colour filters and the  intensity for each colour (wavelength) is recorded. The intensity will be maximum for one particular colour.

From the black body radiation graph (example above) we can then determine the temperature of the photosphere of the star.

 

Why is that helpful for finding the distance?