Star colour

wiensdisplacement Black body temperature curves. Credit: Wikipedia The reason that we see some stars as predominantly blue-ish in colour, and that others are more red, is because the intensity of the emitted light can be different for different wavelengths.


Ideally, a star emits light as a theoretical object called a black body, for which the intensity over the spectrum depends on the temperature of the star’s outer layer. This makes it possible to determine the temperature of that outer layer by observing the intensity of the various wavelengths in the spectrum.

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. This follows Wien's Displacement Law.


Astronomers use sets of standard colour filters to determine through which filter the maximum energy (light intensity) is measured. That filter (wavelength) tells us the temperature of the star’s outer layer that is called the photosphere.


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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.



jewelbox bessell


Famous for its display of a great variety of colours, this open cluster of stars deserves its name: The Jewel Box.

One of the bright central stars is a red Super Giant, in contrast to the many blue stars that surround it. The cluster contains just over 100 stars, and is about 10 million years old. It is at a distance of about 7500 light-years and is about 20 light-years across. It can be seen with binoculars towards the southern constellation of Crux.