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Colours of metals, flames and stars

(a) Why is the blue flame is hotter than yellow flame?
(b) The colour of steel which is heated to a very high temperature will change, for example from red to yellow. Can you all tell me the sequence of the changing colours and why could it change?
(c) Why are certain stars yellow, some in white and some blue? Is it caused by temperature? If we stay outside the atmosphere can we still see the same phenomenon (that is stars have different colours) and why? If not then what will we see?
(d) Are questions (a) (b) and (c) related with each other?
(e) What are the difference and similarities between (a), (b) and (c)?


All hot objects emit radiation the colour of that radiation depends on the temperature of the object. For example a hot (say 200oC) object will emit mostly infrared radiation while one at 1000oC will emit a lot of visible radiation as well.

There are laws of thermal radiation emission that give the exact distribution of energy across the spectrum that is how much energy will be radiated at different wavelengths (colour regions) at different temperatures. If you are really interested look up Wien's Law in a Physics book (its on this site of course and I will add an extract from the text part of the site for you.

If the peak of the energy-wavelength curve for a black body lies in the red region of the spectrum the body will appear red hot, and as it gets hotter this peak will move towards the violet end of the spectrum. However this does not mean that the body will look "violet hot". The reason for this is that at the higher temperature all visible wavelengths will be present to some extent and so the body will appear "white hot". A white-hot body will give high emission across the whole range of the visible spectrum.

If we know the value of the maximum of the curve for one black body at a known temperature we can use Wein's law to calculate the temperature of another black body providing the wavelength at which maximum energy is emitted is known. This has been used extensively in astronomy for finding the temperatures of stars.

Stars like our Sun are very average, they have a maximum in the green region of the spectrum and a surface temperature of around 6000oC compared with white-hot stars like Sirius or Rigel with surface temperatures of more than 30 000oC.

The colour of the stars will be virtually unaffected if you go outside the Earth's atmosphere, although they will be brighter - no atmospheric absorption or scattering of the light.


© Keith Gibbs 2020