MadSci Network: Astronomy

Re: Why are there no green stars?

Area: Astronomy
Posted By: David Barlow, Private individual, Grad education in Physics/Astrophysics and Comp. Support
Date: Mon Mar 24 05:46:24 1997

In actual fact there are no green stars at all. Stars come in all shapes and colours but never Green. There is a very good reason for this. A star is nothing more than a hot ball of gas. The exact temperature, at the surface, of the star depends on its Mass and Radius. When plotted on a graph all stars fall on a number of lines, this is called the Hertzsprung-Russel (HR) Diagram. In order to do this plot you have to assume the Star is a perfect Black-Body radiator. That is, it obeys the Boltzmann equation for heat radiators. This describes the ranges of frequencies of light a body will emit for a given temperature. When all the above is put together and number crunched it is found that no combination of Mass, Radius and Temperature for a star emits light predominantly in the green part of spectrum.

The maths and diagrams to proove my statement are simply too much to go into at the moment, it requires a books worth of proofs. So, I would reccomend reading a few books on Gas Laws and basic Astrophysics to see what I mean. A few good ones I have are,

I have been unable to find any good online sources showing exact details of this unfortunately. There are some web sites showing the HR diagram and probably a few on the Boltzmann Equation. But neither explain the dependencies and show what it all means.

Added by Marc Herant (astro moderator) to expand and complete what David Barlow has said:

The color of a star is determined by the relative amount of various frequencies of light that it emits. This amount of light vs. color, or emitted energy vs. frequency (or wavelength) is what is called a spectrum. Most stars nearly emit what is called a "black body" spectrum. The name black body is not important, it is just a label which designates an emitted spectrum with a certain shape. Depending on the temperature of the surface of the star this black body spectrum peaks at a certain wavelength (or equivalently color, or frequency) which means that the light emission is greatest at that specific color, but there still is emission in other colors.

So, a high temperature star (say surface temperature 8000 Celsius) will have a black body spectrum that peaks in the blue and will look blue. A low temperature temperature star (4000 C) will have a black body spectrum that peaks in the red and will look red. However an intermediate temperature star that has a black body spectrum that peaks in the green (and there are some) will not look green. why?

This has to do with the way that the human eye perceives color. The human eye uses 3 different type of cells (called cones) in the retina which respond to different colors. Now recall that even though a black body spectrum may peak in the green, this is mixed with other colors (red and blue). It turns out that the specific mix of colors given off by a black body spectrum that peaks in the green is interpreted by our retinal cones as yellow-white. Since there are no star that emit pure green without the other wavelengths, there is no such thing as a green star.

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