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Q & A: why is starlight so visible?

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Most recent answer: 02/25/2012
Q:
Hi, We're learning Astrophysics in class now and we learned that the peak emission wavelength of star's emission spectra is around the visible region.. My question is.. The visible region is so small, why would the emission spectra be exactly there? Is it just chance or is there some reason for it? Thanks
- Omri (age 18)
Stockholm, Sweden
A:
It's not a coincidence at all. In between evolution and the laws of physics, this approximate result is inevitable. There are several relevant facts. One is that it's obviously advantageous to evolve eyes that are sensitive to the most abundant light. So having eyes that work well in the range given off by the neighboring star pays off. Eyes sensitive to gamma-rays or even the some far-ultraviolet wouldn't have much to work with. There is a lot of infrared light around, as you can see with infrared cameras, but the detection process is somewhat more problematic since we ourselves are good sources of infrared.

So what sets the wavelength/frequency scale of the stellar light? It's not the core stellar temperature, which is far too hot and would indeed involve lots of gamma rays. They, however, are absorbed by all the freely moving electrons in the stellar interior. It's the surface temperature that matters. And that temperature is set by the temperature at which the gases (mainly hydrogen) become transparent. That amounts to the temperature corresponding to the energy scale kT. where k is Boltzmann's constant and T is absolute temperature)  of the binding energy of electrons to hydrogen atoms. So the frequency of the light peaks right around the range where a single photon has an energy (hf, where h is Planck's constant and f is frequency) typical for the energy of chemical changes. That also makes it straightforward for evolution to find molecules whose chemical state changes upon absorbing a photon of the light.

Mike W.

(published on 02/25/2012)

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