# Wavelength of light in a gravitational field

I'm having a light source which can emit a particular wavelength of light, on an extremely massive planet.
Light will be affected by gravity, So will the wavelength of the light source increase?
As per this equation?
$$p=\frac{h}{\lambda}$$ If it does so, then is this the reason why extremely massive stars emit red light?

• Extremely massive stars (like O and B stars) are hot and blue not red. Extremely large stars tend to be red because they are usually red giants and quite cool. Commented Nov 22, 2015 at 16:33
• @JohnRennie But,Will the wavelength increase? Commented Nov 22, 2015 at 16:36
• @SathyaramGanapathy: It is almost always a mistake to put two or more questions into a single question here. Its a law: people will only answer what you consider a minor subsidiary question and ignore the question you really wanted answering. Commented Nov 23, 2015 at 11:15

## 2 Answers

The star emits red light because it has a surface temperature somewhere around 3,500 Kelvin. Our sun has a temperature of about 6,ooo Kelvin so it emits yellow/white light. Like any black body radiation, the temperature determines the frequency of the emitted photons. As for the gravity potion of your question, I may be wrong but I think you talking about photons that have been linearly slowed down (not their oscillating frequency) so that there wavelength has shifted as with the Doppler effect.

Be careful with the terminology. The emission of light depends from the temperature of the emitting body. The bandwidth goes from infrared to red and blue to ultraviolet than higher the temperature.

Red and blue shift means, that the well known spectrum from elements like hydrogen is shifted to the blue or red side. And yes, this happens due to the doppler effect AND due to gravitational potential. Than more mass has a star than more the spectra are moved to red.