> Sun should be capitalized when referring to the star at the center of our solar system, as opposed to other suns -- the stars at the center of other planetary systems.
> As we came to understand that the stars in the sky are other suns [...]
- http://science.nasa.gov/astrophysics/focus-areas/exoplanet-e...
See also the 2nd definition at http://www.merriam-webster.com/dictionary/sun :
> : any star that has planets which move around it
> Sun should be capitalized when referring to the star at the center of our solar system, as opposed to other suns -- the stars at the center of other planetary systems.